[sdiy] (Semi-OT) cap polarity in microphone

[John L Marshall]

Scott,

Your meter is anything but typical.

Typical cheapie DMM's have a fixed input impedance of 1 Mohm.

A 10 Mohm meter load across a 1 Mohm source resistance will cause the 10
Mohm source to appear as 9.1 Mohm. Nearly a 10% error. Ouch!

Take care,
John

---- Original Message -----
From: Scott Gravenhorst <music.maker at gte.net>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Monday, March 18, 2002 2:08 PM
Subject: Re: [sdiy] (Semi-OT) cap polarity in microphone


> But I wonder by how much the measurement would be different.
> Aren't voltmeter circuits designed to be very high impedance
> specifically so that the meter's load is negligable in most
> cases?
>
> I just looked at the specs for the meter I own, a Fluke 8060a,
> not that it's typical, but the manual says that for DC voltage
> measurement on the 200mv and 2v ranges, the input impedance
> is greater than 1,000 megohms, typically being 10,000 megohms.
> For the other ranges, they state a nominal input impedance of
> 10 megohms.  Even 10 megohms doesn't seem like much of a load.
>