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Measuring "high" voltage

K

Ken Smith

Jan 1, 1970
0
One trick I've used to mitigate Zener capacitance is to use a regular
small-signal diode in series with the Zener (with the appropriate
polarity). Typically the forward capacitance of the small-signal
diode is much lower, so you get two capacitors in series. Very handy
for a rude-and-crude clamp at an opamp's output, but I've never
tried it in a transient-suppression application.

You have to provide biasing.

Fortunately, the diodes fail shorted, usually.
 
K

Ken Smith

Jan 1, 1970
0
It will never see that high a potential in the ripple checker I
spoke of. Transient rarely rise above 200V, and that is with a 15kV
input.

Earlier, there was mension of the series capacitor arcing over. In that
context, the 1KV fast rise time spike makes a lot more sense.
 
R

Rich Grise

Jan 1, 1970
0
Ken Smith wrote...

They may, but we don't care because the scope input is toast. We need a
TVS, transient voltage suppressor, AKA zener diode, with its massive
sub-ns response to overvoltage. They do have a capacitance penalty, but
in this case we can live with it.

i.e., *MONGO* Zener, AKA TransZorb. :)

Cheers!
Rich
 
S

Steve Goldstein

Jan 1, 1970
0
Steve, neat trick. Wouldn't you have to pre-bias the zener to get it near
the operating voltage?

We used this as a slow protection clamp on an op-amp output to protect
some downstream stuff. Something like an amplifier with +-15V driving
some other bits with +-5V supplies and feeble ESD diodes. So we didn't
care a lot about accuracy, and we could wait a few us for it to turn on.
The primary concern was to make sure the op-amp didn't oscillate, which
it would have done had we just used the Zeners.

Had we wanted stiffer clamps we would have prebiased the Zener at a
cost of more power and complexity, but this wasn't needed.

BTW, I've just changed my posting address (getting the hang of this
Forte Agent thing). To reply directly you'll need to remove the food
item from my address.

Steve
 
P

Phat Bytestard

Jan 1, 1970
0
Fortunately, the diodes fail shorted, usually.


Diodes typically fail shorted. Diodes caused to fail by means of HV
can also fail open.
 
K

Ken Smith

Jan 1, 1970
0
Diodes typically fail shorted. Diodes caused to fail by means of HV
can also fail open.

"can" vs "usually"

I think it is really the high current pulse that opens them not th ehigh
voltage as such.
 
P

Phat Bytestard

Jan 1, 1970
0
"can" vs "usually"

I think it is really the high current pulse that opens them not th ehigh
voltage as such.

A high voltage breach usually punches through actually removing
media. That plasma will always involve maximum available current.

At low voltage, a high current will simply melt the junction.

Not that media can't be lost that way as well, it is just that an HV
blow out really does blow out the material.
 
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