G8MNY > TECHNI 22.06.18 10:45l 82 Lines 3261 Bytes #9 (0) @ WW
BID : 26121_GB7CIP
Subj: High Voltage Probes
Sent: 180622/0836Z @:GB7CIP.#32.GBR.EURO #:26121 [Caterham Surrey GBR]
To : TECH@WW
By G8MNY (Updated Feb 17)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Although you can buy a commercial probe, here is one that you can make for
either an oscilloscope or a DMM. It is designed to fit an old plastic single
piece clear pen tube. For general high voltage work there is no need for great
accuracy, just safety & kit protection.
240V - 10kV MAX
DC to 50Hz only. Not for LOPTY AC stages!
100Mê input load.
Use 1kV rated Rs!
10:1 DIGITAL VOLTMETER PROBE
This assumes the DMM is 10Mê input.
<------------- 90Mê -------------->
Wire ,/~~~³~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~³ Flexible DMM
Probe`\___³__________________________________________³ insulated wire 10Mê
thick plastic like clear uncracked pen ³
Apply mains to DMM & note reading. Add the series probe & the reading should
have gone down to 1/10. If to high add more Rs to suit, it too low replace an R
with a low value to suit.
100:1 SCOPE PROBE 100
This assumes the scope is 1Mê input.
<-------------- 99Mê ---------------->
Wire ,/~~~³~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~³ Flexible Scope
Probe`\___³_________________________________________³ insulated wire 1Mê
The 9Mê is just a low value 10Mê selected on test. Or you could just use the
same DMM probe for both & recalibrate the scope to suit when used.
If scope has 100V/cm range, apply 240V mains at 350V peak 700V peak-peak &
measure with scope directly, it should give about 7cm p-p. Now with probe in
series adjust the scope's Y front panel gain for the same 7cm P-P display on
the 1V/cm range. Or you can adjust the R values as in DMM calibration.
MEASURING HIGH VOLTAGES
With all power off, & any HV point safely dumped to earth, connect the wire
probe. Ensure the meter/scope is safely connected to the same circuit as
With hands clear power up the HV & take readings etc.
For B/W TV/MON/SCOPES EHT be aware that 10kV will easily track any slightly
dirty/damp surface! Plastic sprays like "Damp Start" can be useful, but be
aware of non connection to surfaces coated with it!
If the testgear end gets disconnected power down the HV unit & make sure any HT
is safely discharged before working on the loose testgear end of the probe.
This longitudinal resistor design should not flashover, but is extra testgear
protection is needed put a wire ended NEON tube across the DMM/SCOPE input.
That should limit the DMM or SCOPE to see only 80V (e.g. probe can only measure
up to 800V on DMM or 8kV on SCOPE).
See my Tech Buls "Scope Probes" & "Scope & DMM Calibrator"
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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