Author Topic: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers  (Read 3610 times)

Offline AFreshLad

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Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« on: September 15, 2021, 08:40:19 AM »
I have built my first SSTC a while ago now and have been running it perfectly fine using an external clock signal at 100% duty cycle at high power. But now I want to add an interrupt circuit to be able to play music on it, and thus need the coil to run on a feedback signal. So I attached a CT at the primary and successfully got a clean enough feedback signal that was able to drive the coil. But the problem is that the moment it strikes an arc to ground, the coil shuts down because the power supply shuts off (its a switch mode PSU I used for low power testing). I'm not sure what the problem is, is it that the frequency of the feedback will decrease when an arc strikes ground and basically cause the primary to become a short?

Also, my feedback circuit consists of the CT output put across a shunt, and its voltage amplified by a Darlington array, which creates a good enough square wave, so why is a Schmitt trigger always used, does it have some other advantage I don't know of? And the other feedback circuit I've seen is the use of a 4046 IC, which I've researched a bit, but still don't really understand why its used. Is it because the 4046 will continue to output the default 'center' frequency if the tesla coil feedback crashes and goes outside of the lock range? I'm just looking to find the best and most stable way to get a feedback signal and any help will be appreciated.
Thanks!
« Last Edit: September 15, 2021, 03:16:05 PM by AFreshLad »

Offline davekni

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #1 on: September 16, 2021, 06:19:40 AM »
Quote
But the problem is that the moment it strikes an arc to ground, the coil shuts down because the power supply shuts off (its a switch mode PSU I used for low power testing).
It may be electrical noise from the arc confusing the supply rather than overload.
David Knierim

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #2 on: September 16, 2021, 07:41:05 AM »
I'm not sure how that could occur though, because when I run the coil with an external signal, even when I strike an arc to ground its completely fine.

Offline Duane B

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #3 on: September 16, 2021, 11:05:56 PM »

why is a Schmitt trigger always used

A Schmitt trigger has hysteresis so it is somewhat immune to noise. Say you have 30 mv of noise on the signal you are switching on, the Schmitt trigger will only switch once at the crossover point, but a regular gate may switch several times in the 30 mv window where the noise is..
Duane Bylund

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #4 on: September 17, 2021, 12:41:16 AM »

why is a Schmitt trigger always used

A Schmitt trigger has hysteresis so it is somewhat immune to noise. Say you have 30 mv of noise on the signal you are switching on, the Schmitt trigger will only switch once at the crossover point, but a regular gate may switch several times in the 30 mv window where the noise is..
Thanks for the explanation of the Schmitt trigger but I'm still confused as to why a 4046 IC would be used? Does it also just provide filtering of some sort or does it have more features? Thanks.

Offline Duane B

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Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #6 on: September 17, 2021, 02:20:14 AM »
More info on the 4046:

https://highvoltageforum.net/index.php?topic=460.0

From what I understand from reading the link is the 4046 only for providing a feedback signal within a limited frequency so that if the feedback goes beyond the limits, its return to the center default frequency?

Offline Magneticitist

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #7 on: September 17, 2021, 03:12:39 AM »
There are varying levels of intuitive design when it comes to PLL circuits but the ones on the simpler side just require you set that frequency range and hope you can find the secondary resonance within it. I believe fine tuning how small a window this range is will allow you to fine tune the phasing better. If you've ever looked at PLL SSTC's and thought "wow that's nice fat output" it's probably because of the tuned phase angle which is allowing maximum power throughput. When I played with the basic PLL circuit (Scopeboy's) I found it very easy to use an antenna and just change the timing capacitor to give me something around maybe 150-200khz window with my secondary resonance fairly close to the middle.   

The VCO on the PLL will output a 50% square wave at all times with a frequency depending on the timing components. Tuning that in til it finds a lock by getting resonant feedback will then allow you to use the same potentiometer for frequency tuning to then adjust the phase angle. If for whatever reason it drops out of lock it may find it again but not necessarily during the time you're loading it causing it to drop out. I think for me I had to play with the values a little til noticing I was able to load the coil various ways while maintaining lock and was then able to crank full voltage. Before that tuning I could bring my hand close or something and it would drop out. Steve Conner and Steve Ward I think have both designed more intuitive PLL circuits that are robust enough to handle ground strikes and various loading without difficulty but those seemed pretty complicated to me.

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #8 on: September 17, 2021, 05:26:51 AM »
By loading the coil do you mean just bringing objects close to the coil or just straight shorting it out with a grounded arc. I can understand how loading it by increasing the topload capacitance can keep the feedback inside the lock range, but I don't really understand what happens to the feedback frequency when a ground arc is made. From my experience I'm guessing the frequency decreases or increases way outside what it should be, and outside the lock range of a 4046. Could it also depend on the type and placement of the feedback, because as you said, you used an antenna which gets feedback from the secondary, whilst I used a CT on the primary, and as my tesla coil is not a DRSSTC, I'm guessing when the secondary is shorted with a ground arc, the primary doesn't really follow the resonant frequency of the secondary and rather follows its own resonance with whatever parasitic capacitances and inductances it has. I'm guessing this because the one tesla tesla coil is a DRSSTC and they got away with just a CT on the primary with no 4046, allowing them to use the resonance of the primary if the secondary were to short.

Also, concerning the phase angle, I'm guessing the feedback signal should be somewhat phase shifted from the signal I want to output, as the 4046 cannot give you a signal with the same frequency and phase as the output (I think) as the VCO needs some voltage from the phase comparator. So to just make things easier, as tuning a 4046 seems very finicky, should I just have a separate circuit that phase shifts the signal? I have already done this somewhat crudely with my current feedback circuit, by adding a capacitor and potentiometer in series across the feedback signal, allowing me to adjust the capacitive and resistive component and give me theoretically 0 - 90 degree adjustable phase shift, and it does work to an extent.

