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Re: ScopingSSTC

Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi,
I have run a lot of scopes with the ground hot as you mention (the scope is grounded though the AC line Ground so you need to defeat that). The scope chassis and all adds a lot of capacitive loading to the circuit which may very well be bad for gate drive measurements. Also, the scope's power supplies are still referenced to ground through the AC lines. The isolation at that point is not very well guaranteed. So scope loading and isolation from the AC line are issues. Tektronix scopes seem to be the "luckiest"... Dig through the specs sheets where they give clues about how hot you can run.
Also, try not to run the scope chassis on the RF side but rather the fixed voltage rail side so the chassis is not oscillating at 250kHz. Scopes don't like that...
It is a pretty messy business really. I have also pried many scopes off grounded metal workbenches when they slip a little off whatever we had them on and they arc weld themselves to the bench. It usually does not hurt the scope "much" though! We also had a bunch of "extra scopes" and the repair costs were "acceptable" at the time. I would never run any scope "I" paid for hot...
Of course, don't touch the darn thing... Some little hand held scopes are battery run and fully isolated too and cost less than a good dif probe! 

http://www.amazon.com/gp/product/B0007ZPNH8/ref=nosim/104-6863387-9336701?n=228013

I suppose, someday, Amazon will sell CD caps too :o)))

We did toy with the idea of making our own probes at one time:

http://hot-streamer.com/temp/DiffProbe03.gif
But we got all tied up in trying to make them have super high bandwidth and stuff and it never really went anywhere... We should have kept it simple and doable... 

Cheers,

        Terry



At 02:18 PM 12/14/2005, you wrote:

Hi everyone!
I cannot understand the purpose of differential measurment. Why can't I check the form of the signal by connecting the neutral probe of the scope to one end of GDT and the other probe to another eng of GDT?????? I think that it would be OK, because i DON't touch anything, but the wires of GDT (i mean the secondary winding, of course) 

Thanks.