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Messages - davekni

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21
Voltage Multipliers / Re: Suggestions for CW HV feed-through
« on: November 30, 2019, 05:22:19 AM »
Interesting full-wave schematic.  Requires two AC inputs, presumably 180 degrees out of phase.  When reduced to a voltage-doubler, that does not match the normal circuit called a "full wave voltage doubler", which uses a single AC input:


Of course, what circuit is called what doesn't matter.  The voltage multiplier you are building is the one that makes sense for your project (and for mine and likely most others).

22
Voltage Multipliers / Re: Suggestions for CW HV feed-through
« on: November 29, 2019, 09:12:32 PM »
Steve,

I'd thought that "full-wave" applied to only voltage-doubling.   Is there a circuit for higher multiplication factors that's called "full-wave"?

At 1.7nF and 14 stages, parasitic capacitance will have some effect, but probably not excessive.  (I have 16 stages starting with only ~550pF.)

Another thought for your interconnect at the bottom:  How about using a pair of battery-contact springs on the bottom of the multiplier and two pieces of nickle or nickel-plated sheet metal at the bottom of the housing?  Alignment could have very loose tolerances that way.  I expect the weight of your multiplier is well more than twice the contact force of a typical AA battery holder.

23
Voltage Multipliers / Re: Suggestions for CW HV feed-through
« on: November 29, 2019, 07:23:28 PM »
Banana plugs/jacks sounds like a good solution as long as you have the alignment issue solved.  The stamped metal "banana-peel" part typically produces sharp edges of contact, which likely have enough pressure to break through the oil film.  You could bend the banana-peel tabs a bit to create even more pronounced edge contact rather than contact along the surface of the stamped metal.  I'd suggest having the male plug parts on the multiplier in case you need to tweak the tab bending and/or find a part with stronger (higher insertion force) tabs.

BTW, have you ran any simulations on the multiplier including parasitic capacitance between the two capacitor columns?  I was surprised how much voltage loss came from capacitive coupling between columns.  (If you are making a high-power unit with relatively large column capacitors, stray capacitance may not be significant.  I was purposely using as small capacitors as possible for safety - using it as a Van de Graaff alternative.  Also added a string of 30 x 10meg 10kV resistors in series with the output.)

24
My guess for the higher-power instability is power supply capability.  The pulsing load can be difficult for power supplies to handle.  Scoping the supply output will show if the voltage is dropping during each pulse when the noise starts.

The goal is to have roughly 0 degrees phase shift for the entire circuit.  Positive H-Bridge output voltage when the current is positive and negative voltage when current is negative.  Since there are some delays (HC14, gate driver, FETs), a bit of phase lead in the current feedback would be optimum, perhaps 20-30 degrees.  That's for optimum output power.  If using IGBTs where there's more value (efficiency) to switching just before their zero-current point, the optimum phase shift may be slightly different.  Depends on inductance of Tesla primary coil.

With this controller circuit, phase will change as the power increases at the start of each burst.  At the beginning of a burst, the current feedback will have almost 90 degrees phase lead.  That's because the CT output voltage will be under 5Vpp, so the diodes won't conduct, so infinite load impedance.  Current into an inductor creates the 90 degree lead.  As the power builds, the CT output increases and the diodes conduct more of the time, so the effective load resistance on the CT drops towards 1k.  12.6mH at 220kHz is 17.4k reactance, so phase shift is arctan(1k/17.4k) = 3.3 degrees.  That's the limit at infinite power.  So, the phase lead is dropping from almost 90 towards 3.3 at the start of each burst.

R1 in my schematic is just further isolation of the HC14 input from CT feedback voltages below 0 or above 5V.  The 1N4148 diodes clamp the voltage to one diode drop below 0 and above 5V.  That's the same as the HC14 input, so the input and the 1N4148 diodes share the current.  R1 forces most of the current to stay in the 1N4148 clamp diodes.  Modern HC14 inputs can handle input current fairly well without disrupting operation, so R1 isn't all that important.  Old parts were more sensitive.  (The original schematic had a note about changing the clamp diodes to schottky parts, which is another way to accomplish the same goal, as they have lower forward voltage drop than the HC14 input  The suggested 1N5818 part is rather high capacitance, however, so I'd not recommend that particular schottky diode.)

