Measuring Voltage on a high impedance load

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ckeene
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Measuring Voltage on a high impedance load

Post by ckeene » Fri Mar 20, 2009 5:46 am

Hello,

I'm trying to measure a DC voltage at a very high load (somewhere around 15Meg). At this level, do I need to be cautious about my DMM's impedance, lest the reading become inaccurate?

Thanks

-ck

rodabod
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Re: Measuring Voltage on a high impedance load

Post by rodabod » Fri Mar 20, 2009 9:12 am

ckeene wrote: I'm trying to measure a DC voltage at a very high load (somewhere around 15Meg). At this level, do I need to be cautious about my DMM's impedance, lest the reading become inaccurate?
Yes, you'll load-down this resistance with your meter (say 10 Meg) so take this into consideration.
Roddy Bell
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The Scum
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Post by The Scum » Fri Mar 20, 2009 9:50 am

What's the circuit in question? How is the high impedance being used? Is it a 15 Meg guitar input on something? A 15 Meg resistor used within a circuit, such as to get some finicky filter to behave?

The pedantic answer: yes, you'll load the 15 M with the 10 M of a typical meter.

The pragmatic answer: 10 M and 15 M are such light loads that either/both may only have a tiny influence...no current going into either of them, thus no real net load.

The Murphy's law answer: it's a high impedance node...sticking an antenna (er, a meter probe) on it is going to cause such disastrous parasitic effects (AM radio reception, etc) that you can't measure what you're after. A touch of Heisenberg & Shroedinger there, too...

If you're certain that it's a 15 M load, you could possibly use an ammeter on it to measure current, then apply Mr. Ohm to derive the voltage.

ckeene
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Post by ckeene » Fri Mar 20, 2009 10:07 am

Specifically, it's a Wurlitzer 200. This instrument, if you didn't know, has a reed and tine mechanism for generating sound. the reed part is a metal plate with a 140-170v fixed DC charge applied to it. So it kind of works like a condensor mic. (You probably knew all this stuff.)

Anyway, there's a 10M resistor inline on the DC reed supply, so I think that's affecting my measurement. There are a couple other big resistors near the filter caps too (22M and 8.2M).

Thanks!

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