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Registered Member #3505
Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108
I picked up a cheap Oscilloscope(BK Precision,Model 2120) and now I'm trying to figure out how to use it. I've watched several youtube vids on the basics and now I'm trying to figure out If I'm semi retarded (Possible) or if I got a dud. Lets say I wanted to scope the GDT output wave form where should I hook up the probe and should I use the ground clip or not? Do I need the Mosfets hooked up?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
If the circuit is isolated from the mains by a transformer then 'scope the gate-source of the mosfets. If not then only 'scope the isolated low voltage circuitry. The 'scope body is/should be internally earthed and the probe ground/earth clip should be used.
check how your 2 channels have ground configured. they might be the same on some scopes which means you can cause shorts if you arent weary. something to keep in mind if you are trying to scope multiple gate drive signals at once you can check the gate drive signals safely by powering the bridge with a low voltage current limited source. mainly on a sstc you would scope the gate drive outputs and use the scope for tuning depending on how you achieve tuning. ie pll vs antenna vs current transformer if your scope has a test hook thingy you can clip your probes to it and it should show a 1 khz square wave which you can then use to compensate the probes and make sure the channels work and are accurate. you might need to use a plastic trim pot tool for that
Registered Member #4108
Joined: Sun Sept 25 2011, 11:43PM
Location:
Posts: 149
When I scope my sstc a just rap 1turn of wire above the primary,and connect that to that scope, then just apply like 30volts to the fets, you could do something similar with the gdt. But I have a tube scope... Are they less fragile?
Registered Member #3505
Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108
Being that I'm new to this solid state stuff and didn't have a signal generator I took my tesla tuner and hooked it up to a small coil to generate a variable frequency output. I dumped this into my driver and this is the waveform I got out of my GDT(273Khz input to driver 2v/div @ 1us with 10X probe). After looking at Richie Burnett's website it looks like I have excessive low frequency ringing and I need to designed my GDT. What do I need to do to fix this wave form? I'm 95% sure I have the proper toroid so what am I doing wrong? I'd appreciate any help.
Registered Member #1902
Joined: Fri Jan 02 2009, 07:59PM
Location: Lancaster, NY
Posts: 75
There is overshoot and ringing on your Gate signal, redesign your Gate drive transformer with twisted pairs of wire. The objective is to have minimal leakage inductance in series with your primary and secondary windings. Even a properly designed GDT will need a dampening network in the form of a low value gate resistor in series with your secondary winding(s). Preferably 1 to 20 ohms of resistance may be required. Also use 12 to 15 volt Zener diodes in anti-series from Gate to Source to clamp the Gate signal to a safe level.
Registered Member #3505
Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108
After playing with several different wire types and a couple of different toroids I managed to get this waveform. I think it might be slightly over dampened. Do you guys think this will be adequate or should I keep tweeking?
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Oscilloscopes and SSTC
