Multimeters
Seeing as how a common meter movement can be
made to function as a voltmeter, ammeter, or ohmmeter simply
by connecting it to different external resistor networks, it
should make sense that a multi-purpose meter ("multimeter")
could be designed in one unit with the appropriate switch(es)
and resistors.
For general purpose electronics work, the
multimeter reigns supreme as the instrument of choice. No
other device is able to do so much with so little an
investment in parts and elegant simplicity of operation. As
with most things in the world of electronics, the advent of
solid-state components like transistors has revolutionized
the way things are done, and multimeter design is no
exception to this rule. However, in keeping with this
chapter's emphasis on analog ("old-fashioned") meter
technology, I'll show you a few pre-transistor meters.
The unit shown above is typical of a
handheld analog multimeter, with ranges for voltage,
current, and resistance measurement. Note the many scales on
the face of the meter movement for the different ranges and
functions selectable by the rotary switch. The wires for
connecting this instrument to a circuit (the "test leads")
are plugged into the two copper jacks (socket holes) at the
bottom-center of the meter face marked "- TEST +", black and
red.
This multimeter (Barnett brand) takes a
slightly different design approach than the previous unit.
Note how the rotary selector switch has fewer positions than
the previous meter, but also how there are many more jacks
into which the test leads may be plugged into. Each one of
those jacks is labeled with a number indicating the
respective full-scale range of the meter.
Lastly, here is a picture of a digital
multimeter. Note that the familiar meter movement has been
replaced by a blank, gray-colored display screen. When
powered, numerical digits appear in that screen area,
depicting the amount of voltage, current, or resistance
being measured. This particular brand and model of digital
meter has a rotary selector switch and four jacks into which
test leads can be plugged. Two leads -- one red and one
black -- are shown plugged into the meter.
A close examination of this meter will
reveal one "common" jack for the black test lead and three
others for the red test lead. The jack into which the red
lead is shown inserted is labeled for voltage and resistance
measurement, while the other two jacks are labeled for
current (A, mA, and �A) measurement. This is a wise design
feature of the multimeter, requiring the user to move a test
lead plug from one jack to another in order to switch from
the voltage measurement to the current measurement function.
It would be hazardous to have the meter set in current
measurement mode while connected across a significant source
of voltage because of the low input resistance, and making
it necessary to move a test lead plug rather than just flip
the selector switch to a different position helps ensure
that the meter doesn't get set to measure current
unintentionally.
Note that the selector switch still has
different positions for voltage and current measurement, so
in order for the user to switch between these two modes of
measurement they must switch the position of the red test
lead and move the selector switch to a different
position.
Also note that neither the selector switch
nor the jacks are labeled with measurement ranges. In other
words, there are no "100 volt" or "10 volt" or "1 volt"
ranges (or any equivalent range steps) on this meter.
Rather, this meter is "autoranging," meaning that it
automatically picks the appropriate range for the quantity
being measured. Autoranging is a feature only found on
digital meters, but not all digital meters.
No two models of multimeters are designed to
operate exactly the same, even if they're manufactured by
the same company. In order to fully understand the operation
of any multimeter, the owner's manual must be consulted.
Here is a schematic for a simple analog
volt/ammeter:
In the switch's three lower (most
counter-clockwise) positions, the meter movement is
connected to the Common and V jacks through
one of three different series range resistors (Rmultiplier1
through Rmultiplier3), and so acts as a
voltmeter. In the fourth position, the meter movement is
connected in parallel with the shunt resistor, and so acts
as an ammeter for any current entering the common
jack and exiting the A jack. In the last (furthest
clockwise) position, the meter movement is disconnected from
either red jack, but short-circuited through the switch.
This short-circuiting creates a dampening effect on the
needle, guarding against mechanical shock damage when the
meter is handled and moved.
If an ohmmeter function is desired in this
multimeter design, it may be substituted for one of the
three voltage ranges as such:
With all three fundamental functions
available, this multimeter may also be known as a
volt-ohm-milliammeter.
Obtaining a reading from an analog
multimeter when there is a multitude of ranges and only one
meter movement may seem daunting to the new technician. On
an analog multimeter, the meter movement is marked with
several scales, each one useful for at least one range
setting. Here is a close-up photograph of the scale from the
Barnett multimeter shown earlier in this section:
Note that there are three types of scales on
this meter face: a green scale for resistance at the top, a
set of black scales for DC voltage and current in the
middle, and a set of blue scales for AC voltage and current
at the bottom. Both the DC and AC scales have three
sub-scales, one ranging 0 to 2.5, one ranging 0 to 5, and
one ranging 0 to 10. The meter operator must choose
whichever scale best matches the range switch and plug
settings in order to properly interpret the meter's
indication.
This particular multimeter has several basic
voltage measurement ranges: 2.5 volts, 10 volts, 50 volts,
250 volts, 500 volts, and 1000 volts. With the use of the
voltage range extender unit at the top of the multimeter,
voltages up to 5000 volts can be measured. Suppose the meter
operator chose to switch the meter into the "volt" function
and plug the red test lead into the 10 volt jack. To
interpret the needle's position, he or she would have to
read the scale ending with the number "10". If they moved
the red test plug into the 250 volt jack, however, they
would read the meter indication on the scale ending with
"2.5", multiplying the direct indication by a factor of 100
in order to find what the measured voltage was.
If current is measured with this meter,
another jack is chosen for the red plug to be inserted into
and the range is selected via a rotary switch. This close-up
photograph shows the switch set to the 2.5 mA position:
Note how all current ranges are power-of-ten
multiples of the three scale ranges shown on the meter face:
2.5, 5, and 10. In some range settings, such as the 2.5 mA
for example, the meter indication may be read directly on
the 0 to 2.5 scale. For other range settings (250 �A, 50 mA,
100 mA, and 500 mA), the meter indication must be read off
the appropriate scale and then multiplied by either 10 or
100 to obtain the real figure. The highest current range
available on this meter is obtained with the rotary switch
in the 2.5/10 amp position. The distinction between 2.5 amps
and 10 amps is made by the red test plug position: a special
"10 amp" jack next to the regular current-measuring jack
provides an alternative plug setting to select the higher
range.
Resistance in ohms, of course, is read by a
logarithmic scale at the top of the meter face. It is
"backward," just like all battery-operated analog ohmmeters,
with zero at the right-hand side of the face and infinity at
the left-hand side. There is only one jack provided on this
particular multimeter for "ohms," so different
resistance-measuring ranges must be selected by the rotary
switch. Notice on the switch how five different "multiplier"
settings are provided for measuring resistance: Rx1, Rx10,
Rx100, Rx1000, and Rx10000. Just as you might suspect, the
meter indication is given by multiplying whatever needle
position is shown on the meter face by the power-of-ten
multiplying factor set by the rotary switch. |
