[Elecraft] Resistance Readings with Digital Volt Meter

[Timothy A. Raymer]

For a long time, there were Volt-Ohm-Meters (VOM) that were very 
common.  The Simpson 260-8XL is a very common example of one of these that 
is still available.

Those types of meters normally have an analog meter that displays the 
readings.  The problem with them is that the meter itself participates in 
the amount of resistance that the circuit is presented with.  Usually there 
will be an indication of this on the meter face, such as 20,000Ohm/V or 
something along those lines.  So, this is usually a giveaway.  There were 
also a few Volt Ohm meters with digital readouts that had circuitry to 
emulate Analog Volt Ohm meters.  These were not real common, but show up 
occasionaly.  The more expensive meters in days past were Vacuum Tube Volt 
Meters.(VTVM)  The RCA WV-98C is an example that many of us may have seen 
in an electronics lab.  They presented the measured circuit with a "self" 
impedence of several Megaohms, but were very expensive.  And because they 
had a vacuum tube in them, they did not lend themselves to being very 
portable.  Digital Multimeters came along, and gave us the best of both 
worlds: relatively inexpensive, essentially infinite input impedance, and 
for an additional cost, automatic ranging.  And so it goes.

To answer your question a little more directly: a digital Multimeter or DMM 
will put a voltage on a circuit to measure resistance.  Typically they will 
have the positive voltage on the positive lead.  If you know what to expect 
in terms of the reading, (100Kohm was our earlier example) the next largest 
scale on the meter should give you the most accurate reading.

The difference is that in the older analog Volt Ohm meter (VOM) the meter 
movement itself can affect the measurement you are taking, due to its 
relatively low impedance.  In essence, it can "load" the circuit under test.

In addition, because the voltage on them tended to have the positive 
voltage on the black lead, this can cause strange readings in complex 
ciruits with diodes and other semiconductor devices in them because the 
voltage is opposite of what a Digital meter usually uses.

An analog VOM with a digital readout would have the same loading 
characteristics as a VOM with a needle meter display, and would have the 
same 20,000 ohms/V rating on it as an analog VOM.

If you look at the label for your meter, the odds are good that it says 
Digital Multimeter. As long as you select the appropriate scale for the 
testing you are doing, you should get comparable results to the manual.

Tim Raymer
73 de KA0OUV

At 16:29 01/02/2002 -0500, rlewis@staffnet.com wrote:
>Okay, I've got some questions. How does a "true DVM" differ from an 
>"analog" DVM with a digital readout? How does a "true DVM" measure 
>resistance without putting a voltage across the device being measured?


Timothy A. Raymer               <traymer@mail.state.mo.us>
Missouri Department of Health   <raymet@dhss.state.mo.us>
and Senior Services