# Measuring "high" voltage - Page 3

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Re: Measuring "high" voltage

On Wed, 2 Aug 2006 13:46:03 +0000 (UTC), snipped-for-privacy@green.rahul.net (Ken
Smith) Gave us:

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

Re: Measuring "high" voltage

"can" vs "usually"

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

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Re: Measuring "high" voltage

On Fri, 4 Aug 2006 01:03:28 +0000 (UTC), snipped-for-privacy@green.rahul.net (Ken
Smith) Gave us:

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.

Re: Measuring "high" voltage

I guess its all a matter of priorities.  Pretty colors vs working scope,
you choose.

As I suggested elsewhere in this thread, a SIDAC would be good for this
job.  You will need to keep the operating voltage at the input of the
scope well away from the voltage limit to allow room for the SIDAC, but
that wouldn't be a major problem.

One of the nice things about SIDACs is that when tripped, they "breakover"
and pull the voltage near ground.  This reduces the amount of energy they
have to absorb.

We also need an impedance in the path from the HV that we are fairly sure
won't fail shorted.  A lowish valued resistor would be good for this.
Without it,  we just get to see the TVS explode a few nS before the scope
does.

If not:

Signal ----------+-----------+-----------------Scope
!           !
V          ---
---          ^
!           !
+200V --/\/\---+           +---/\/\-- -200V
!           !
+---[TVS]---+
!           !
---          V
^          ---
!           !
GND         GND

The TVS and 4 diodes are likely to be destroyed if the fault happens.

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Re: Measuring "high" voltage

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

Cheers!
Rich

Re: Measuring "high" voltage

On Tue, 1 Aug 2006 15:44:41 +0200, "Fred Bartoli"
us:

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.

It is meant to clamp LOW overvoltages such that scope probes get
protected.  Pretty simple.

Re: Measuring "high" voltage

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

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