Multimeter readings -sometimes false

[brett]

Hi.  This is a reminder about multimeters and their readings.  Newer members might not have discovered this yet.

On the weekend I was reading the bias voltage on Q1 in a RM axis fuzz.  The ideal reading is about 3.6V, but mine was showing 3V.  So I figured that something was wrong, and I better start fiddling with the bias resistors.  

It took me an hour of messing around before I realised that the meter I was using has an impedance of about 1M - low enough in relation to the 680k resistor that I was measuring across to affect the measurement.  The resistance of the meter and 680k resistor together was about 400k.  No wonder the bias voltage was too low when I measured it - the meter was lowering the measured value by about 1V.  :roll:

Note to self:  NEVER trust a voltmeter when working with resistances to earth of more than 100k.

[Brett Clark]

This is one reason that I have always advocated Flukes over the cheap imported meters - better meters typically have very high input impedances (>10M Ohm ), and the manual will tell you what they are.

[niftydog]

NEVER trust a voltmeter when working with resistances to earth of more than 100k.

Never trust a CHEAP or ANALOGUE multimeter when working with resistances of more than 100k!

A good quality digital multimeter will last you forever and it will be useful for readings up to about 1M.

[Paul Perry (Frostwave)]

There is another side to this coin... I've been caught out measuring AC voltages on transformer windings, AC warts etc by DMMs wiht huge input impedances. When you are looing at a broken winding, having a 50Meg input impedance can make the capacitance between windings look a relatively small impedance, and you get these spurious readings. Sometimes I have to wire a 10K resistor across the inputs as a sanity check!! of course, testing a circuit, the higher the impedance the better.

[Brett Clark]

Some Fluke models have a feature called V-Check which provides a low impedence input option for this purpose.

[Brett Clark]

Never trust a CHEAP or ANALOGUE multimeter when working with resistances of more than 100k!

Actually (IIRC), there were some not-cheap analog VTVM's (Vacuum Tube Volt Meters) back in the day that had stupid-high input impedences (>100M)!!!

But cheap analog meters may be as low as 20K....