Page 73 - Teach Yourself Electricity and Electronics

Multimeters 53

It can be difficult to interpolate for high values of resistance, unless the right scale is se-
lected. Engineers and technicians usually connect an ohmmeter in a circuit with the
meter set for the highest resistance range first; then they switch the range until the me-
ter is in a part of the scale that is easy to read. Finally, the reading is taken, and is mul-
tiplied (or divided) by the appropriate amount as indicated on the range switch. Figure
3-9 shows an ohmmeter reading. The meter itself says 4.7, but the range switch says 1
KΩ. This indicates a resistance of 4.7 KΩ, or 4700 Ω.

3-9 An example of an ohmeter
reading. This device
shows about 4.7 1K
4.7 K 4700 ohms.

Ohmmeters will give inaccurate readings if there is a voltage between the points
where the meter is connected. This is because such a voltage either adds to, or sub-
tracts from, the ohmmeter battery voltage. This in effect changes the battery voltage,
and the meter reading is thrown way off. Sometimes the meter might even read “more
than infinity” ohms; the needle will hit the pin at the left end of the scale. Therefore,
when using an ohmmeter to measure resistance, you need to be sure that there is no
voltage between the points under test. The best way to do this is to switch off the equip-
ment in question.

Multimeters

In the electronics lab, a common piece of test equipment is the multimeter, in which
different kinds of meters are combined into a single unit. The volt-ohm-milliammeter
(VOM) is the most often used. As its name implies, it combines voltage, resistance and
current measuring capabilities.
You should not have too much trouble envisioning how a single milliammeter can be
used for measuring voltage, current and resistance. The preceding discussions for mea-
surements of these quantities have all included methods in which a current meter can
be used to measure the intended quantity.
Commercially available multimeters have certain limits in the values they can mea-
sure. The maximum voltage is around 1000 V; larger voltages require special leads and
heavily insulated wires, as well as other safety precautions. The maximum current