RE: HV Measurement - The Divider Problem (fwd)

[<sroys@xxxxxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Wed, 22 Sep 2004 09:20:33 -0500
From: Carl Litton <Carl_Litton@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: RE: HV Measurement - The Divider Problem (fwd)

Just built one myself.  It works perfectly to give 1/100 reading AC or
DC.  Only 2 cheap resistors needed.  On a $30.00 DMM, I've measured up
to 15 kV so far and regularly 2 to 8 kV with MOT voltage double/tripler
experiments without incident.   Less than $5.00 invested.


1 Ohmite SLIMOX 100Meg thick film 2 Watt 15 kV (Mouser $4.20) + 1 single
1.2 Meg resistor of any type (1/4Watt is fine as current will be in
micro amperes) - can also take several smaller resistors in series to
give 1.2 Meg and treat them as a single resistor.

Solder both resistors in series.  Connect high end of circuit to the
free end of the 100 Meg Ohmite and the opposite end to ground.  Place
DMM leads  - one between the the two resistors and on the low (ground)
end.

Reading on DMM is multiplied by 100 to give exact voltage.




Carl Litton

-----Original Message-----
From: High Voltage list [mailto:hvlist@xxxxxxxxxx] 
Sent: Wednesday, September 22, 2004 8:40 AM
To: hvlist
Subject: HV Measurement - The Divider Problem (fwd)

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

-- 
Matthew Smith
Kadina Business Consultancy
South Australia