Thanks for all the help!!

Offline johnf

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #9 on: September 17, 2021, 06:45:00 AM »
Be very careful as to which 4046 you use
not all vendors ones are the same with some nasty quirks
I standardised on the Philips one as it had a very good datasheet

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #10 on: September 17, 2021, 07:26:06 AM »
I have already ordered mine from texas instruments. The datasheet isn't as extensive as the Phillips one but I'm hoping that the IC being from a reputable brand will be suitable.

Offline Magneticitist

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #11 on: September 17, 2021, 08:51:50 AM »
I'd heard from others that the PLL can be real finnicky but I was surprised when playing with some cheap CD4046's I got from ebay. I could pretty much do anything with the antenna, it was unremarkable the location or orientation etc. It's just a short piece of wire poking out randomly now. I had a long antenna fall over and touch the secondary a little then just lay flat and still, I saw no change in output at the time. I'd never pulled arcs directly to ground with it but that's what I heard was problematic, not so much that it should inherently have trouble getting a lock back but that it could cause the chip to try locking onto all types of dirty edges way outside of the f range so yes the secondary feedback seems to be problematic there. I suppose you could do the phase shifting externally but I sort of just find the CD4046 kind of handy if I just want to run 12v logic using just that IC and a couple gate drivers for example. If you just want reliable feedback, using a CT on the secondary seems reliable enough. How you are implementing primary feedback currently may be your problem though. I would think unless you've actually got the primary running as an LC tank at a frequency close to your secondary resonance then there could be all kinds of phase shift issues. Gate drivers with enables like the UCC seem to reliably allow me to run secondary feedback and interrupt without a startup oscillator because of the enable pulse on the driver outputs. Likewise when running CW I can switch the enable high and the pulse will start the feedback loop so long as there is voltage on the bus to do it. As for your dropouts it's hard to say as I guess it depends on the PSU but I can say I've never really seen issues with running secondary feedback, a decent CT, and something like a 74HC14.

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #12 on: September 18, 2021, 03:32:45 AM »
I would think unless you've actually got the primary running as an LC tank at a frequency close to your secondary resonance then there could be all kinds of phase shift issues.
Do you mean phase shift issues or frequency issues, because my guess was that the frequency of the primary just crashes too high or too low when the secondary is shorted with a ground arc. I have also been able to get the primary exactly in phase with the secondary by finely adjusting an external frequency source. Also, as my primary isn't an LC circuit, I can see that putting the CT on the secondary is probably better for feedback, but I don't know what would happen to this feedback if the secondary is shorted with a ground arc. Since this arc is basically shorting the capacitor that is the topload, wouldn't the LC circuit just fall apart, and stop resonating.

I'd never pulled arcs directly to ground with it but that's what I heard was problematic, not so much that it should inherently have trouble getting a lock back but that it could cause the chip to try locking onto all types of dirty edges way outside of the f range so yes the secondary feedback seems to be problematic there.
Also, I don't understand how your 4046 IC locked onto any signals outside the F range, as I thought it was supposed to only lock onto frequencies within the range you set.
Overall, I think I am going to go with the 4046 IC and not a 74HC14 although it seems reliable with feedback from the secondary as I want to be able to draw ground arcs with a grounded rod.
Thanks!

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #13 on: September 18, 2021, 09:08:00 AM »
I have been researching the 4046 more and looking into many people's 4046 circuit designs for tesla coils and I realized what you meant by the circuit locking onto random noise, as the datasheet says the phase comparator 1 has a tendency to lock onto harmonics, although I'm still not sure why it does. Also, I saw that the phase comparator 2 has many advantages over comparator 1, with no locking into harmonics, doesn't require 50% duty cycle input, and always has 0 degree phase difference, so why is phase comparator 2 never used. The only reason I can see is that the Vco of phase comparator 2 will go to 0 when no lock is made, which I'm guessing cannot work as the frequency of a tesla coil always decreases, not increases due to objects placed near it/ loading it. But I feel like this is such a small problem that has some hacky workaround, and I would really like to use comparator 2 as it seems so much better. Especially concerning the phase shift, comparator 1 shifts from 0 to 180 degrees as it goes from Fmin to Fmax, so it seems its not as suitable as comparator 2 in keeping the tesla coil perfectly in phase.

Also with the circuits that play audio, I see that the audio signal is injected into the signal feedback from the phase comparator to the VCO where the Low Pass Filter is, and I don't exactly get how that works. Does it just force the VCO signal into ground or VCC which forces the tesla coil out of resonance and 'turn off' the arc, and then return the tesla coil to its own feedback turning the arc 'on' again. Or is it actually modulating the VCO in with an analog signal from the audio, causing the tesla coil to shift off of resonance proportional to the audio wave amplitude and creating analog audio waves. If so, for a tesla coil capable of 100% duty cycle, an interrupt circuit wouldn't be necessary to play music right?

Offline AstRii

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #14 on: September 18, 2021, 10:42:45 PM »
Also with the circuits that play audio, I see that the audio signal is injected into the signal feedback from the phase comparator to the VCO where the Low Pass Filter is, and I don't exactly get how that works. Does it just force the VCO signal into ground or VCC which forces the tesla coil out of resonance and 'turn off' the arc, and then return the tesla coil to its own feedback turning the arc 'on' again. Or is it actually modulating the VCO in with an analog signal from the audio, causing the tesla coil to shift off of resonance proportional to the audio wave amplitude and creating analog audio waves. If so, for a tesla coil capable of 100% duty cycle, an interrupt circuit wouldn't be necessary to play music right?