I'm a bit puzzled as to why the scope signal amplitude ramps up at the start of the burst so much faster than it ramps down at the end.  Is this normal for SSTCs?  If so, will someone please explain why?

Passing the Tesla secondary ground lead once through the CT core as you pictured is effectively 1 turn.  The "turn" is just quite large, counting the path to the ground/counterpoise and back through the air to the top-load.  2 turns would be one more loop of wire, so the ground wire goes through the core twice.  As long as one turn produces a signal large enough for reliable starting, more just adds to heating in the 1k resistor.

25
Induction launchers, coil guns and rails guns / Re: Sense coil fabrication?
« on: November 28, 2019, 06:46:53 AM »
There's no need for a second coil if the test frequency is high enough (if minimal magnetic field penetrates the disk).  Just measure the primary coil inductance as a function of disk height above the coil.  Call the no-disk inductance "Linf" (for disk-at-infinity).
    Coupling factor K = sqrt(1 - L / Linf).
Here "L" is the measured drive coil inductance with the disk above the coil.

The assumption about the disk blocking most of the field is probably valid for the relatively large disks and high frequency that klugesmith has been discussing.  It was for my old experiments with 50mm disks on a 14uF 20kV cap.  It isn't a valid assumption for my penny launcher - smaller disks and lower frequency.  That's why the zinc-filled pennies go about half as high as the older mostly-copper pennies.  (Still 5% zinc for pre-1982 pennies, a brass alloy.)  Dimes go even higher, as they are "pure" copper inside, so block more of the field.

26
Spark gap Tesla coils / Re: SGTC MK1 - An Accomplishment in Progress
« on: November 28, 2019, 06:20:37 AM »
Your small flyback transformers may be designed for a bit lower voltage and higher current.  As Steve suggests, it's probably OK to run the ZVS input voltage a bit higher.  The biggest issue may be the unconnected focus pin, if it decides to arc inside the potted windings.  Those small flybacks look new enough that they are likely from a color TV or monitor, which suggests they'd be good for at least 20kV output.

Measuring the turns ratio would help decide what input voltage is safe.  One way is to feed your signal generator sine wave to the flyback input through a resistor, and measure the flyback HV output with your meter.  Hopefully your signal generator can product enough power to get at least 100-200V on the HV output with only the meter load.  Set the signal generator for something in the 20-50kHz range, lower if you can get enough output voltage.  Measure the voltage across the flyback primary with the scope, to see what actual voltage is achieved from the signal generator and resistor.  Adjust the generator and/or resistor to get 100Vdc on the meter.  Record the flyback primary peak voltage from the scope (or P-P and divide by 2).  Adjust the resistor or generator for 200V, and record that input voltage.  The turns ratio is then (200V - 100V) / (peak_input_for_200V - peak_input_for_100V).  The output diode forward drop cancels in the calculation by using deltas (changes in voltages).  (This presumes a 1.0 coupling factor.  The real turns-ratio will be higher by 1/K.)

An alternative method:  Charge a capacitor (0.1 to 5uF) to 100V.  Then connect the capacitor to the flyback secondary, negative to the HV output wire and positive to the HV return pin.  Measure the flyback primary waveform with your scope.  Repeat with the capacitor charged to 200V.  For the primary peak voltage, use the initial fast edge voltage, not the following ring-down.  Calculation is the same as above, except that the actual turns-ratio will be lower by a factor of K.  (It's worth repeating each capacitor discharge multiple times, taking the scope reading from the trace with the cleanest waveform - initial fast step followed by ring-down without subsequent fast steps.  Mechanical touching of wires often makes mechanical bounce.  The good traces will avoid that noise, or at least have it well past the initial step.  Or, use a TRIAC, such as BTA8 or BTA16 or BTB16 or ... as the switch.)

K (coupling factor) is probably between 0.8 and 0.85 based on the couple flybacks I've measured.  If you want to measure K, perhaps the easiest is to run your ZVS at as low a voltage as it can handle.  Measure the frequency with the flyback secondary open (not arcing), then again with the secondary shorted.  If it's like what I see, the frequency change will be ~2.5:1.

