ORC Radio Chatter

Experimenter's Corner => Test Equipment => Topic started by: K9DJT on August 12, 2013, 09:15:44 AM

Title: VOM, VTVM and DMM Differences - Part 1
Post by: K9DJT on August 12, 2013, 09:15:44 AM
With our fall swapfest soon approaching, we all have an opportunity to maybe find a deal on a multimeter.  But do we know the difference between the offerings?  How does a VOM (Volt-Ohm-Meter) differ from a VTVM (Vacuum Tube Voltmeter), or either to a DMM (Digital Multimeter)?  Do they all perform the same functions?

One of the earliest multimeters manufactured was the Simpson-260 which was introduced in the 1930’s and has since evolved into a series of them.   As a matter of fact, they are still being produced and sell for about $299 new.  You will find many old-timers out there who still believe there is nothing better.  It measures AC and DC voltages up to 1000 volts and DC currents up to 10 amps.  In addition, it will measure resistance from zero to 20 Meg ohms.  Accuracy is reasonable and is specified as a percent of full scale reading.  When looking at the spec’s of a traditional VOM such as the Simpson, you will notice it is rated as a certain “Ohms-per-Volt,” e.g., 20,000 ohms per volt DC, and 5000 ohms per volt AC.  This relates to the input impedance (resistance) of the meter when making a voltage measurement and the impact it has on the measurement itself.  It is also considered an indication of the sensitivity of the meter, i.e., the higher the ohms-per-volt, the greater the sensitivity.

Looking at the above VOM example (Single click 1st pix below), you can see that there is a different resistance used for each voltage range.  The actual lead-to-lead resistance is different for each voltage range you might use in making a measurement.  Now consider the impact of that resistance you are placing in the circuit under test.  What is the chance of it altering the circuit enough that your measurement really isn’t correct?  It most definitely will in low current electronic type circuits.  It is referred to as the Loading Effect.

Looking at the circuit example below(Single click 2nd pix below), you can see what happens when you use a low impedance VOM on a high resistance circuit.  You do actually alter it unknowingly BY applying a parallel resistance path in the circuit and obtaining the equivalent circuit.  This is the reason you want a high impedance meter to do electronic diagnostics.

We will discuss high impedance meters such as a VTVM and DMM next month.

73, Gary
Title: Re: VOM, VTVM and DMM Differences - Part 1
Post by: WA9JOB on August 13, 2013, 02:59:09 PM
Great topic Gary. Meters 101.  Basics are always good to review.