Exactly as you stated, the VCO is modulated with analog audio which shifts the frequency off resonance. The output power is then modulated with the audio signal and that creates perfect analog audio playback.
I have made a few of these analog audio modulated coils, you can see them here:
https://www.uhvlab.org/sstc-ii
https://www.uhvlab.org/ctu-sstc


Marek Novotny
Czech Republic, Czech Technical University in Prague
www.uhvlab.org

Offline Magneticitist

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #15 on: September 18, 2021, 11:33:37 PM »
My setup never locked onto bad frequencies but this is what I hear can happen as the IC looks to track the various dirty edges. It essentially confuses the chip as it tries to lock onto something it wont. Theoretically I suppose you don't need a dual resonant primary to use primary feedback but if you had some method of doing it by amplifying the feedback signal enough as well as filtering the really high frequencies the primary may want to ring at it CAN work. (right?) Assuming you had this ironed out I would imagine only phase shift issues could become evident if this shift became too much during loading.
I'm not really sure how you're going about this but I'd think secondary feedback would in fact do you a little better. Pulling arcs to ground has never seemed an issue to me with a good SSTC, more so a problem with a DRSSTC without any kind of OCD.

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #16 on: September 19, 2021, 04:14:18 AM »
Thanks for all the feedback!
After researching more into PLL and running a circuit sim on it, I think I have quite a good idea on how the XOR gate and VCO work together, and I sort of derived the equation for calculating F capture range of an XOR phase detector. I'm pretty sure the locking onto harmonics problem of an XOR phase detector is limited to the range of the VCO which is adjustable, and should be avoidable by making the VCO range small. But still, I want to try to find a way to use phase comparator 2 due to its ability to keep a 0 phase shift. From what I know, phase comparator 2 will not return to center if frequency lock is lost, but will still go all the way to the rails, either high or low depending on whether the frequency in is below or above center frequency. Thus, if I set the limits correctly and design a good feedback that keeps the frequency within these limits, then it should work fine, right? and even if it were to go outside the range, it should keep the frequency limited to a safe level at least. I have looked into steve's driver design from:
https://www.stevehv.4hv.org/CWcoil/PLLSchem.JPG
and he has used phase comparator 2, but I don't understand why he used an external delay circuit on the feedback from the VCO to the phase comparator to change the phase of the output waveform rather than biasing it like done on a type 1 phase comparator. To me it logically makes sense that it would still work with biasing, and I confirmed that using a circuit simulator. The only thing I noticed was that the phase shift varied quite a bit depending on if the feedback frequency proportional voltage for the VCO was closer or further from the biasing voltage, as that would determine how much the biasing voltage would effect it the circuit, but still it works. I think I will try to use phase comparator 2 just for the 0 phase shift ability and try to design a feedback signal from the secondary that won't drop out under any circumstance.

Offline davekni

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #17 on: September 19, 2021, 06:06:44 AM »
Quote
To me it logically makes sense that it would still work with biasing, and I confirmed that using a circuit simulator. The only thing I noticed was that the phase shift varied quite a bit depending on if the feedback frequency proportional voltage for the VCO was closer or further from the biasing voltage, as that would determine how much the biasing voltage would effect it the circuit, but still it works.
Yes, biasing phase comparator 2 does work.  I did that once decades ago, though not for a Tesla coil.  Ideal version would have phase comparator 2 output be + and - current sources rather than switches.  Then a DC current source would provide linear phase adjustment.  I did use a current source for biasing, even though 4046 phase comparator 2 output is not current source.

As the datasheet mentions, noise sensitivity is the primary disadvantage of phase comparator 2.  Input signal glitches count as valid edges, increasing input frequency.  I'd suggest a schmitt-trigger input buffer or inverter before the 4046.
David Knierim

Offline Magneticitist

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #18 on: September 19, 2021, 06:32:18 AM »
I agree about using both IC's. Teslista555 and others have seemed to benefit from adding the schmitt trigger stage. This was going to be how I went about another PLL if I got around to it.

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #19 on: September 19, 2021, 10:15:24 AM »
Yeah, I have ordered Schmit triggers alongside the 4046s will include them in the circuit between the PLL input and CT, as it doesn't really have any negative effects.

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #20 on: September 19, 2021, 03:16:40 PM »
Quote
To me it logically makes sense that it would still work with biasing, and I confirmed that using a circuit simulator. The only thing I noticed was that the phase shift varied quite a bit depending on if the feedback frequency proportional voltage for the VCO was closer or further from the biasing voltage, as that would determine how much the biasing voltage would effect it the circuit, but still it works.
Ideal version would have phase comparator 2 output be + and - current sources rather than switches.  Then a DC current source would provide linear phase adjustment.  I did use a current source for biasing, even though 4046 phase comparator 2 output is not current source.

I've been giving it some though and I don't really understand why current sources would be ideal rather than voltage sources. I understand that the biasing works of principles of voltage dividers but don't know how a current version of that would be put together, especially when feeding the signal into the VCO which is voltage controlled. Could you send an example of a circuit that uses current sources so I could try to understand it. Thanks!

Offline davekni

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #21 on: September 19, 2021, 08:19:01 PM »
Last time I used a 4046 was ~4 decades ago, so I don't have any circuits around to share.

Concerning why current is better for phase comparator 2:  Consider the nominal case when VCO is center-frequency (VCO input pin 9 at center between supply voltages).  Look at figure 5 of the Philips HEF4046B datasheet.  Phase comparator 2 output is mostly open (zero current).  At VCO output edges, it momentarily pulses high or low, causing a current of +-VDD/2/R3 to slightly charge or discharge low-pass filter capacitor C2.  The charge added to or subtracted from C2 is proportional to phase error (fixed current of +-VDD/2/R3 times varying pulse width based on phase error).

