Input Impedances for VTVMs, DMMs, etc

[Michael Donnelly]

Hey all,

At the risk of getting off topic there seem to be so many questions about
VTVMs, DMMs, VOMs, etc and what to use and what the specs mean that maybe
some basic definitions are in order (If I'm wrong in doing this feel free
to flame me):

VTVM: DC volts input impedance - 10 megohm meter, 1 megohm probe for a
total of 11 megohm. Typical DC accuracy - 3%. AC volts input impedance -
1 megohm shunted by about 200pf at the probe tip. Typical AC accuracy -
5% over a specific frequency range (25Hz to 1Mhz). Typical of the breed,
Heath IM-28 (Heath tie-in!). A VTVM is a very good all around measurement
tool for most troubleshooting, unless you are very heavily into digital
circuits. In those cases it may not be accurate enough.

VOM (Volt/ohm meter): Rather than using an 'input impedance' spec VOMs
express it as a 'sensitivity' of 'ohm/volt'. Typical are 20K ohm/volt DC
and 5K ohm/volt AC. Examples are Heath MM-1 and Simpson 260. In general,
these specs translate into a much lower input impedance and less
accuracy. IMHO, VOMs are not suitable for serious troubleshooting. They
can load down a circuit which results in inaccurate readings. But need to
know if 120VAC is present on that outlet? Good choice here. And another
caveat: many VOMs use ohms batteries much greater than 1.5 volts. They
can damage semiconductor junctions.

Good Quality DMM (DVM): emphasis on good quality. Input impedance for
both DC volts and AC volts - 10 megohm or greater. DC basic accuracy -
.3% or better. AC accuracy - 1.0% or better, within a specific frequency
range. Typical of the breed, Fluke 70 series, Fluke 80 series, TEK DMM900
series. These DMMs even have an analog bargraph to spot trends. Best
choice for your troubleshooting needs, even on vintage equipment.

So....if you want/need an accurate voltmeter, what to do? If you have a
VTVM, use it! It will serve you well, unless you're into digital. Want a
DMM? When shopping around ask two basic questions. Basic DC accuracy -
better be .5% or better. Input impedance - better be 10 megohm or
greater. I'll bethca many of the $40 DMMs don't qualify. After all, when
you are staring at a piece of non-working equipment what is usually the
first test equipment you grab? Your voltmeter. Invest in a good one.

Shameless plug - Yesterday I received a Tech America Flyer. On the front
page is a Fluke 77 Series 11 DMM on sale for $99.95. That is an absolute
bargain! Fluke is closing out the Series 11 and coming out with the
Series 111. I have no connection what so ever with Tech America. Just
passing on that info.

Mike Donnelly
MJATS at JUNO.COM

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