RE: high voltage measurement w/ divider

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Original poster: "Mccauley, Daniel H by way of Terry Fritz <teslalist-at-qwest-dot-net>" <daniel.h.mccauley-at-lmco-dot-com>


I tried replying to this post last night, but for some reason, the message
got kicked back to me.

Building a HV Divider is much more than just stringing some high meg
resistors in series and forming a
classical voltage divider.  There is much much more to it to design and
build one that will work well.
Because a high voltage divider is relatively large and the impedances are
very high, the parasitic capacitance and
inductances can be significant.  If the string is relatively long, the
inductance could be relatively high (on the order of a few uH).  This isn't
enough to cause concern at low frequencies, but if you are looking at your
higher frequency ring ups/downs, it could greatly affect your output.  Also
at 60Hz, a 10pF (small) parasitic capacitance has an impedance of
approximately 260M!  So you can see, that a good high voltage dividier
design will minimize these stray capacitances / inductances and compensate
for their affect.

A link to a HV Divider design I created is here:

http://www.spacecatlighting-dot-com/hvdivider.pdf

But I guess if you just trying to qualitatively look at some high voltages,
than a simple resistive divider network would probably work well enough at
least at 60Hz.


Dan



Hey all,

I was wanting to build a voltage divider out of some spare 10
meg resistors that I had laying around to safely measure the
charge voltage of my 200 uFD, 10 kV energy discharge cap bank
for my can crusher/quarter shrinker assembly. I was wondering
if I could accurately measure the known fraction of the total
voltage by placing 10 of these resistors in series and measur-
ing 1/10 of the total voltage across just one of the resistors?
I know this principle works because of Ohm's law and all, but
what my real question is would the 100 megs be too much resis-
tance to get an accurate and reliable reading on my Sperry DVM?
I think most DVMs have at least 20 kOhms /volt deflection so
it seems that measuring up to 1000 volts (1/10 of the 10 kV)
should be ok since 20 megs (20 K X 1000 volts) is greater than
the 10 meg for each resistor. BTW, these are the Digikey 10 meg
resistors that many of you are using as bleeders for your MMCs.

Also, I think these are 1/2 watt resistors and if my math is
right, they should be dissapating 1 watt when the caps are
charged to the full 10 kV (10*4 V/10*8 Ohms= 10*-4 amps or
0.1 mA and therefore 10 kV X 0.1 mA = 1 watt. I think this
doubling of their wattage rating on such an intermittent ba-
sis should be ok?

Thanks for any help,
David Rieben