Instead of centered frequency, consider C2 at VDD/4.  Now phase comparator 2 output current is +3VDD/4/R3 or -VDD/4/R3.  PLL loop gain is now asymmetric, higher for increasing frequency and lower for decreasing frequency.  Switched current sources would fix this asymmetry, making PLL loop gain constant.
David Knierim

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #22 on: September 20, 2021, 04:28:03 AM »
I think I get what your saying, but when you say current sources, do you mean constant current sources, because otherwise, it doesn't make sense to me. But assuming you do mean constant current sources, I'm guessing then the biasing would also work as a constant phase shift that wont change across the 2 F lock range as it would bias the capacitor the same amount across the whole range. But if I were to hypothetically build a current based system, wouldn't I need power rails beyond the logic levels the system works at, as otherwise, I'm guessing the current will taper off at limits of the logic levels (unless a theoretical system with 0 resistance is used somehow). Also, just out of curiosity, this phase shift that is created, is it a constant delay, like a constant time period that the output wave is shifted from the input, or is it a constant phase angle shift? My guess is that its an angle shift as adding a current bias to the comparator output means the VCO will have to compensate that by creating a phase shift. As phase comparator 2 when dealing with a phase shift and same frequency outputs only either positive or negative current sources, in this case the opposite polarity of the current biasing to cancel it out, it will create the perfect phase shift that creates an average current that cancels the bias current. But this same delay between the two comparator input signals, if replicated for a higher frequency means the current output will occur more frequently, and thus increase the average current above that of the biasing current. Thus to cancel out the effect of the bias, it would have to reduce the length of the current spikes/ reduce duty cycle, hence decreasing the delay between the two waves, and vise versa for lower frequencies. This to me isn't definitive proof of it shifting the phase angle a set amount, but does point more towards a phase angle shift rather than delay, as from my understanding the delay changes with frequency.

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #23 on: September 20, 2021, 04:59:17 AM »
I have also been thinking about the XOR phase comparator in terms of a 'current' mode rather than voltage, and am I correct in saying that the only time a current mode will be useful is when biasing, as with voltage biasing it seems the phase shift varies faster from 0  - 180 degrees that without biasing, (due to asymmetric amounts of current from the biasing system at different frequencies) limiting the F lock range of the XOR gate below the VCO range, whilst a constant current biasing system would just introduce a constant bias that would just slightly clip the F lock range as the 0 - 180 phase shift range of the XOR gate has been slightly shifted, with a part of it outside the VCO range. But if no biasing was used, my guess is a current mode on an XOR comparator is useless, as, from what I know, an XOR comparator doesn't really have a gain, but rather, when two different frequencies are input, just outputs a sort of PWM signal where the duty cycle varies sort of sinusoidally, (which gets filtered into a sort of sine wave by the LC filter) and if this sine wave manages to clip the VCO voltage proportional to the frequency of the input signal, will lock onto it, no gain or anything like that. This concept was also how I was able to derive the equation of the F capture range of a type 1 PLL circuit.

Offline davekni

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #24 on: September 20, 2021, 05:31:57 AM »
Yes, your reply #22 looks accurate.  A constant bias current would produce a constant phase offset.  This is true IF phase comparator 2 put out currents.  With a real 4046, phase comparator 2 currents depend on R3 and on VCO voltage, so phase shift will also depend on VCO voltage.

For me, it is easier to think of phase comparator 1 as putting out a voltage rather than current.  Unlike phase comparator 2, phase comparator 1 is never high-impedance.  Think of it as generating an average voltage using PWM.  Range is 0 to VDD for 0 to 180 degrees phase input.

Yes, a current source that handles 0 to VDD requires supply voltages beyond 0 and VDD.  You can get close with rail-to-rail opamps without additional supply voltages.  Look for app-notes and opamp datasheets with current-source examples.
David Knierim

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #25 on: September 20, 2021, 06:03:57 AM »
Thanks for the input
I understand that a type 1 comparator never has a high impedance mode, but wouldn't the concept of a constant current source still apply, where the comparator will just output + and - current signals instead of positive and negative voltages, which are biased by a constant current. Further, even after the low pass filter, doesn't the whole concept of an XOR comparator depend on the voltage output having some oscillation. The F capture range of an XOR comparator on the datasheet is determined by the RC filter, where a 'stronger' RC filter with a lower cutoff frequency means the capture range is smaller, and thus if it were giving out a perfect, flat voltage, wouldn't it fail to lock onto a frequency from a state of not locked.

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #26 on: September 20, 2021, 10:39:59 AM »
Hi
I have just finished attaching and testing the PLL circuit with a Schmitt trigger and it works perfectly. It was not at all finicky and the phase comparator 2 works perfectly. It keeps a lock on the signal through any situation, ground arcs etc, and even when shorting the secondary to ground recovers instantly after removing the rod. Further, by fine-tuning the phase shift using a circuit similar to Steve Wards linked here:
https://www.stevehv.4hv.org/CWcoil/PLLSchem.JPG
I was able to get 0 current switching to happen on my half-bridge, meaning I can finally attach the interrupt circuit without creating huge voltage spikes on my bridge. The 4046 IC was even run on a breadboard, with long looping wires connecting things at it still ran just fine (although the coil was running at very low power so less interference). The only problem I can see with phase comparator 2 is that I have set F min at ~200KHz whilst my coil resonates at ~296KHz which means the coil won't start by it self, instead I have to short the secondary and draw out an arc to start the coil. This problem might be solved at higher powers, but either way, it could also become a problem during interrupts if the secondary oscillations die out during the turn off period and doesn't restart oscillations again. This could also be solved by increasing F min.

