RE: Measuring HV capacitor voltage (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 19 Jul 2007 14:38:34 -0600
From: S&JY <youngs@xxxxxxxxx>
To: 'Tesla list' <tesla@xxxxxxxxxx>
Subject: RE: Measuring HV capacitor voltage (fwd)

David,

Corona is your enemy in voltage dividers.  It ionizes the air which makes
the air act like additional resistance across your resistor string, and you
end up with faulty readings.  And if the corona leads to an arc, you are
likely to turn your meter to toast.

So, you need to use a lot of resistors for a 10 KV divider, e.g. 40 to 50 of
them.  For my 20 KV divider I use 80 1 meg resistors.  Each resistor should
be limited to about 250 volts across it - 500 at most.  Layout is important
- lay them out in a zig-zag fashion that keeps them away from each other.
Or better yet, solder them end to end and put them inside a vinyl tube.  If
you do that, you can wrap the string around a form (e.g. short piece of 3/4"
PVC) for more compactness.

For safety, put a power diode between the meter end of your voltage divider
and ground.  If your divider ever breaks down and shorts, the diode will
conduct and will prevent your panel meter from having dangerous voltages to
it.

Jameco (www.jameco.com)sells bags of 100 1/4 watt resistors for 99 cents a
bag - great price!

Your analog meter cannot have an impedance of more than 1 K or so, as it
uses a coil of very fine wire.  Use a digital ohmmeter and measure it.
Also, usually on the meter face is an indication of the full scale (fs)
current.  Knowing these values, you can calculate the required resistance of
your divider string.  Mine has a fs of 83 microamp and a resistance of 1.2K.
Put a pot across your meter (e.g. 1K) so you have a way to calibrate it.  

Now for the bad news.  Let's assume your meter has a coil resistance of
nearly zero.  Then 15 volts across your 15K series resistor means it will
take about 1 mA for full scale.  That means your voltage divider must
dissipate 1 ma x 10 KV = 10 watts.  This means your voltage divider will get
hot and will need some air circulation.  Or, you must limit your measurement
time to just a few seconds.  I think you would do better to get a meter with
a full scale current of around 100 microamp.  Then you wouldn't have to
worry about voltage divider heat.


--Steve Y.



-----Original Message-----
From: Tesla list [mailto:tesla@xxxxxxxxxx] 
Sent: Thursday, July 19, 2007 12:10 PM
To: tesla@xxxxxxxxxx
Subject: Measuring HV capacitor voltage (fwd)


---------- Forwarded message ----------
Date: Thu, 19 Jul 2007 15:15:51 +0000
From: David Rieben <drieben@xxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Cc: drieben@xxxxxxxxxx
Subject: Measuring HV capacitor voltage

Hi all,

I'm having a little issue with measuring up to 10 kV across a capacitor
on a standard panel mount, analog 0-15 VDC voltmeter from Radio 
Shack. Obviously, I was trying to make it read 1 volt for each measured
1000 volts. I tried using a voltage divider of 10 seriesed 10 meg resistors 
and a 100 K pot but found that the 100 megs was just too much resist-
ance to allow for sufficient current for reliable movement of the volt
meter. 
I then tried a series-parallel arrangment of 10x10 of the 10 meg resistors 
for a total of 10 meg and then still used the 100 K pot and varied it to try

to bring in the proper calibration. I was checking the voltage measurement 
with my HV probe and DVM. The resistors didn't like being bunched up 
10 at a time and started flashing over externally. Also, I was having
trouble 
getting the analog meter readings to be accurate throughout the 0 - 15 volt 
range according to the measurement of the HV probe. I think the meter is
designed for 10 meg impedance and it also comes with a 15 K resistor that
is supposed to be in series with the voltage source. Anyone have any sug-
gestions for reliable and reasonably accurate measurement of high voltage
through a low voltage analog voltmeter via a voltage divider network? 

Thanks
David Rieben




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