Re: HV Measurement - The Divider Problem (fwd)

[<sroys@xxxxxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Wed, 22 Sep 2004 09:14:32 -0500
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: HV Measurement - The Divider Problem (fwd)

Matthew,

Did you take into account the effect of the input resistance of the 
DMM across the divider?

-- Bert --
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High Voltage list wrote:

> Original poster: <sroys@xxxxxxxxxxxxxxxxx>
> 
> 
> 
> ---------- Forwarded message ----------
> Date: Wed, 22 Sep 2004 12:05:39 +0930
> From: Matthew Smith <matt@xxxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: HV Measurement - The Divider Problem
> 
> Hi All
> 
> I recently built a voltage divider, using a string of 10 x 33M, 0.5W HV resistors.
> 
> 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.
> 
> 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.
> 
> Assuming that the supply wasn't delivering what I thought it should, I put the 
> DMM straight across it.  Bang.  No more DMM.
> 
> So, the voltage WAS what I had calculated, it's just that the divider was 
> reading very low.
> 
> I have had a similar experience before (and another fried DMM), but thought that 
> I was dealing with a fault divider - this is why I checked and double-checked my 
> resistances.
> 
> Would anyone care to point out what I'm missing here?  I really can't afford to 
> buy quality DMMs on a consumable basis ;-)
> 
> Cheers
> 
> M
>