Your new flyback has focus taken from an intermediate stage instead of the HV output.  My larger flybacks are that way too.  I'd still suggest grounding that pin (pin 7) to be safe.  Does the specification list anything about output voltage or current?  How about the design input Vcc voltage?

For comparing the flyback arc characteristics, measuring HV return current would be informative.  A series resistor, perhaps 100 ohms to ground.  Measure the voltage with your scope, or add a parallel capacitor and measure DC value with your meter.

Based on how well you are understanding all the information, it's hard to picture that you are just starting.  Impressive progress!

27
Presuming the units of Al are nH, then 800nH * 32T^2 = 819uH for the CT secondary, which is low.  At 220kHz (1.38meg radians/sec), the impedance is 819uH * 1.38meg = 1.13k ohms.  That will produce about 45 degrees of phase lead at high-power when the 1k resistor is the dominant load, and more at low power.  If you can get more wire, and it fits, you could compensate for the low Al core by winding 100 turn secondary and 2 turn primary.

Thinking about Al more, startup is the more stringent condition than full-power.  Until the CT output voltage is high enough for the clamp diodes to conduct, the CT secondary is mostly unloaded, so 90 degree phase lead.  The higher Al range that Mads suggests (Al around 5uH/T^2, or or 100/2 turns on a lower Al core) will help the CT output get to higher voltage sooner.

28
Agreed, great simulation.  Are you going to expand to dynamic field with the disk present?  Or is that beyond hobby work?  Somewhere years ago I had a time-only simulation based on measured drive coil inductance-reduction vs. position.  No actual geometry.

For fields below saturation, iron is quite helpful.  At least it is on my little penny launchers.  On my higher-power version, I have 10mm x 11mm stack of transformer I laminations for launching 19mm diameter disks (pennies).  Did have trouble with the rebound force.  Initially just clamped the laminations together, backed by a thin rubber pad opposite the coil end for damped support.  The rebound force against the pad slowly worked the center laminations upwards, so the penny no longer fit in its place.

29
Solid state Tesla coils / Re: Feedback current transformer doesn't work
« on: November 27, 2019, 06:16:11 AM »
I'd suggest removing the second blocking cap - it's the perfect place for a second 1k resistor.  BTW, the second resistor should not affect performance, just reduce HC14 input current in high-power situations, improving reliability.  (As it stands, the HC14 pin 1 is not at a defined DC level because of the second cap.  The HC14's internal input clamp diodes conduct once oscillation starts, making that second coupling cap voltage go to roughly zero.)

The startup issue is this:  When the enable pulse comes, the gate drive transformer is driven to one state or the other (one pair of H-Bridge FETs turned on).  Depending on what charge is left on the H-Bridge output DC blocking capacitor, that initial state may or may not generate much current (or voltage, so this applies to antenna feedback too).  Mads suggests adding a ~5-10k power resistor across the H-Bridge output, at least for DRSSTCs.  (For SSTCs, across the DC blocking capacitor works too.)  This makes the initial H-Bridge output state centered, so the initial enable edge generates a half-voltage edge.  That single half-voltage edge must generate enough current signal to change the HC14 state, which then makes another H-Bridge output transition, making another current half-cycle, etc.  (Oscillation starts.)

R2 in the second circuit Mads shared does two things.  It defines the DC level of the HC14 input between enable pulses.  It then changes the level on the HC14 input after the enable edge.  That makes a full-voltage H-Bridge output transition to initiate oscillation.  If R2 were placed across the HC14 (pins 1 to 2), then the HC14 will oscillate even without enable, so will create H-Bridge edges after enable goes true, until oscillation takes over.

30
Spark gap Tesla coils / Re: SGTC MK1 - An Accomplishment in Progress
« on: November 27, 2019, 05:58:09 AM »
My guess about insufficient voltage is that the MMC is charging to barely the spark-firing voltage.  When the voltage is marginal, a spark gap can fire or not depending on just how a corona streamer forms - what dust particles happen to be in the air, UV photons that happen to ionize some air, etc.  If the issue was limited current, the MMC would reach spark-gap firing voltage, just more slowly.

