Author Topic: VOM, VTVM and DMM - Part 2  (Read 4958 times)

Offline K9DJT

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VOM, VTVM and DMM - Part 2
« on: September 11, 2013, 11:09:29 AM »
Last month I touched base on the VOM (Volt-Ohm-Meter) and ohms-per-volt.  As you should remember, the VOM is a useful measurement tool on a low resistant circuit but questionable on a high resistant circuit because of the “loading effect” which can result in an erroneous reading.  So how can we confidently make a voltage measurement in a high resistant circuit?  Originally it was accomplished by using a VTVM (Vacuum Tube Voltmeter), and most recently a DMM (Digital Multimeter).The VTVM was first introduced in 1942 by David Packard, Model 400A, and was manufactured until 1958.  Many other companies followed with their own designs and became the choice of instruments by engineers and technicians through the 60’s.  Basically the meter operates by using a tube amplifier to generate the current required to deflect the meter pointer, hence the name Vacuum Tube Voltmeter.  By using this technique, an input resistance or impedance (the resistance applied in parallel with the circuit under test) of up to 20 megohms can be achieved.  The key benefit is that the 20 megs is independent of the range selected which is complete opposite of a VOM.  Now by using ohms law, if you were to place 10 to 20 meg ohms of impedance across a high resistant circuit, you will conclude it will have a negligible effect, and therefore provide a accurate measurement.  A value of 10 meg ohms for input impedance is now the accepted standard.

The above is the upside which the technical community felt outweighs the negative side of requiring an AC line cord for the power supply, the inability to measure current and the need to deal with a little larger test probe.  In the years to come, the tube amplifier was replaced using a FET solid state device which brought back the portability, but still the need for a larger probe and no current measurement.  But wait, couldn’t we measure a current if we measured the “voltage-drop” across a 1 ohm resistor of proper wattage placed in series with our circuit?  You bet we can.  With that being said, it is not always practical but still a solution in many cases.

There is not enough space to write about all the features and benefits of purchasing a DMM this month, but I will say it has the best of both the VOM and VTVM in one package, i.e., it typically has a 10 meg input impedance, it is portable (no line cord), makes use of a variety of slim probes and  in most cases can measure a current up to 10 amps.

73, Gary