One of the most common laboratory instruments is the multimeter. This versatile three-in-one device transforms from a voltmeter to an ammeter to an ohmmeter literally at the press of a button.

How does it work? What changes inside when a different function is selected? What do you need to do to measure an amp, an ohm, or a volt?

What's Inside the Multimeter?

Every multimeter, whether digital or analog, is constructed around a "basic meter movement" capable of measuring a very small current or voltage. This same meter movement (display unit) is used whatever the quantity being measured. Since the meter movement does not change, something else must be switched inside the meter when a different function is selected.

In the illustration at right, the multimeter begins as a simple 200 uA movement. This meter can measure current over the range 0..200 uA. Despite its limited functionality, a useful measurement device can now be built around this simple movement.

Note the meter movement shown has an internal resistance of 1000 ohms. This implies that any circuit or component placed across its terminals will effectively have 1000 ohms in parallel with it. It also implies that when exactly 200 uA is passing through the meter, there is 200 mV across this resistor. So, in fact, this same movement may equally well be labelled a 0..200 mV meter. Still not especially useful, unless that is all you need to measure.

VOLTMETER

Selecting the VOLTS function effectively places some resistance in series with the internal meter movement and the external connections. The total internal resistance is now (1000 + R) ohms. Of course, the meter still shows a full scale reading when 200 mV is across its internal 1000 ohm resistor.

IDEAL VOLTMETER:
Internal Impedance = infinite

As an example, let this resistance be R=9000 ohms. Any voltage applied to the terminals will be split in the ratio 9:1 across the two resistors. The total internal resistance is now 10000 ohms.

Now, if 2000 mV is placed across the terminals of this new device, there will be 800 mV across the 9K resistor and 200 mV across the 1K resistor. The basic meter movement always reads full scale with 200 mV across it, so the overall meter now reads full scale with 2000 mV across its terminals. The meter can now be used to measure 0..2 V. Only the decimal point needs to move in the display.

SUMMARY: To select different voltage ranges, different values for an internal series resistance R are chosen by pressing different buttons on the multimeter.

Because a voltmeter must be placed across a voltage to be measured, and the meter should not affect the measurement, the best voltmeter has a very high internal resistance.

AMMETER

Selecting the AMPS function effectively places some resistance in parallel with the internal meter movement and the external connections. The total internal resistance is now (1000 || R) ohms. Of course, the meter still shows a full scale reading when 200 uA flows through its internal 1000 ohm resistor.

IDEAL AMMETER:
Internal Impedance = zero

As an example, let this resistance be R = 1000/9 = 111.1 ohms. Any current flowing through the terminals will be split in the ratio 9:1 across the two resistors. The total internal resistance is now 100 ohms.

Now, if 2000 uA is placed across the terminals of this new device, there will be 200 uA flowing through the 1K resistor and 1800 uA though the other. The basic meter movement always reads full scale with 200 uA through it, so the overall meter now reads full scale with 2000 uA flowing between its terminals. The meter can now be used to measure 0..2 mA. Only the decimal point needs to move in the display.

SUMMARY: To select different current ranges, different values for an internal parallel (shunt) resistance R are chosen by pressing different buttons on the multimeter.

Because an ammeter must be placed in series with the current to be measured, and the meter should not affect the measurement, the best ammeter has a very low internal resistance.

An ammeter has near-zero impedance, therefore NEVER PLACE A VOLTAGE SOURCE DIRECTLY ACROSS AN AMMETER It follows that you should never connect the meter before checking what function has been selected. Ammeters generally have a fuse in series with the terminals to avoid permanent meter damage. (This fuse is often found blown in undergraduate laboratories.)

OHMMETER

Unlike current or voltage measurements, where some 'active' electric quantity is to be measured, there is nothing about a simple component like a resistor which would affect a meter movement. No amount of series or parallel components will make any reading appear on the basic meter movement.

When the OHMS function is selected a small test circuit is configured within the meter, with the unknown resistor placed into the test circuit at the meter terminals. To this end, an internal power source is required. Portable multimeters use a battery to create a current or voltage though the test resistor. Using Ohm's Law, (V=IR), a known current through an unknown resistance produces a measurable voltage at the internal meter movement.

Observe that to measure a resistor in this manner requires that it be isolated from any circuit it might already be part of. In other words, do not measure resistors in-circuit.

SUMMARY: Resistance measurements require some internal source to create a test circuit for isolated resistors. To select different resistance ranges, different values for an internal series resistance R are chosen by pressing different buttons on the multimeter.

R.Tervo 1997
University of New Brunswick - Department of Electrical and Computer Engineering