March 1972 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
For most needs to measure voltage,
current, and resistance, modern users of test equipment do not need to give much thought
to the electrical characteristics of the instrument being used. Other than setting the
function switch to the proper position (ohms, volts, amps, milliamps, etc.) and not exceeding
the safe input limits, measurement accuracy can usually be assumed to be good to within ±2
to ±5 of the least significant displayed digit. I.e., if the digital display shows
10.000, then the actual value is likely in the range of 9.995 to 10.005. Autoranging
even removes the need to manually determine the proper range setting. For critical measurements,
of course, you need to actually read the documentation to get actual accuracy ranges.
One critical electrical parameter of a meter is its input impedance, since its value
affects the voltage / current division at the measurement point. An extreme example is
where you want to measure the voltage of a unit under test (UUT) that has a 100 kΩ
impedance and an open circuit voltage of 1 volt, and the meter also has in input impedance
of 100 kΩ, the reported voltage will be 0.5 V (50% error). Increase the meter
input impedance to 100 MΩ and the reported voltage will be 0.999 V (0.1% error).
The need for accuracy when measuring high impedance circuits motivated the design of
vacuum tube voltmeters (VTVMs) that placed a tube in series with the input to achieve
megohm levels as opposed to mere kilohms for standard meters.
Test Equipment Scene: VOM's, VTVM's and TVM's
By Leslie Solomon, Technical Editor
The "bread and butter" instrument used by more hobbyists
and service technicians than any other single piece of electronic gear is the ubiquitous
multirange tester known as the voltohmmeter, or VOM.
The VOM has been around for many years and was one of the first widely usable test
instruments. Now being manufactured by a number of companies (Eico, RCA, B&K, Simpson,
Triplett, Sencore, Heath, and others), the VOM is readily available from most electronic
The modern VOM has many features and uses not possessed by its predecessors. The VOM
of the past had relatively low input impedance, somewhat limited ranges, and a small
meter size. It thus became less and less useful as circuit impedances increased and the
levels of measurement for voltages and resistances decreased. Thus, the older VOM is
now largely relegated to appliance servicing, where exact measurements are not necessary.
As the need for more precise, non-loading measurements emerged, VOM's underwent some
drastic changes. It was necessary to have input impedances of 50,000 ohms per volt or
more; accuracies of 1% were necessary and voltages under 1 volt had to be measured.
There were also important mechanical changes to be made. Not only did the physical
size of the meter increase (to permit better interpolation of readings), taut band meters,
which are inherently rugged and reliable, were incorporated. The low friction involved
in the taut-band movement also provided highly repeatable measurements.
Another physical problem, the high mortality of VOM's due to case breakage, was attacked
by a number of manufacturers (Heath and Weston, for example) by the introduction of high-impact
plastic cases that resist breakage. Some modern VOM's even come with circuit breakers
and diode meter protectors to eliminate accidental burnout. Even the rotary function
switch has given way, in many cases, to multiple pushbutton selection of ranges and functions.
One manufacturer (Sencore) has even come out with a meter with 112 ranges!
Enter the VTM. Updated VOM's are good, but there arose a need for
an even more precise, multirange instrument that would load the circuit even less and
could cope with more difficult measurement problems. Thus the vacuum tube voltmeter (VTVM)
The VTVM differs from the VOM in that the meter is driven by a vacuum tube circuit.
In this way, a very high input impedance (11 megohms in typical) is obtained on almost
all ranges, resulting in negligible circuit loading. Since the tube circuit acts as a
buffer between the meter and the circuit being tested, the VTVM has a built-in meter
protector. And, since the tube circuit has gain, a more sensitive measuring instrument
can be designed. Another advantage of the VTVM - especially on the ac ranges - is that
a tube circuit can be tailored to have a very wide frequency response. For example, a
modern VTVM not only spans the audio range, but can reach high r-f when the proper probes
are used. There are even special-purpose ac VTVM's - such as Eico's combination broadband
ac VTVM and amplifier - to serve dual functions.
The biggest asset of most VTVM's is in working with semiconductor circuits. Because
of the low voltages involved in this type of gear, most VTVM's have a low-end range of
0.5 volt or less. With the very high input impedance of the VTVM, this makes it easy
to check low-level transistor voltages. At the high end, most VTVM's can read 1 kilovolt
(5 kV or more with a probe) making them doubly useful for servicing vacuum-tube circuits.
Of course, VTVM's run somewhat higher in price than standard VOM's.
Then Came the TVM. It's only natural that semiconductors are replacing vacuum tubes
in test equipment - as they have in many other applications. Especially significant has
been the introduction of the field effect transistor (FET) with its high input impedance
- an ideal characteristic for measurement. Thus the transistor voltmeter (TVM) has come
into being. Essentially a solid-state version of the VTVM, the TVM usually incorporates
all the good features of its predecessor. Since the semiconductor elements are small
and require little power, TVM's can be made highly portable (using battery power supplies
in many cases). Making use of all the latest advances in circuit design, TVM's have increased
ranges, excellent sensitivity, and high input impedance. However, when selecting an instrument
for your own use, there are a few things that should be kept in mind.