Another possibility occurred to me:  The flyback secondary winding has enough internal capacitance to resonate, and that is coupled to your primary ZVS resonant circuit.  That makes two resonant frequencies, one where the two winding voltages are in-phase and one where they are 180 degrees out-of-phase.  (The two resonant frequencies are discussed in some of the Tesla coil discussions, as the Tesla coil primary and secondary are two coupled resonant circuits.)  Perhaps the ZVS is occasionally locking into the higher-frequency out-of-phase mode.  I saw that occasionally a couple days ago in a ZVS-driven flyback experiment of my own - when the output was loaded more heavily.  The flyback is inefficient in that mode.

Concerning the nature of arcs, I'm just learning with my recent Jacob's ladder project.  My only previous experience was with spark-gap sudden discharges, not with continuous arcs.  Perhaps others here can assist.  If I had to guess, I'd say the fire-y arcs are higher current.  I doubt frequency matters as long as it's in the 10+kHz range.  The ionized air path definitely decays significantly in 1-2ms, but I don't think it decays much in <100us.

Many meter resistance ranges top-out at 20meg, so a 264meg resistor may have shown up as open.  An easy way to look for high resistances is with a DC voltage source and a volt meter.  Meters often have 10meg or 1meg input resistance on voltage ranges.  Apply a DC voltage to the HV output wire, then measure voltage on the pins.  If the DC voltage is negative and above ~30V, then the HV return pin can be found that way.  The internal HV diodes often have 20-30V forward drop, so at least that much voltage is required to see continuity from HV wire (positive output, which is the diode cathode) to the HV return pin.

If you want to head down the analytical path, I'd suggest measuring the flyback output turns.   There are a few ways to do so depending on what tools you have around.  Scope?  Probes good for a few hundred volts?  AC signal generator (some source of low voltage in the kHz range)?

31
Spark gap Tesla coils / Re: SGTC MK1 - An Accomplishment in Progress
« on: November 26, 2019, 05:54:16 AM »
If it's not firing consistently, then it sounds like insufficient voltage rather than insufficient current.  The latter would cause it to be slow (low repeat frequency), but not inconsistency.  At risk of frying the little flybacks, a bit higher ZVS input voltage would help.

The pin arcing may be caused by a feedback resistor within the flyback.  Several of my flyback transformers include a high-value resistor from the positive HV output lead back to a pin adjacent the negative HV return pin.  For example, the one I've been experimenting with recently has 264meg ohms from output lead to pin 7.  Pin 6 is the HV return.  I don't know if the 264meg resistor was for feedback to regulate voltage, or part of a voltage-divider to generate focus voltage (~2kV).  Either way, if your flyback has such an internal resistor, it's better to tie the associated pin to HV return.  (I have 20meg from pin 7 to pin 6 to monitor HV voltage.)

If that HV feedback pin is left floating, it may arc to the HV return or not.  The problem is that it might arc inside the flyback rather than at the pin.  Internal arcing will damage insulation and perhaps start a more extensive internal failure.  That's why I'd suggest tying it to the HV return, directly or through a resistor.

32
Solid state Tesla coils / Re: Feedback current transformer doesn't work
« on: November 26, 2019, 04:30:23 AM »
That's a good question.  Transformer windings (and inductors in general) have zero DC voltage across them (ideally - ignoring wire resistance).  In other words, the average voltage is zero.  That's because the current through an inductor is the integral of the voltage across it (scaled by 1/inductance).  If the average voltage is positive, current will ramp up until it pulls the average back to zero.

With the diodes clamping the CT output directly, the negative voltage is clamped to ~-0.7V (one diode forward drop).  The positive voltage is clamped to +5.7V.  This ramps CT current up until the duty cycle is lopsided enough to make the negative clamp time ~8 times as long as the positive part to get zero average.

BTW, here's the attachment that failed previously, basically the same as what Mads posted, except the cap and resistor are reversed, which makes no difference.  Hopefully it shows up this time.


Note R2 in the second schematic Mads posted.  That helps kick-start oscillation.  Putting the resistor (R2) between pins 1 and 2 of the HC14 should work instead.  That has the advantage of repeated kick-starting if the first one fails.

