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RE: Primary Scoping

I said,
>>I have thought about using a 1000:1 divider ratio using 4 of the dale
>>100Megohm resistors in series to a  5 parallel combination of 2megohm 
>carbon
>>comp resistors. I'll then put some transorbs to ground before the scope
>>connection. All components will be physically well mounted to prevent
>>circuit disconnection.
>>BTW, transorbs can be thought of as a 5ps.(thats not a typo: 5*10^-12
>>second) zener with a many thousand Amp ratting. By using 2 of the
>>unidirectional devices in antiparallel I can get any clamp voltage between 
>5
>>and 500 volts with only a 200pF capacitance penality.

Bryan said,

>You've just built a lowpass filter with a cutoff frequency of 2 Hz!  So you 
>won't see too much of the signal you are trying to measure (100's KHz).
>
>Fc = 1/(2*pi*R*C) = 1/(2*3.14*400e6*200e-12) = 2 Hz.
>
>With such large resistors, any capacitance is going to severely reduce the 
>frequency response.  This includes the cables and scope input capacitance.
>
>Bryan Zublin
>bzublin-at-gi-dot-com
>

Its bad, just not that bad. You forgot the thevinin equivalent of the 400meg
in parallel with the 400K; call it 400k. Fc= 1/(2*pi*400K*200e-12)=2.0Khz.
Still nothing worth looking at unless I want to watch the 60 cycle go by!

 I was thinking out loud. Since then I've found that a 12" wire at 8 feet
works very well. Great isolation too.


                                       jim