Re: Differential voltage probes

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

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

Hi Steve,

At 03:13 PM 6/28/2003 +0100, you wrote:
>At 18:28 27/06/03 -0600, you wrote:
>>Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
>>
>>Hi All,
>>
>>Finn and I have been thinking about differential voltage probes that 
>>would be useful for solid state Tesla coil work.  Rather than spend $$$$ 
>>on commercial ones, say from Tek, we were wondering about just making our own.
>
>Hi Terry, I like your design (tres simple) and I would add a couple of 
>suggestions:
>
>*Clamp diodes from the op-amp inputs to the rails

Should not be needed.  The TL082 is pretty tough.  I was thinking a lot of 
little things like that could be forgotten in favor of just keeping it 
super simple.  The TL082 is pretty "old", but it probably does as well as 
any very common chip in this case.


>*A trimpot on one of the 1K resistors for CMRR trim (maybe T-network type 
>thing)

I would like to not have any adjustments.  Have to really try it and see if 
it needs any additional parts to make it work right.


>*A variable capacitor somewhere for HF CMRR trim (maybe put a small 10pF 
>fixed capacitor across one 1K resistor and a 2-20pF trimmer capacitor 
>across the other)

A few caps may indeed be needed.  have to try and see.


>*Metal film resistors because carbon ones have a voltage coefficient of 
>resistance.

Should not matter too much.  I was thinking the probe would be like 5% if 
one selected the resistors a bit.  Even at 10% it would still do 
practically everything we need.


>I have all the parts lying around and I'll give it a try

Cool!  I am not sure I will have time this weekend.

>I just remembered hewlett-packard (I think) make a 'linear optocoupler' 
>which is like an ordinary optocoupler but with 1 LED and 2 isolated 
>photodiodes. You use one photodiode for feedback stabilization and the 
>other for output.
>
>You can use one of these plus two op-amps to make an opto-isolated scope 
>probe with about 1MHz bandwidth. This approach would probably give better 
>performance when looking at small signals like gate voltages etc.
>
>Agilent part# HCNR200
>http://literature.agilent-dot-com/litweb/pdf/5988-4104EN.pdf

Sounds like it is getting too complicated ;-))  If we could just figure out 
the "basics", then folks could always add fancy stuff as they please.  More 
worried about phase shift, frequency response, safety... right now.  Steve 
Ward blew a channel on his scope this morning :-((  A probe like this would 
have easily prevented that...  "Normal" scope probes and stuff just do not 
like to work with voltages over about 100V.  In the case of AC circuits, 
the grounding and all can easily blow things up unless you are super 
careful.  A simple probe like this would fix all those problems.  In most 
cases, extreme accuracy and bandwidth are simply not needed.  Steve was 
just trying to check AC voltage but the probe got left in the 1X 
position...  Hopefully just a few blown scale resistors...

A very simple device like this would allow people to check AC stuff and 
<1000V stuff very easily and safely.  Floating scope cases and using low 
signal level probes is just too risky when a simple thing like this would 
solve all that.

Cheers,

         Terry


>Steve C.
>