I'd also suggest an additional resistor between HC14 pin 1 and the clamp diodes D1/D2, ~1k ohms.  That further reduces current to the HC14 input, forcing the clamp diodes to handle most of the current when voltage exceeds 0-5V.

33
I could picture frequency rising if the copper shape changes significantly during melting, spreading out to a shape that blocks more of the magnetic field within the work coil.  If the frequency goes up, then the impedance goes down, not up.  The capacitors stay the same, so higher frequency comes from lower inductance, which is lower impedance.

Copper resistance roughly doubles as it melts.  That's the most likely explanation for power dropping.
https://www.sciencedirect.com/science/article/pii/S0022369717304341

At the start, before the copper heats significantly, the electrical conductivity of the copper being heated and the work coil are roughly the same.  That makes direct induction heating quite inefficient.  I don't know much about the graphite crucibles, what their electrical conductivity may be.  Conductivity may be too low for much heating from the induction field.  If the graphite is conductive enough to generate much heat, that will reduce as the copper melts and its shape fits the crucible inner wall.

34
Dual Resonant Solid State Tesla coils / Re: Problems with my first DRSSTC
« on: November 24, 2019, 10:10:06 PM »
Great!  All your hard work to get to this point will seem less burdensome now that you have results.

35
Solid state Tesla coils / Re: Feedback current transformer doesn't work
« on: November 24, 2019, 02:58:48 AM »
I was just looking at that SSTC schematic with current feedback to answer another post.  It appears to me to have an error.  The two 1N4148 clamp diodes on the current transformer secondary should be on the HC74 input (pin 1) rather than directly on the transformer secondary.  Even better would be to add a second 1k resistor in series with the HC74 input, then place the clamp diodes between the two resistors:
 [ Invalid Attachment ]

Does anyone else have experience with the "http://www.loneoceans.com/labs/sstc2/sstc2schematicv10.jpg" circuit with CT feedback?  Anyone have a reason that it makes sense to have a diode directly across the CT, making it carry a net DC current?

36
Did you scope the HC14 output (pin 2) directly?  If it's not burned out, it should oscillate.  Was the antenna still connected to pin 1?  If not, a small 5-10pF capacitor from pin1 to ground would simulate the antenna's capacitance.  With no antenna and no capacitor, the oscillation will be higher-frequency, so may not get through the gate driver chips.  It should still show up on the HC14 output (pin 2) if you scope there.

Your comment about noisy oscillation makes me think of another possible issue.  Perhaps the antenna is picking up a lot of 50 or 60Hz signal from line power wiring in the vicinity.  That's a common situation - line frequency fields everywhere.  Anyone know of common circuits to reject low-frequencies from antenna-feedback SSTCs?  If not, and if you want to continue down the antenna path, I'll come up with a suggestion.

Probably not directly related to existing issues, but I'll mention anyway:  It's normal to tie unused CMOS chip inputs to ground or power or some other signal, not to leave them floating (unconnected).  For HC14 and such, I often wire the inverters in a chain to keep it simple.  In your case of using only the first inverter (pins 1 to 2), you could wire pins 2-3, 4-5, 14-13, 12-11, and 10-9.  That would connect all 5 unused inputs to something.

I'd encourage your switch to current feedback.  I have no personal experience with antenna feedback, but it certainly seems more susceptible to noise sources.  If you go that direction, however, the schematic you attached appears to have an error.  The two 1N4148 clamp diodes on the current transformer secondary should be on the HC74 input (pin 1) rather than directly on the transformer secondary.  Even better would be to add a second 1k resistor in series with the HC74 input, then place the clamp diodes between the two resistors:


Does anyone else have experience with the "http://www.loneoceans.com/labs/sstc2/sstc2schematicv10.jpg" circuit with CT feedback?  Anyone have a reason that it makes sense to have a diode directly across the CT, making it carry a net DC current?

Anything around 1uH/turn^2 or more should be fine for the CT core.  That would keep the secondary above 2.5uH, which is 3.5k-ohms reactance at your 220kHz, far enough above the 1k load resistance.