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #27 on: September 21, 2021, 06:04:52 AM »
Quote
Further, by fine-tuning the phase shift using a circuit similar to Steve Wards linked here:
https://www.stevehv.4hv.org/CWcoil/PLLSchem.JPG
Yes, feedback delay is another way of adjusting phase lead.  Steve also has a switch to hold VCO frequency constant during interruption.

Was thinking more about phase comparitor 2 linearity.  Both phase comparitor 2 and phase adjustment can be made linear without using current sources, by using an opamp integrator instead of a plain R and C filter:



This keeps the integrator input voltage at VDD/2, so R3 and phase adjust R6 feed into a constant voltage.  Makes resistor currents constant without needing current sources.

Note that the opamp integrator is inverting.  To correct for this, the two phase comparitor input signals are swapped in the above schematic.

Of course, since you have something working now, no need to change anything.

Congratulations on success!
David Knierim

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #28 on: September 22, 2021, 05:26:40 AM »
Congratulations on success!
Thanks!
I am not very familiar with Op amps and integrator circuits, but from the little I know, this looks similar to an inverting amplifier, but with a capacitor C2 rather than a resistor. If it were a resistor, I know the output will be just an amplified and inverted signal from the input, with the input acting as a virtual ground, in this case getting fixed at VDD/2. So, I guess it sort of makes sense that putting a capacitor there, which would only let through changes in voltage, aka dV/ dt, rather than the actual voltage means that to keep the virtual ground at VDD/2, the output must become the integral of Voltage over time. Is this correct? Also seeing that the capacitor differentiates, I'm guessing any information about DC offset is lost, so the output integral has an offset / 'C' value of 0. And this is where I think this actually is useful for adding linear phase shift, as by adding the phase adjust pot on the output of the capacitor which has removed DC offset, the effect of the offset pot is constant, as the same current will 'leak' into the pot at any frequency for the VCO which creates a constant shift on the output. I'm guessing the actual integration function of the circuit is useless.

And yes, the constant time period shift created using Steve Ward's circuit works just fine, so I won't be adding any of this Op amp stuff, I'm just curious as to how it works. Also, I didn't require the circuit that keeps the coil at resonance during interrupts, as when interrupted it has no problem 'pinging' the secondary to create enough feedback to start resonance. Further, in my brief look at the secondary feedback during interruptions, it doesn't really die down enough that the signal is lost between interrupts, so I don't think that safeguard was necessary for my coil at least.

Thanks a lot for all the help so far, I think now I'm going to try to put all this stuff on an actual circuit board rather than a bread board, try to reduce the parasitic inductances and finally, move to a full bridge design.

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #29 on: September 22, 2021, 07:07:48 AM »
Quote
aka dV/ dt,
I think you are confusing differentiation (dV/dt) with integration, which is the opposite.

Quote
means that to keep the virtual ground at VDD/2, the output must become the integral of Voltage over time.
Yes, this part is correct.

Quote
I'm guessing the actual integration function of the circuit is useless.
No, integration is the goal.  Current into a capacitor is also integrated.  Integrators are a type of low-pass filter with theoretically infinite DC gain and constant 90 degree phase shift.

For normal PLL use, a pure integrator would form an unstable loop.  It's 90 degree phase shift combined with 90 degrees of the VCO (phase is the integral of frequency) makes for 180 degrees total (0 degrees phase margin).  However, for Tesla coil use, a PLL is locking to a phase-shifted version of its own VCO output, not to an independent reference frequency.  That bypasses the 90 degrees of frequency-to-phase.
David Knierim

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #30 on: September 22, 2021, 10:41:20 AM »
Doesn't a capacitor act as a differentiator. It only lets through changes in voltage, and the current through it is proportional to the rate of change of voltage, right? That's what I meant by the capacitor acting as a dV/dt differentiator of the output of the op amp, which if fed into the input like it is, means in order to keep the input at VDD/2 (virtual ground), the output of the opamp must be the integral of the signal, such that after the capacitor 'differentiates' this integral output, the original signal is present, hence keeping the input at VDD/2.

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #31 on: September 23, 2021, 05:48:40 AM »
Quote
Doesn't a capacitor act as a differentiator.
A capacitor by itself has two equivalent properties.  Current is the derivative of voltage with time (times capacitance value), and voltage is the integral of current over time (divided by capacitance value).  Both are equally true, different ways to state the same fact.  This opamp circuit is an integrator.  Input voltage is converted to current by R3.  R3 current feeds C2.  C2 voltage (opamp output voltage) is the circuit output, the negative time integral of input voltage.  (This presumes input and output voltages are measured relative to VDD/2.)  If R3 and C2 were swapped, the circuit would become a differentiator (except fur opamp stability issues).
David Knierim

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #32 on: September 23, 2021, 11:12:25 AM »
Thanks for the answers, but I'm still not fully getting it. I think I'll wait till I hopefully study this in uni before I burn myself out trying to understand it without basic knowledge.

