[Amps] Good Source for HV Probe?

[Ian White, G3SEK]

Richard wrote:
>
>
>>Hey Gang,
>>
>>In doing a search around the web I'm coming up pretty empty for a source
>>for a good (hopefully inexpensive) HV Probe.  My DVM is rated for 1KV
>>max.  Any ideas?
>>
>€  I made one in a small plastic box with a 400M‡, 50kV-rated resistor.
>I put a 20t trimpot across the output to zero in the calibration for a
>10M‡ DMM load.  The division ratio is 1000 to 1.  Fluke sells a probe
>that does pretty  much the same thing.

As Rich says, a home-made 1000 to 1 probe can work just fine.

Mine is very similar to Rich's. It uses a string of ordinary 10M 0.25W 
350V metal film resistors in the barrel of a plastic pen... must have 
been about 20 of them, so the probe is good up to about 7kV. The pen 
case is glued into a very small plastic box which acts as a handle. The 
resistor string extends to the bottom of the box where there is a 500K 
10-turn pot for calibration.

One end of the pot track goes to the string of resistors, and the other 
end of the track goes to a clip-on ground lead which is also connected 
to the negative of the DMM. The slider of the pot goes to the positive 
input of the DMM.

The advantage of this construction is that there are no high voltages 
inside the box part that you  might be holding, and the hot end of the 
probe is a long way away.

Calibration is best done with a DC voltage near the top end of the meter 
range, just under 200V for a typical 3-1/2 or 4-1/2 digit meter. This 
minimizes the effect of any errors on the last digit. For example if the 
meter reads 191.5 volts on its 200V range when connected direct, then 
with the 1000 to 1 probe connected you should adjust the trimpot to read 
191.5 millivolts on the meter's 200mV range. Then switch to the meter's 
20V range to read voltages up to 20kV (within the breakdown limit of the 
resistors).

This isn't a perfect calibration method, but it's a lot better than 
relying on the accuracy of special high-voltage resistors (which are 
hard to find) and the accuracy of the DVM's input impedance.

-- 
73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                           'In Practice' columnist for RadCom (RSGB)
New e-mail: g3sek@ifwtech.co.uk
New website: http://www.ifwtech.co.uk/g3sek