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Registered Member #580
Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410
How do you measure the current (in circuit, not short circuit current) from an isolated high voltage DC source using an analogue and/or digital multimeter without it going crazy! I read online people have readings of microamps, I have readings before i even finish the circuit, therefore invalidating the magnitude of the results.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
You have to isolate any leakage paths through the meter, e.g. Put a 50 uA (or as required) moving coil meter in a metal box; connect one side of the meter to the metal; bring out the other lead from the box; (no special insulation between lead and box required as it's only the meter volt-drop. Less than 1V)
Connect the box to the HV source ------DANGER the box is now at HV Connect the other wire to the load.
IF the load can be isolated it's much easier and safer to connect the load to ground via the meter.
WHICH EVER METHOD YOU USE, PUT A COUPLE OF DIODES IN INVERSE-PARALLEL WITH THE METER
This will protect the meter when unexpected (?) arcs to ground etc. occur.
Registered Member #350
Joined: Mon Mar 27 2006, 05:14PM
Location:
Posts: 106
The safest way is to have one lead of the multimeter grounded. Ground the negative output of your HV supply and also the negative lead of the multimeter. That way you don't need a metal box around your DMM. Then connect a capacitor - e.g. 100nF should work but no electrolytic - in parallel to the DMM. That will prevent any current peaks through the DMM. Then connect your load between the positive terminal of the HV source and the positive lead of the multimeter. And make sure the DMM is far away from the positive terminal.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
A small word of caution here: Don't be lured into a false sense of safety by connecting one lead of your DMM to ground and trusting the whole DMM to be close to ground potential. If the circuit unexpectedly opens somewhere the thing will be at full potential. Better to measure at the hot end and take full precaution.
Registered Member #593
Joined: Tue Mar 20 2007, 12:32AM
Location:
Posts: 50
Wow Terry, as inexpensive as the Velleman is, I would be scared of letting it ride on the toroid, especially on that raised up "pan" type platform yours seems to have there.
Still though, now the temptation is there. I want to get several more of those little scopes. I love mine, and I've really learned about my coils from it.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Followup question; how could you measure output power of an HV transformer without an HV meter. Say I could estimate the voltage by spark gap distance. What do I then measure the current between? The HV electrode and air? Simply pulling an arc will give the short circuit current, but voltage will be unknown. Or do I have to go via input power - estimated losses?
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measuring current of HVDC