37
Great point about flash-over risk.  That's why my coin shrinker is in the far corner of my garage.  Charging and triggering is from inside the house.  It is a compact (and lower energy - only 14uF) system.  When it has flashed over, the bang is much louder than a normal coil shrink.

38
Spark gap Tesla coils / Re: SGTC MK1 - An Accomplishment in Progress
« on: November 23, 2019, 06:34:56 PM »
Oh!  I'd seen the phrase "Terry filter" on this forum, but had no idea what it was.  Thank you for the insight.

39
Spark gap Tesla coils / Re: SGTC MK1 - An Accomplishment in Progress
« on: November 23, 2019, 05:01:46 AM »
Thinking a bit more about filtering between flyback output and spark-gap:  A simple R/C filter is probably sufficient, avoiding the cost/work of soldering 100 inductors.  In this case, the R/C filter is removing high-frequencies generated by the spark gap, so would have the R on the spark-gap side:


Something like the above filter will waste a few % of input power, but should protect the flyback from fast spark-gap falling edges as well or better than the inductor string.  The resistor is constructed from 10 in series to avoid the cost/trouble of finding a single 20-30kV capable resistor.  Even with 10 resistors, each will see 2-3kV.  Resistors rated for a few kV aren't too hard to find, but any 1-2W rated resistor will likely be fine.  The voltage is there for a very short time.  I used 2W 10k resistors, six per string, for my small Marx generator (9kV per string).  That's only 1.5kV/resistor, but I did some testing to much higher voltage on those 2W parts.

Values for the above low-pass filter aren't critical.  Take some care in finding a capacitor, however.  Many cheep ceramic 20-30kV capacitors have bad capacitance vs. voltage curves, dropping down by 80-90% at rated voltage (only 10-20% of capacitance remaining).

One other caution, which I think you're already doing.  Make sure there's some load (MMC or arc gap) on the secondary when the ZVS starts up.  ZVS oscillators often produce a startup-burst that is ~2x the normal run voltage.  Energy builds up in the input inductor until oscillation starts.  Than that energy transfers to output oscillation until used up.

40
Looks like you are way past the stage of looking for crude estimates, but I'll share this idea anyway.  If you can find an 850nm LED, use it as a photodiode.  A 940nm light source will produce almost no response.  An 850nm light source will generate a reasonable response.  780nm light will likely produce a yet-higher response, unless the LED package (for the photo-diode one) has filtering.

I haven't personally tried the above test with IR LEDs, but have with both visible and UV.  So far, all the LED's I've used as photodiodes behave as expected, responding to wavelengths shorter than or equal to their emission wavelength, but not to longer wavelength light.

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post Re: SGTC MK1 - An Accomplishment in Progress
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davekni
December 10, 2019, 06:04:28 AM
post Re: Duty cycle when driving a CRT TV flyback transformer
[Transformer (ferrite core)]
davekni
December 10, 2019, 05:28:54 AM
post Re: Possible use for large inductor (laminated core)
[Transformer (iron core)]
davekni
December 10, 2019, 04:12:29 AM
post Duty cycle when driving a CRT TV flyback transformer
[Transformer (ferrite core)]
John123
December 09, 2019, 10:52:59 PM
post Replacement guide for Windows Media Center
[Computers, Microcontrollers, Programmable Logic, Interfaces and Displays]
MRMILSTAR
December 09, 2019, 08:29:16 PM
post Re: Sense coil fabrication?
[Induction launchers, coil guns and rails guns]
Uspring
December 09, 2019, 04:23:33 PM
post Re: Possible use for large inductor (laminated core)
[Transformer (iron core)]
MRMILSTAR
December 09, 2019, 03:59:02 PM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
jturnerkc
December 09, 2019, 03:41:42 PM
post Possible use for large inductor (laminated core)
[Transformer (iron core)]
kamelryttarn
December 09, 2019, 09:43:11 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
davekni
December 09, 2019, 12:53:57 AM
post Re: CW multiplier resistor string suggestions
[Voltage Multipliers]
davekni
December 08, 2019, 09:57:51 PM
post Re: CW multiplier resistor string suggestions
[Voltage Multipliers]
MRMILSTAR
December 08, 2019, 05:28:09 PM