Re: HV Measurement - The Divider Problem (fwd)

[<sroys@xxxxxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Wed, 22 Sep 2004 20:46:24 -0300
From: Antonio Carlos M. de Queiroz <acmdq@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Cc: hvlist <hvlist@xxxxxxxxxxxxxxxxx>
Subject: Re: HV Measurement - The Divider Problem (fwd)

High Voltage list wrote:

> From: Matthew Smith <matt@xxxxxxxxxx>

> I recently built a voltage divider, using a string of 10 x 33M, 0.5W HV resistors.

The output resistance is too high for direct connection to a
conventional
meter.
 
> It's not quite a divide-by-ten due to inconsistencies in the resistance values
> (all off same tape).  This isn't a worry - I have measured the resistances and
> know the exact factor to apply.

Including the input resistance of the meter?
 
> I hooked this up to a small HV supply I'd been working on, and put my Wavetek
> DMM across the 1/10th resistor.  The voltage read far below what I was
> expecting.  I checked again at the 1/2 point and still got a reading much lower
> than expected.

Probably because the imput resistance of the meter reduced the last
resistor
to effectively less than 10 MOhms.
 
> Assuming that the supply wasn't delivering what I thought it should, I put the
> DMM straight across it.  Bang.  No more DMM.

... ... ...

Use the meter to measure the current through the string of resistors,
not the
voltage over one of them.

Antonio Carlos M. de Queiroz