[Elecraft] Resistance Readings with Digital Volt Meter

[Ron D' Eau Claire]

> 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?

Never heard of a "true DVM", there are "true rms ac voltmeters", but I
suspect this refers to a digital multimeter or "DMM" compared to an analog
"Multimeter" or "Volt-Ohm-Milliammeter" (VOM).

The difference is that the DMM circuit does not require as much power to
operate the metering circuit, so it takes less current from the circuit
under test to measure voltage or current and applies less voltage to the
circuit to measure resistance.

In an analog VOM or "multimeter", the circuit whose voltage is being
measured provides the power to move the meter pointer. In ANY circuit being
tested, measuring the voltage or current alters the voltage or current to
some extent. The "quality" of an "analog" VOM was largely measured by the
sensitivity of the meter movement. A good VOM meter movement may require
only 50 microamperes to deflect it to full scale. Cheaper meters may require
as much as a milliampere to produce full scale deflection. That's current
that must come from the circuit under test.

In many situations the amount of power taken from the circuit to measure the
voltage or current is insignificant, but not always so. Very high impedance
circuits are one example. Even a very sensitive VOM with a 50 microampere
movement will draw enough current to change the voltage in such a circuit so
that any measurement made is essentially meaningless.

This problem was overcome by the advent of the "vacuum tube voltmeter" or
VTVM. In a VTVM a vacuum tube provides the power to operate the meter. The
meter is controlled by applying the voltage from the circuit under test to
the grid of a vacuum tube, which has a VERY high impedance and requires only
the tiniest amount of current to control the meter. Now one can make
accurate measurements of extremely high impedance circuits. The current
required from the circuit under test by a VTVM is almost entirely that
needed by the voltage dividing resistors used to set the voltage scales.
These can be very large resistors. Most VTVM's have input resistance of 10
or 11 megohms for d-c measurements.

Then came the "Digital Multimeter" or DMM. It is a digital version of the
VTVM, using a field effect transistor at the input. The FET also has a very
large input impedance and most DMM's have a 10 megohm input resistance,
similar to VTVMs.

Today, almost all multimeters you see with an analog movement will be a VOM,
not a VTVM, and they require a lot more current from the circuit under test
than a Digital Multimeter.

Measuring resistance, in every meter that I have used, digital or analog,
involves applying a voltage to the circuit and measuring the resultant
current. The meter or readout actually measures the current, but it is
calibrated in Ohms.

In older VOM's, it was common to use a 22 or 45 volt battery in the ohmmeter
circuit. That meant that you might apply 22 to 45 volts to the circuit under
test and the meter might supply several milliamperes at that voltage. That
was no problem for a tube circuit designed to work at several hundred volts,
but it is deadly to modern solid state gear.

VTVM's were not quite so bad, typically using 1.5 volts for the ohmmeter
functions. But that can be dangerous to some solid state circuits.

Modern DMM's use much less voltage and current. My 'bench' DMM delivers at
the most 0.3 volt to the circuit being tested and never allows more than 1
mA to flow in the circuit being tested for resistance. That the maximum
values, and it is usually much less. Those levels are safe for any solid
state circuits. The process is the same. A known voltage is applied to the
circuit, the resulting current flowing  is measured, and the result is
displayed as ohms on the readout.

However, even a few tenths of a volt is sufficient to cause some solid state
junctions to conduct, somewhat. That conduction will cause the resistance re
ported to be something less than infinite. That's why the Elecraft manuals
say that circuits where you'd expect the resistance to be in the range of
many megohms usually say to check for a resistance of >100K ohms.

Different DMM's use slightly different voltages and so they will report
substantially different resistance across such circuits.

Ron AC7AC
K2 # 1289