Author Topic: Can you use low impedance volt measurement to discharge capacitors  (Read 2935 times)

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Offline SkippyTopic starter

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Just watched Dave's Agilent multimeter review. I see it's got a low impedance volt measurement input.

Could you use it to discharge  filter capacitors in power supplies? So, you've got 300V in your cap, you attach the probes, it's got 10k impedance across it, and you can watch the power drain out? Or would your multimeter melt? Or am I a numpty in some other way?
 

Offline Excavatoree

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Re: Can you use low impedance volt measurement to discharge capacitors
« Reply #1 on: April 11, 2013, 01:36:10 pm »
In SOME cases, you might get away with it,  but honestly, why risk an expensive meter when a cheap resistor (chosen correctly) is the best way to do it?

 

alm

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Re: Can you use low impedance volt measurement to discharge capacitors
« Reply #2 on: April 11, 2013, 03:03:13 pm »
The low impedance mode is not very resistive at high voltages. The Agilent U1272/1273A, for example, is specified as having a 2 kohm input impedance in lowZ mode on the 1000 V range. 1000 V across 2 kohm would dissipate 500 W, not going to happen. So there is something like a PTC in series that goes up in resistance as the current through it increases. It is designed to discharge the stray capacitance of wiring, I would not rely on it dissipating the energy from a large cap. It might very well go high resistance and stop discharging the cap.
 

Offline ddavidebor

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Can you use low impedance volt measurement to discharge capacitors
« Reply #3 on: April 11, 2013, 03:38:46 pm »
In SOME cases, you might get away with it,  but honestly, why risk an expensive meter when a cheap resistor (chosen correctly) is the best way to do it?
+1
David - Professional Engineer - Medical Devices and Tablet Computers at Smartbox AT
Side businesses: Altium Industry Expert writer, http://fermium.ltd.uk (Scientific Equiment), http://chinesecleavers.co.uk (Cutlery),
 


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