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Shaun

Tue Nov 13 2007, 11:28PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

15V zeners and 10 IRFP260s are on the way (from AlliedElec, wayyyyy cheaper than Digikey). I have also built a 300kHz +/- 5Khz adjustable oscillator from a 555 to try my luck at open loop, just to make sure the feedback is really the problem.

I can't see why the GDT would be a problem; I even went as far as using the exact core steve specified on his site. However, I will check the gate waveforms anyway.

Also, the coil is grounded to mains ground. I have an 8' copper rod in the ground from my SGTC, but I wanted this thing to be portable. That's okay, right?

Zum Beispiel

Wed Nov 14 2007, 10:06AM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

Shaun wrote ...

Also, the coil is grounded to mains ground. I have an 8' copper rod in the ground from my SGTC, but I wanted this thing to be portable. That's okay, right?

That depends... If your house has good grounding system it should work.
I wouldn't dare to ground any kind of TC to mains ground, but that is just my opinnion. I have 2.5m of copper driven into the ground just for my coils, because I don't want to accidently set the house on fire (Old house = dodgy wiring).

If you already have a rod driven into the ground, try using it instead. One place less for problems.

Bauerb2

Wed Nov 14 2007, 05:44PM

Registered Member #973 Joined: Tue Aug 28 2007, 07:32PM
Location: Pittsburgh, PA
Posts: 66

i ran a wire out my window and connected it to a 5 foot pipe i pounded into the ground. i wouldn't want to connect to mains ground though...

Tom540

Wed Nov 14 2007, 09:12PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
Location:
Posts: 617

When it comes to grounding ti all depends on where you live. Ive never had problems with using the mains ground.

Ken M.

Wed Nov 14 2007, 10:09PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

I Agree with Tom, I use a mains ground with a RFI filter inbetween the only issue I get is that any sound system with an amplifier and is on, the return frequency causes the speakers to buzz at maybe 60dB.

Shaun

Fri Nov 16 2007, 01:19AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Hi all,

My IRFP260s and Zener diodes came in today, and I have since rebuilt my bridge. I have not given it any power yet, not until I test some things. First, I scoped the gates of both transistors and they look exactly like the GDT primary waveform, except one is inverted. So far so good. Here are the waveforms for both gates:

Both traces are 5V/div and the timing is 0.5ms/div.

Couple of notes: I am doubling the size of my antenna, making it about a foot long.
I am still on mains ground, but I dramatically shortened the wire length. Its now about 6" from seondary base to ground.
I have built and tested a 300kHz+/-5kHz 555 oscillator to try my luck running open loop.

I will update soon with results!

Shaun

Fri Nov 16 2007, 02:32AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Here's what's going on at this point: I did what I said I would in my last post, but the coil does not work. The good new is, my FETs are still intact. I never went above 30 on the variac when I saw there was not breakout. Also, no fluorescent bulb flashes this time.

This first pic is the GDT primary running open-loop with freq set to a little less than 300kHz.

This is 1 ms/div and 5V/div. It shows periods of HF oscillation followed by off-time from the interrupter.

Here is a zoomed-in image of those HF oscillations:

Notice the duty cycle is not exactly 50%. Will this cause problems?

Here's the FET gate running open-loop with no power to the bridge (this is where things get sticky):

This one is 1 ms/div and ONLY .5V/div. VERY WEAK.
Notice the HF periods are barely visible.

And finally, a zoomed-in image of those weak oscillations:

This one is only 0.2V/div and 1us/div
I only see a short period of ringing where the HF square wave transitions should be. This is the problem, methinks.

So I'm beginning to think the GDT is the problem. Probably important to mention is that I heard NO NOISE at all from any part of the setup during these tests.

Can anyone tell me what is going on?

...

Fri Nov 16 2007, 05:04AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

The scope shot you took on 11/15 looks very wrong, I would suspect a phasing problem. Try swapping the primary, or the inverting/noninverting drivers...

Short of that, try taking the antenna and hook it up to the output of the 555 oscillator (if that isn't what you are doing already). In theory, you shouldn't need to have metal/metal contact to pin3, just brining it close to the output (maybe wrap the wires arround eachother a few times for good luck).

Shaun

Sat Nov 17 2007, 07:50PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

I have a quick additional question...if I am scoping the FET gates or GDT primary with no power going to the bridge, can I expect the antenna feedback to kick in?

My major problem right now is UCC pin 2 is picking up a 60 Hz square wave from the mains; still trying to find a way to overcome this.

Dr. Dark Current

Sat Nov 17 2007, 08:25PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Shaun wrote ...

I have a quick additional question...if I am scoping the FET gates or GDT primary with no power going to the bridge, can I expect the antenna feedback to kick in?

No you cant! The weak oscillations you see on the output of the GDT are only the interrupter pulses (at interrupter frequency), I think.

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