Sensitivity. The first thing to look for in selecting a VOM, VTVM,
or TVM is the number of ohms-per-volt specified for the ac and dc ranges. Assume, for
example, that a VOM is rated for 1000 ohms per volt. This means that the loading resistance
of this instrument is 1000 ohms times the scale indication. Thus, with 10 volts input
on the 10-volt range, the VOM resistance is 10,000 ohms. That's pretty high but stop
and consider that, when indicating 10 volts, the meter takes 1 milliampere from the circuit
under test. That may be OK for testing a power supply but the effect is quite different
when the meter is connected across a load of a half a megohm in a grid or base biasing
circuit. In that case, will the meter indicate the true voltage value? Will the loading
seriously affect the performance of the circuit? Remember that from an electrical viewpoint,
the meter looks like a 10,000-ohm resistor. Think what a measurement of a very low voltage
across such a low resistance looks like and you will see why the VTVM and TVM with their
11-megohm input resistances became popular.
Why are so many test meters specified at a much lower ohms-per-volt rating for ac
measurements? Simply because they have dc meter movements and the quantity being measured
must be rectified. And that means higher loading.
With ac measurement, you must also consider the relationships between rms, peak, peak-to-peak,
and average values. Most ac measuring instruments, unless otherwise specified, use rms
(root mean square) values as the basis for sine wave measurements. If you have a need
to convert from one value to another, remember the following relationships:
peak value = 1.414 X rms value
rms value = 0.707 X peak value
peak-to-peak value = 2.83 X nns value
= 2 X peak value
average value = 0.637 X peak value
Rectifier-type ac meters do not indicate true rms except for sine wave inputs. Actually,
they respond to average rectified values. For half-wave rectification, the average is
0.637 times the peak value, while rms is 0.707 times peak. In most cases, the meter scale
has been calibrated to indicate about 10% higher than average so as to indicate rms values.
Accuracy. This refers to the meter's ability to indicate true voltage,
current, or resistance. Accuracy is normally specified as some percentage of full-scale
deflection. For example, consider a 3% meter measuring 100 volts on the 150-volt scale.
The accuracy would be within 3% of 150 volts, or 4.5 volts (maximum) at any point on
the scale. Thus, in measuring 100 volts, you may have a reading as low as 95.5 or as
high as 104.5. That's not too bad, but suppose you were measuring 10 volts on the 150-volt
scale. You could hit 5.5 volts on the low end or 14.5 on the high end - an error possibility
of about 50%. That is why you should always try to make all measurements as near as possible
to the high end of the scale.
Ranges. In the days when the vacuum tube was the predominant active
element in most circuits, most voltages to be measured, even those on grids and cathodes,
were over one volt; and this fact was reflected in the use of 2.5 or 3 volts for the
lowest range on most test instruments. Now, in the solid-state age, many voltages under
one volt must be measured. A glance at the schematic for any semiconductor device will
show what low levels have to be measured. This new low level of measurement is reflected
in the 0.5-volt or less full-scale ranges on a modern VTVM or TVM. These instruments
also have ranges up to 1 kilovolt or so for use in testing vacuum tube circuits.
Resistance Measurements. Resistance is measured by impressing a voltage
across the unknown resistance and measuring the voltage drop produced by the current
flow. In most cases, the greater the voltage sensitivity of the instrument, the higher
the measurable resistance values. Most VOM's, VTVM's and TVM's are perfectly capable
of measuring the usual spread of resistance values found in electronic equipment. However,
there is one point to be remembered: the usual ohmmeter uses 1.5 volts or more to make
a resistance measurement; and this voltage, if applied to a resistor in a semiconductor
circuit, may be high enough to forward bias the associated semiconductor junction. This
makes any resistance measurement invalid and also may lead to the accidental burning
out of the junction.
Many modern instruments use very low voltages to make in-circuit resistance measurements
to avoid the forward bias effect. Remember that a silicon junction will switch on at
about 0.6 volt forward bias, while a germanium junction requires only about 0.3 volt.
Keep this in mind when using an ohmmeter to make in-circuit resistance measurements.
This also applies when using an ohmmeter to "test" transistors. It is possible to deliver,
unknowingly, enough current through a forward biased junction to completely destroy it
through the thermal effect. The solution is to use an intermediate resistance range so
that neither current nor applied voltage is excessive. The use of special low-power ohmmeter
circuits will solve the problem.
Uses and Abuses. There is no reason why a VOM, VTVM, or TVM should
not provide good service for many years if it is properly handled. Just don't forget
the basic rules: always make sure that you are in the correct function (connecting an
ohmmeter or current meter across a voltage source can be disastrous) , and always start
on the highest range, working your way down until the meter indication is as far upscale
as possible. If you have a meter with color-coded banana jack inputs, check that the
leads are properly connected. Black is usually ground, and red is the "hot" lead. With
no power on the meter, make sure that the needle rests at zero. There is usually an adjustment
screw on the front of the meter for zeroing.
There is also a small thing called static charge that can accumulate on a plastic
meter face (especially on large ones) that can cause erratic meter deflections. Most
units are treated to remove this effect; but if you do run into trouble, there are several
anti-static compounds that can be used.
Posted December 14, 2017