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #33 on: September 26, 2021, 10:32:14 AM »
Hi, I'm back with a new problem and I don't think this warrants a new thread so I'll put it here.
I have rebuilt my whole bridge and driver circuit as the old version was quite bad in terms of parasitic inductances etc. But now, with the new bridge, it runs perfectly, and I do get zero current switching, with no voltage spikes on the turn off of an interrupt cycle. But on the turn-on, there are huge voltage spikes. I probed the voltage across the primary and the feedback signal from the secondary and, while the coil is running, see a 90 ish degree phase shift between the feedback and the primary voltage, which makes sense. But during the off-time of the interrupt, the primary voltage induced by the coupling between the secondary and primary has a 180 ish degree phase shift from the feedback signal, changing instantly from the original 90-degree shift the moment the coil turns off. I'm not exactly sure why this occurs, but my guess is that it has something to do with the series capacitors with the primary that are present in steve ward's coil design (I don't have them) linked here:
https://www.stevehv.4hv.org/CWcoil/PLLSchem.JPG
These capacitors look like they have a similar job to the capacitor between the current transformer and the clipping diodes. I don't exactly know what exactly it does though, my guess is that it provides a consistent ish load for the CT or in this case for the primary coil rather than just loading it when the voltage reaches the rails where the clipping diodes/ body diodes of the mosfets start conducting. Also, due to the large currents that flow through the primary and high duty cycles, I'm guessing I'll need a very high-performance capacitor with low ESR, and I don't have any such capacitors on hand. Also, concerning these caps, what voltage should they be rated at, do they need to be rated at the voltage the bridge runs on or lower since I'm pretty sure that voltage will never fully fall across the capacitor.

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #34 on: September 26, 2021, 02:26:25 PM »
I did some thinking and some research and my previous explanation doesn't really seem right. I read that adding those series capacitors cancels out the inductance of the bridge maybe? I'm now quite lost on what these capacitors do and if they will fix my problem at all, but as far as I can see this is the only fundamental difference between my design and Steve Ward's design, which implements interrupts. Here is a scope capture of what I was talking about in the previous post about voltage spikes.

Pink is the voltage across the drain and source of one of the Gate MOSFETs (I'm running a half-bridge)
Yellow is the gate to source voltage on the same MOSFET
Blue is the raw output from the CT on the secondary before any delays and filtering stuff is done.

You can see how after the yellow gate voltage shuts off, the voltage on the MOSFET gate to drain still oscillates due to, I'm guessing, it being back driven by the secondary that's oscillating by itself. But these oscillations are now suddenly shifted from where they were before with respect to the blue feedback from the secondary, which is why I think the voltage spikes at turn on are created as now the MOSFET switches at the wrong time and I'm guessing causing some sort of inductive discharge, but I'm sort of lost on this.
Any help will be appreciated, Thanks!

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #35 on: September 26, 2021, 07:51:40 PM »
Scope traces look roughly as would be expected.  I think most SSTCs are interrupted with longer off-times, so that ring-down is finished before the next enable edge.

For better understanding, I suggest learning an analog simulator (SPICE variant).  There are several free ones.  I use LTSpice.  There are simpler on-line simulators too.  No need to simulate your entire circuit.  Start with just a pulsed voltage source into a series L/C circuit.
David Knierim

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #36 on: September 27, 2021, 05:26:36 AM »
Oh, that makes sense as I'm creating my interrupts with an arduino and the off-time is quite short. But I still don't understand why the phase of the voltage on the primary shifts so suddenly from the feedback when the gate drive signal is off. Is there any way to keep the on times high and the off times low as I would like the maximum output from this during music playback (as it is a SSTC).
Thanks for the help!

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #37 on: September 28, 2021, 10:03:43 AM »
I've ran a simulation on my tesla coil circuit and I haven't seen any phase shift like present on the scope capture. I'm completely lost on what's causing it and my best guess is it has something to do with a smaller coil in the middle driving a larger coil one the outside (secondary driving primary) being different somehow to the large coil driving the smaller coil in the middle (primary driving secondary). Do you have any idea what's going on?
Thanks

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #38 on: September 29, 2021, 05:28:01 AM »
Please share your simulation schematic and result plots, and for reference which simulator you are using.  Then it will be easier to help.
David Knierim

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #39 on: September 29, 2021, 06:36:30 AM »
I'm not too familiar with simulations so I just used a web-based one linked here:
https://www.falstad.com/circuit/
When I first used it, and fixed the primary gate drive signal at close to resonance (I simulated the MOSFETs as well), I got the primary and secondary to be in phase, and when the signal was disconnected it remained in phase, which is where I was stuck. But now, while tinkering with it more, I somehow got the primary to drive the secondary whilst being 180 degrees out of phase, and when the primary gate drive was turned off, cause the primary to flip 180 degrees when back driven by the secondary, just as in real life, but I have no idea why. I'm going to experiment more with the simulation to try to figure it out. I don't know what simulation schematics and result plots are, I only have a screenshot of where it did flip 180 degrees.
Thanks!

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #40 on: September 30, 2021, 05:34:40 AM »
That screen shot shows the results plot of two signals.  Can you make a screen capture of the schematic (circuit diagram) also?
David Knierim

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #41 on: October 06, 2021, 08:08:50 AM »
Hi, sorry I didn't reply sooner, I have just been trying to design and test a new circuit.
The problem about the phase shift stuff before I realized wasn't a problem, I must of had a brain fart for 3 days before I realized that of course the voltage would shift 180 degrees as the 'voltage source' for the primary is not the MOSFET side but the transformer side during the off time. I scoped the current on the primary and saw that it remained mostly in-phase, but did shift enough such that during the turn-on event, it would switch whilst a reasonable current was flowing through the primary, which I think is causing the voltage spikes.

