Re: HV Measurement - The Divider Problem (fwd)

[<sroys@xxxxxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Wed, 22 Sep 2004 22:58:05 +0800
From: Peter Terren <pterren@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: HV Measurement - The Divider Problem (fwd)

If you had an open circuit in the chain, then this would explain all your 
readings and the demise of your meter.
Was it a 50Hz supply or a flyback because the spiky nature of the latter 
will give low rms or standard voltage readings despite the high peak 
readings (which is what blows the meter).

In addition your DMM probably has 10M input impedance which is significant 
compared with the 33M resistors.

My meter is from my old x-ray unit (with a dial reading of 100 kV full scale 
deflection - FSD for 125 uA), I added a resistor chain (720 M Ohm, 72 M Ohm, 
7.2 M Ohm, 800 K Ohm) to allow FSD of 100 kV, 10 kV or 1 kV.  The current is 
full wave rectified at about 100V level with fast diodes (BYV-29   500 Volt, 
9 A, 60nS) and is protected by a gas arrester and diodes across the meter 
itself.  A capacitor smoothes the output of the bridge rectifier giving the 
ability to read DC and (sort of)peak AC. Voltages up to 1000 V agree with my 
digital voltmeter to within around 5% on DC and peak AC 50 Hz.  Peak reading 
is about 80kv before arcing over occurs.  On the contrary, capacitative 
coupling causes my homemade meter to read 15kV of flyback as 100kV.
http://tesladownunder.iinet.net.au/Other_HV_stuff.htm#Voltmeter

Peter

> ---------- Forwarded message ----------
> Date: Wed, 22 Sep 2004 12:05:39 +0930
> From: Matthew Smith <matt@xxxxxxxxxx>
> 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
>
> -- 
> Matthew Smith
> Kadina Business Consultancy
> South Australia