RE: HV Measurement - The Divider Problem (fwd)

[<sroys@xxxxxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Wed, 22 Sep 2004 12:01:13 -0500
From: Carl Litton <Carl_Litton@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: RE: HV Measurement - The Divider Problem (fwd)


Some variations on a theme:

I have also used a 2 Meg rotary  potentiometer from an old scrap
oscilloscope instead of the 1.2 Meg low end resistor for extremely
precise calibration of the divider, then read the resistance across that
side of the pot and built a series chain of 1/4 watt carbons to give the
same resistance.

Mouser now also has a 1 GigOhm Ohmite SLIMOX 2Watt in stock for the
exact same price ($4.20).  Coupling with a 12 Meg low end resistor or
chain thereof allows DMM to read directly in Kilovolts.

Finally, this all good up to about 15 kV.  The 6 to 10 Watt 40 to 80 KV
capable resistors are available but I find them all in the $20-$30
range, making the project costly enough to warrant just buying a HiVolt
AC/DC probe for about $100.00.

If anyone knows of a cheaper source, I'd like to know.


Carl Litton
901-374-5747



-----Original Message-----
From: High Voltage list [mailto:hvlist@xxxxxxxxxx] 
Sent: Wednesday, September 22, 2004 9:42 AM
To: hvlist
Subject: RE: HV Measurement - The Divider Problem (fwd)

Original poster: <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