Author Topic: Feedback current transformer doesn't work  (Read 1330 times)

Offline nick

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Feedback current transformer doesn't work
« on: November 21, 2019, 01:38:13 PM »
Hi,

I've built a coil using Loneoceans' SSTC 2 https://www.loneoceans.com/labs/sstc2/sstc2schematicv10.jpg. My circuit is the same for all intents and purposes. The only difference - and source of my problem - is the feedback current transformer. In short, nothing works when using ~50 turns around a core through which the bottom of the secondary passes (as described in the linked schematic). What's interesting is that when I use an antenna instead (which works GREAT) and scope the output of the CT, I see exactly what I expect: the coil's resonant frequency at more than sufficient voltage. Any ideas what the problem could be?


To hijack my own post, I had another question - where can one find a nice toroidal top-load? I'm currently using 2 smooth salad bowls glued together (lol) - which works OK - but would love to have something nice like in Loneoceans' coil: https://www.loneoceans.com/labs/sstc2/toroid.jpg

Thanks!

Offline Weston

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Re: Feedback current transformer doesn't work
« Reply #1 on: November 22, 2019, 02:43:03 AM »
Have you tried reversing the phasing of the current transformer? That would be my first guess.

The three places I know where you can get toploads from are ebay, information unlimited (amazing1), and eastern voltage research. They are somewhat pricey, after years of making tesla coils I recently bought my first real spun aluminium topload.

Offline nick

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Re: Feedback current transformer doesn't work
« Reply #2 on: November 22, 2019, 11:58:46 AM »
Thanks for the reply! I'm sure I've tried reversing the polarity but will do so again tonight. Unfortunately I'm pretty sure I've already done this and it didn't work :(

I'm kind of getting tired/bored of my current build (it always feels like the journey is so much more fun than the destination...) so I think this weekend I will upgrade to a full H bridge. I now have 9 or 10 60n60 IGBTs - I might as well put them to work!

Shame about the (lack of) availability of nice toroids... My next step might be to try aluminium ducting that I see others use but I'm surprised that the relatively rough surface works well.

Offline davekni

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Re: Feedback current transformer doesn't work
« Reply #3 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?
David Knierim

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Re: Feedback current transformer doesn't work
« Reply #4 on: November 25, 2019, 09:11:13 AM »
It could just be a copy/paste error from the Steve Ward SSTC5 / mini SSTC that he used as inspiration. The antenna single-wire feedback was suddenly replaced by a two-wire CT and it just got referenced to ground.



The diodes are there for clamping the induced high voltages on the antenna, since corona on the antenna is pretty normal, maybe he is just missing a DC blocking capacitor like Steve Ward uses with the CT feedback here:

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Offline nick

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Re: Feedback current transformer doesn't work
« Reply #5 on: November 25, 2019, 10:23:17 AM »
Thanks both! Will try this tonight.

Offline nick

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Re: Feedback current transformer doesn't work
« Reply #6 on: November 25, 2019, 05:55:49 PM »
Hello again, apologies for another reply. And also apologies for the noobish question but could you perhaps explain why it would make a difference whether you put the DC blocking cap before or after the diode clamp? Wouldn't it be OK that the cap removed the DC bias regardless, as long as it's in front of the 74HC? What am I missing?

Also @davekni - it would be much appreciated if I could get your feedback as to why adding another 1k resistor will help? I am even wondering why a resistor is needed at all give the CT feedback should produce a low current (albeit at high voltage, but that's what the clamp is for)? Further, why is it beneficial to have another 1k resistor in series - what should this accomplish?

Thank you!

Offline davekni

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Re: Feedback current transformer doesn't work
« Reply #7 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.
David Knierim

Offline nick

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Re: Feedback current transformer doesn't work
« Reply #8 on: November 26, 2019, 12:48:05 PM »
It works! Thanks guys! I ended up with one end of the CT in series with a 100n cap which is followed (in series) by a 1k resistor, before hitting the diode clamp. The other end of the CT is obviously grounded. I did all this before seeing your latest reply Dave, so will try the additional R tonight.

By the way, I still have another 100n DC blocking cap AFTER the clamp - any point keeping it there?

To be entirely honest, I'm struggling to understand all the theory in your post Dave from a first read, but that makes me happy since I can research myself / do simulations, and hopefully learn something new :)

Offline davekni

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Re: Feedback current transformer doesn't work
« Reply #9 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.
David Knierim

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Re: Feedback current transformer doesn't work
« Reply #9 on: November 27, 2019, 06:16:11 AM »

 


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