RE: Differential voltage probes 3

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

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

Hi,

I ordered up all the parts for the probe today.  Unfortunately, the 1Meg 
5kV resistors are back ordered till August 26th :-(  But I can use other SF 
resistors for testing.


>I think it may be a good idea and safety feature to add another transorb to
>the amplifier inputs.  Do you know the nonconducting impedances of the
>transorbs?  Wouldn't like to interfere with RF at the inputs though.

I have 5V 600W surface mount transorbs here.  They will look like 
capacitors (fairly good ones).  Of course, the data sheets don't mention 
this extremely important parameter :-p  My meter measures an ununbelievable 
2.6nF!  Just have to try and see what happens in the real circuit 
there.  But assuming we want greater than 50,000 ohms Xc at 50MHz:

50000 = 1 / (2 x pi x 50E6 x C)   C == 0.0637pF!!

That shows just how sensitive things are in the 50MHz region!

Of course, the thing has to be very safe.  It basically would have to take 
a 240VAC line fault or discharge a say 10Joule HV cap if the resistors 
failed or arced over.  No external metal could go "high" in that case.  The 
scope would always have to be protected.  If the unit did fail the 
resistors, the internal IC and all becomes a "don't care" since the probe 
is "bad" at that point anyway.  Just have to stop the damage inside the 
probe from hurting anything else.


>RWW

>How is this probe going to be packaged?

It would be a plastic box with two banana jacks on one end.  The other end 
would have a ground stud and a scope probe socket that would accept common 
scope probe tips.  There would be a power switch and LED indictor.  It 
would use two 9V batteries.  I would be much like the Tek style probes.

Cheers,

         Terry