Re: Differential voltage probes 1 -> 3

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi,

At 01:45 PM 6/30/2003 -0500, you wrote:

> > Original poster: "Steve Conner by way of Terry Fritz
><teslalist-at-qwest-dot-net>" <steve.conner-at-optosci-dot-com>
> >
> > One that's attached to a Tesla coil spewing sparks and inducing stupid
> > voltages in any ground loop that happens to be in your measurement system.
>
>Hmm, makes sense.
>
>thanks,
>
>KEN
>
>
> > Also handy for looking at gate voltages in the upper half of a bridge.
> >
> > Anyway, I'm going off this whole diff amp thing, since I realized that the
> > CMRR depends on the matching of the two impedances (referred to the diff
>amp
> > box ground) you're probing, and they're unlikely to ever be matched in
>real
> > life. I'm going to investigate the optocoupler route instead.
> >
> > Steve C.

Matching the resistors may not be hard at all since other values could be 
paralleled with the 1K's to "tweak" the resistance.  I'll have to test and 
see, but I think it would be pretty easy to get a 0.1%+ match this way.  I 
have a pretty large collection of surface mount resistors, so I should be 
able to get very close to the right values.

I also plan now on having big MOVs (40pF per side), HV fuses, and 
independent grounds.  That should fix a lot of loop noise and if one plugs 
it into a 30kV tank or something "bad", the probe and scope "should be" 
saved.  Obviously, we play with voltages and powers beyond what the 
"normal" differential probe was meant to face.  So, we need to beef the 
thing up to handle some probable mishaps.  One can never make it totally 
"idiot proof", but I can do a few little things that would dramatically 
increase it's chance of surviving a "big mistake".

I worry a bit about opto-couplers.  The present (Ok, august 26th) resistors 
should take 5000 volts which is beyond most opto solutions.  I think we 
have to worry about the hooking it up to 30,000 tank cap volt situation.  I 
am thinking the MOVs could take the first hit and then high voltage fuses 
could then remove the probe form the "mess".  I have been talking to 
buddies about a simple HV fuses (1mA 30,000V) for such things, and it seems 
pretty easy.

Hey!  If this works good, I can sell my present P5205 for twice what I paid 
for it on E-bay :o))))

Cheers,

         Terry