Re: HV Measurement - The Divider Problem (fwd)

[<sroys@xxxxxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Wed, 22 Sep 2004 10:27:21 -0400
From: Charles Pike <ctpike@xxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: HV Measurement - The Divider Problem (fwd)

Mathew:
According to the manual on my Wavetek 27XT the input impedance is 10 
megohms ( This is typical of most DMMs).  Therefore when you connect the 
meter across a 33 meg resistor its effective value becomes:  R = 
(33*10)/(33+10) =  7.67 meg.  The division ratio is then 39.7 :1 instead 
of 10 to one.  Sorry about the fried meter, I just cooked one myself.

Good luck,
Charlie



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
>
>  
>