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DRSSTC Feedback Problem

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Andri

Wed Jul 01 2009, 01:15AM

Registered Member #1533 Joined: Wed Jun 11 2008, 02:13PM
Location: ReykjavÃk, Iceland
Posts: 46

Hi,

I'm trying to acquaint myself thoroughly with DRSSTCs so I'm building one from scratch using only stuff that I already have or is easy/cheap for me to obtain. Now, everything "works" so to speak except for feedback through a CT (which I had working the other day but now it oscillates at the wrong frequency). Using a simple antenna works but I believe a CT would be a more convenient solution.

My setup:
The CT I'm using is just a toroidal core with a bunch of windings on it. The primary goes through the middle. Between the terminals there is a zener diode. The terminals are also connected to a Schmidt trigger. The zener diode has the anode connected to ground (obviously). This is done so that current through the primary of the CT yields a square wave voltage on the secondary of the CT. The Schmidt trigger is then connected to an and port along with the interrupter. The output of the and port goes to a gate driver, which drives a GDT that drives the H-bridge.

Things I've tried to get it working again:

Switch the terminals of the CT (phase angle needs to be n2pi where n=0,1,2,3...).
Vary the tank circuit capacitance.
Vary the tank circuit inductance.
Add noise to the circuit.

Help would be very much appreciated as I feel as if I've been banging my head against a concrete wall while trying to figure this out.

Here's a link to a video of my TC playing the Doom level 1 theme just to show you something.

One more thing: I took apart big motor control today and found inside, one of these. You think this is any good for a DRSSTC?

Thanks,
Andri

HV Enthusiast

Wed Jul 01 2009, 01:06PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

how do you know its the wrong frequency?

Andri

Wed Jul 01 2009, 01:29PM

Registered Member #1533 Joined: Wed Jun 11 2008, 02:13PM
Location: ReykjavÃk, Iceland
Posts: 46

Well, I know it's the wrong frequency because I don't get sparks. I do get sparks when I use an antenna. I haven't tried looking at the waveform in an oscilloscope. I'll try that when I get home from work.

HV Enthusiast

Wed Jul 01 2009, 03:01PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Andri wrote ...

Well, I know it's the wrong frequency because I don't get sparks. I do get sparks when I use an antenna. I haven't tried looking at the waveform in an oscilloscope. I'll try that when I get home from work.

So again, how do you know its the wrong frequency? Sounds like its just not working properly?

How many turns on your core? It should be quite a bit of turns.

Also, check out Steve Ward's CT design and feedback circuit. You can use that as a baseline to compare to yours.

Andri

Wed Jul 01 2009, 03:36PM

Registered Member #1533 Joined: Wed Jun 11 2008, 02:13PM
Location: ReykjavÃk, Iceland
Posts: 46

You are right. I do not absolutely know for sure that it's the "wrong" frequency. The CT does have quite a bit of turns on it and I have looked at Steve Wards' feedback circuit. That's where I got the idea of using a zener diode. However, Steve uses 4 zeners. The below zero part of the signal is of no interest to me, thus I use only 1. I believe the contribution of the CT to the tank circuit is insignificant due to number of turns and "low resistance" on the secondary side. Perhaps I'm wrong to think so. I am probably very wrong about many things.

HV Enthusiast

Wed Jul 01 2009, 05:51PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

wrote ...

That's where I got the idea of using a zener diode. However, Steve uses 4 zeners.

Well, thats why your circuit doesn't work. Those zeners are used to "square" up the signal and feed it back into the logic circuitry. And they aren't (4) zeners, they are (2) zeners back-to-back with schottky diodes.

I believe this is where your problem is.

Andri

Wed Jul 01 2009, 06:20PM

Registered Member #1533 Joined: Wed Jun 11 2008, 02:13PM
Location: ReykjavÃk, Iceland
Posts: 46

I had this working the other day with only one zener diode as I have already mentioned. I'm wondering why it stopped working. One zener diode will "square up" the signal just as well as two zener diodes. Steve's back to back solution yields a voltage with zero bias. One zener will give a bias of circa half the peak value which is ideal for the Schmidt trigger.

HV Enthusiast

Wed Jul 01 2009, 08:08PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

if you have a negative signal going into some logic chips,you'll damage them.

Andri

Wed Jul 01 2009, 11:09PM

Registered Member #1533 Joined: Wed Jun 11 2008, 02:13PM
Location: ReykjavÃk, Iceland
Posts: 46

Dr. GigaVolt wrote ...

if you have a negative signal going into some logic chips,you'll damage them.

I'm not doing that. Steve's not doing that either. He feeds his zero biased signal through a decoupling capacitor and a 1k resistor and then he shorts anything 0.6V above 5V or below 0V to the respective rail, guaranteeing logic level inputs to his logic chip. I also have logic level guaranteed in a much simpler manner.

HV Enthusiast

Thu Jul 02 2009, 02:06AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Well, we all know Steve's circuit works. Try his first. If it still doesn't work, then you know its not in that part of the circuit.

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