My solution for this is what I've been working on, and I chose to put a CT on the primary as well, and use the feedback from that to time the turn-off and turn on events, whilst the frequency feedback is still driven by the CT on the secondary. But if I continued using the 74HC74 D type flip flop for the interrupt controller, it would be very unstable as if no current was in the primary, e.g. at startup of it the turn off time is long causing the coil to ring down to zero, the coil would just refuse to turn on as no clock edges were fed into the flip flop to tell it to turn on the coil. So I had to redesign the circuit and created feedback that pulled a signal low at zero current and high everywhere else, hence allowing the coil to startup even if the current in the primary is 0. This means I would need an SR latch rather than a D-type flip flop, and I don't have any latches so I couldn't test the circuit on the tesla coil, but the feedback circuit works and produces a usable signal. I also had to add a delay circuit to delay the pulses to match the delay in the driver circuit. My only problem is my circuit is quite 'bulky', uses a lot of components for what I think is a simple task. If you have any better ideas that would be helpful. I attached the schematic below:

The 74HC74 flip flop is for the delay, as I couldn't get enough delay by adjusting the pots on the CT feedback.

Also, I'm bothering with all this rather than just waiting for the coil to ring down as the interrupt generator I'm using in an Arduino running the TMRPCM library, which plays music by using a carrier wave of 32KHz to artificially produce the analog audio waves, and thus the off times are very low.

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #42 on: October 06, 2021, 09:17:29 AM »
I made a mistake with the schematic, the flip flop circuit for delay won't work as it's edge-triggered, defeating the whole purpose of the rest of the circuit. So I now need a different delay circuit design that can delay the pulse by about 800ns. Any ideas will be helpful.
Thanks
« Last Edit: October 06, 2021, 09:26:35 AM by AFreshLad »

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #43 on: October 06, 2021, 02:26:00 PM »
I have just redesigned and built a new circuit which does successfully delay the signal whilst also keeping all the signal information by testing for both the rising and falling edge. However, the circuit is now very complex, and looking at other pulse delay circuits such as the 555 timer ones, its unnecessarily complex. The only problem is the 555 timer delay circuits I've seen delay by a large amount, about 100us, while I was a delay of about 800ns. Do you have any ideas on a simple delay circuit that keeps all the waveform information (as in if the signal in goes low, the signal out should go low and vise versa, rather than just using one rising edge to signal the pulses).
Thanks
I attached the new schematic below.

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #44 on: October 07, 2021, 05:29:35 AM »
Schmitt inverter (HC14), series resistor, cap to ground, another schmitt inverter.  Rising edge delay will be a bit longer due to schmitt thresholds being a bit below VDD/2.  Fix that with a resistor in parallel with capacitor.
David Knierim

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #45 on: October 07, 2021, 08:35:45 AM »
I completely missed that idea. Thanks, I will try it, it should most like work much better than what I have now.

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #46 on: November 05, 2021, 02:28:13 PM »
Hi
I have just finished attaching and testing the PLL circuit with a Schmitt trigger and it works perfectly. It was not at all finicky and the phase comparator 2 works perfectly. It keeps a lock on the signal through any situation, ground arcs etc, and even when shorting the secondary to ground recovers instantly after removing the rod. Further, by fine-tuning the phase shift using a circuit similar to Steve Wards linked here:
https://www.stevehv.4hv.org/CWcoil/PLLSchem.JPG
using CP2 is the only way to get properly working PLL, with stable, adjustable switching phase, to compensate driver/MOSFET delays.   
This schematics is quite good, but not optimal.
Especially single-pole LPF may cause phase oscillations, so in my design I'm using two pole filter.

I was able to get 0 current switching to happen on my half-bridge, meaning I can finally attach the interrupt circuit without creating huge voltage spikes on my bridge. The 4046 IC was even run on a breadboard, with long looping wires connecting things at it still ran just fine (although the coil was running at very low power so less interference). The only problem I can see with phase comparator 2 is that I have set F min at ~200KHz whilst my coil resonates at ~296KHz which means the coil won't start by it self, instead I have to short the secondary and draw out an arc to start the coil. This problem might be solved at higher powers, but either way, it could also become a problem during interrupts if the secondary oscillations die out during the turn off period and doesn't restart oscillations again. This could also be solved by increasing F min.

My solution for proper (re)start is quite simple:  VCO voltage should not be kept constant during OFF period, but reset to value for nominal VCO frequency taken from low resistance voltage divider (1:1).
Also in my design I've added extra deadtime before gate drivers, and make automatic phase compensation for this dT and drivers delay by taking CP input signal from driver's output (and with one capacitor I also compensate for MOSFET turn on time). That's way whole tuning is to set VCO center frequency. 
 

 
Zbigniew Bigaj

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #47 on: December 29, 2021, 04:24:31 PM »
Hi, sorry for not responding in a while, it was our test period for school.
I decided not to use an interrupt circuit as trying to find a good way to restart the coil was turning out to be difficult because I wanted the coil to restart whilst the secondary was still resonating, causing the MOSFETs to be back driven by the secondary until I turn the MOSFETs back on, where a huge voltage spike would occur I'm guessing since it switched at a non - zero current position.

To recap, I do have a flip flop circuit to turn off and on the coil at zero current (timed by a CT on the secondary), which works perfectly when turning off, but when the coil is being back driven, it has a slight phase shift from when it's normally driven (relative to the secondary CTs timing) that causes the MOSFETs to switch whilst current is still flowing. I did try putting a CT on the primary so I would have the true current flow through the primary rather than an estimate created by the CT from the secondary, but trying to filter it and use it was either impossible or so unreliable that it would at some point fail and blow up the MOSFETs. I could also wait until the secondary ringing stops, but the interrupter I'm using, which is an Arduino running TMRpcm plays music using a 32KHz carrier wave, which is too quick to let the secondary ring down. I could find a different interrupter but at this point I decided to go with the simpler method of just biasing the VCO input with an audio signal like done here:

https://highvoltageforum.net/index.php?topic=1759.0

With this it works nicely and sounds pretty good, and with an additional class AB amp (as phone headphone output wasn't powerful enough) and increasing the bass on an equalizer app, the bass is more present and causes the arcs to violently jump in sync with the music which adds to the whole look. So now I'm just finishing up the coil and testing reliability as a whole package so I don't need to have an eye on every component in case something blows up, and so far on low power (60V) the driver seems fine. The secondary coil however is heating up quite a bit, softening the PVC tube its built on so much that it starts to become wobbly.

