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Goodchild

Sat Nov 21 2009, 05:23PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Ok so for some time now I have been trying to scope 2 UCCs in push pull mode (full bridge) using the differential measurement method by using 2 channels on the scope and subtracting to get the voltage difference. But I seem to have a rather annoying problem 60Hz seems to be coupling back from mains into my scope some how and really messing with my readings. Is there a cure for this problem? I thought that maybe some thing as simple as an isolation transformer or line filter may fix the problem but have not tryed this yet.

Scope screen shots:
(This is supposed to me a square wave but the 60hz is modulating the square wave

)

Steve Conner

Sat Nov 21 2009, 05:46PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

What are your scope probes' ground clips connected to?

What is the negative supply rail of your UCC circuit connected to?

Goodchild

Sat Nov 21 2009, 05:51PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

The ground clips have been removed from the probes.

The negative supply rail is connected to a filtered and regulated 12v DC power supply

Bjørn

Sat Nov 21 2009, 05:52PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

First step is to double check your ground connection, and you need to connect the ground on both probes.

Edit: I tried dropping one of the ground connections on that kind of scope once and it caused all sorts of problems.

Goodchild

Sat Nov 21 2009, 05:58PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

So just attach the ground clips to the GND in the circuit?

I have seen people removing them and connecting them together also when testing in differential mode.

Edit: I tryed attaching the ground clips to the circuit GND and this fixed the problem

But what about larger bridges like in a DRSSTC or SSTC would you just attach the GND clips to the
negative power rail?

Coronafix

Sat Nov 21 2009, 09:53PM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Your negative supply must also be grounded, otherwise you will get the 60Hz signal through everything.

Arcstarter

Sun Nov 22 2009, 02:02AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Coronafix wrote ...

Your negative supply must also be grounded, otherwise you will get the 60Hz signal through everything.

Well, if you have the ground leads from the scope connected to the negative supply, it is grounded. Scopes normally have the negative leads connected together, to mains ground.

But yea, you still need to earth ground the negative rail

Steve Conner

Sun Nov 22 2009, 10:22AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

wrote ...

Edit: I tryed attaching the ground clips to the circuit GND and this fixed the problem

Whee, you took my hint.

wrote ...

But what about larger bridges like in a DRSSTC or SSTC would you just attach the GND clips to the
negative power rail?

Now it starts to get more complicated and dangerous, because the negative power rail is not anything like ground. Depending on your design, it might be -160V, -320V or whatever. Either way it would vaporize your ground clips.

What I've traditionally done is to run the SSTC off an isolation transformer, which allows the whole bridge to float. The negative power rail can then be grounded, and the scope probe clips safely hooked to it. My isolation transformer is only 500VA, so I would run my coils at reduced power for scoping purposes.

Another "solution" is to run the scope off batteries and allow it to float with the negative power rail, which is highly dangerous, because the casing and controls become live. If you want to operate the scope, you need to make completely sure that you're insulated from everything else in the surroundings, or electrocution is just a slip away. (Newer scopes with all plastic casings aren't nearly so lethal in this respect.)

Removing the ground wire from your scope's mains lead is even more dangerous: not only is the scope's casing live as before, but your coil's power rail voltage appears across the guts of the scope's power supply. If it breaks down, the scope could be completely destroyed.

Finally, note that while you saw the 60Hz interference on the A and B traces, it was cancelled out on the A-B.

Goodchild

Sun Nov 22 2009, 04:24PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

So just to clarify so that I don't go killing my self or my scope or my bridge, once the tesla coil is isolated from mains I can safely connect the ground clips to the negative power rail and the 2 probe tips to the 2 outputs of the bridge so that I can get a differential measurement.

Steve Conner

Sun Nov 22 2009, 06:34PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes, you got it.

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