The Question part:

I can't yet test with higher power (since the high voltage MOSFETs are still in the mail), but with higher power, the secondary will definitely fall apart. I'm currently using 0.1mm wire with ~2200 turns and I now see that's way too thin, but my secondary coil has only around 25cm of winding area, and I would like to use at most 0.2mm wire so I can have a decent number of turns, which I'm pretty sure will still heat up quite a lot. So can I water cool the secondary by filling the inside of the PVC tube with water and circulating it through a radiator? Either way, I have to create a new secondary so sealing it at the bottom so I can fill it with water for experimenting wouldn't be that hard.

Thanks for all the help so far, the Tesla Coil is really looking good now.

Offline Zbig

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #48 on: December 29, 2021, 08:04:44 PM »
Water will cause some side efects based on water conductivity:
1) extra capacity to secondary coil
2) extra loss  by eddy currents and capacitance currents
3) possible insulation break to the water   

So in my opinion it is not good idea.
Zbigniew Bigaj

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #49 on: December 30, 2021, 05:12:41 AM »
Would using de-ionised water reduce its effect on capacitance and the eddy currents. Or could I use something like mineral oil or the stuff used for cooling transformers?

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #50 on: December 30, 2021, 10:59:44 AM »
Would using de-ionised water reduce its effect on capacitance and the eddy currents. Or could I use something like mineral oil or the stuff used for cooling transformers?

If any, try oil. But if secondary is on thick insulator, it will also reduce heat transfer to oil.  Transformer windings are immersed in oil.
Zbigniew Bigaj

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #51 on: December 30, 2021, 01:13:19 PM »
But if secondary is on thick insulator, it will also reduce heat transfer to oil.  Transformer windings are immersed in oil.

I think I might try the oil, but I don't think the insulation on the secondary is a problem because the immediate problem I'm facing right now is that the PVC pipe itself is heating so much that it is flopping around, the secondary coil wire itself doesn't seem to be too bad, but still will require cooling for higher power.

Also, I want to run my coil on rectified 240V AC but don't have a variac to try it out. I do have a transformer that'll halve the voltage and I've previously run my coil on around 120V AC and was able to "soft start" the system using one of these (BTA - 4000W):

https://www.ebay.com.au/itm/173878712854?chn=ps&_ul=AU&mkevt=1&mkcid=28

which I think is basically a beefed-up light dimmer, but this circuit isn't powerful enough to handle the current at 240V AC as it basically just shorts out the triac inside and just outputs the full sine wave. So is there any soft start circuit I could build for the huge filtering capacitors, or will I have to get a variac?
Thanks!

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #52 on: January 12, 2022, 01:30:38 PM »
Unfortunately, I haven't been able to try oil cooling my secondary coil yet as I've had to self-isolate (couldn't get oil), but my new MOSFETs arrived in the mail and I ran into a new problem.

The new MOSFETs are the Infineon 60R037P7. I was previously using the Vishay G47N60E MOSFETs. The G47N60E and the 60R037P7 only differ, as far as I can see, in rated drain currents, Vds voltage, and the turn on and off times. However, when I use the 60R037P7 without changing anything else about my tesla coil driver, the whole thing 'glitches' due to a huge amount of noise being introduced into the ground and power lines of the driver, which causes the coil to create even more noise. However using the G47N60E and even a cheap Chinese MOSFET TF68N80 (which has much worse turn on and off times), there is no such noise and the driver functions fine. So it must be a characteristic of the 60R037P7 that is causing the noise but I don't know what it is as my knowledge of MOSFETs is very basic.

Any ideas will be appreciated. Thanks!

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #53 on: January 13, 2022, 05:27:40 AM »
Yes, Infineon 60R037P7 looks to be a very fast-switching FET.  It has low Crss (Miller capacitance), so will have rapid Vds rise and fall times.  That is generally great for low losses, but can cause problems, especially with GDT gate drive.  I've had two such projects with fast FET H-bridges fail for this issue, where the fast rise and fall times and GDT inter-winding capacitance cause high-frequency oscillation at each transition.

Several design details can help mitigate this issue:
1) Clean circuit layout with ground planes, for both driver and H-Bridge.  In case you haven't seen it, here's my recommendation for bridge layouts (half bridge initially, full bridge in second post):
https://highvoltageforum.net/index.php?topic=1324.msg9795#msg9795
2) Minimize GDT turns and twist lead wires.  In case it's helpful, my GDT tutorial so far:
https://highvoltageforum.net/index.php?topic=1854.msg13949#msg13949
3) Add common-mode chokes to GDT leads, especially the high-side FET output(s).  One high-side FET for a half-bridge, two for a full-bridge.  Make these GDT output leads longer and wind several turns around another ferrite core.  Two separate cores for a full-bridge, one for each high-side FET.
4) Add gate series resistors if you don't already have such.  May not help all that much given the low Crss of these FETs.

If the above is not sufficient, then pick slower FETs or change to fast IGBTs.  Or, with a bit more complexity, change to an isolated high-side gate driver IC instead of GDT.
« Last Edit: January 13, 2022, 05:30:01 AM by davekni »
David Knierim

High Voltage Forum

Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #53 on: January 13, 2022, 05:27:40 AM »

 


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