Lesson 03 – Measuring with Multimeters

A multimeter is an electronic instrument that can perform several measurement functions. Typical measurement features include measuring voltage, current and resistance. Older type multimeters were analogue with a moving needle but the latest models are digital with a digital readout. Some multimeters can test transistors and semiconductors and can even measure temperature. 

A multimeter has two leads, one red and one black. The black lead is the common (COM) lead and the red lead is the positive lead used to measure Volt and Current. Measuring current is done in series and one must use the separate input socket for current on the multimeter. The multimeter is polarity sensitive so when measuring Current and Voltage make sure the red lead goes to the positive measuring point and the common lead to the negative measuring point. 

Measuring Voltage
AC and DC voltage can be measured with a multimeter, to measure DC voltage select the DC voltage range on the meter. Voltage is measured between two points and in parallel with the circuit or resistor as in the diagram on the right. The + side of the voltmeter represents the red multimeter lead and the – side of the voltmeter the black common multimeter lead. 

Remember voltage is measured with the circuit switched on. If the polarity of the leads is reversed the multimeter will give a negative reading to indicate that. If the multimeter doesn’t have auto ranging you will need to select the correct voltage range to ensure that the meter doesn’t overflow.

AC Voltage can also be measured by selecting the AC voltage range but care must be taken when measuring high voltages such as grid supply voltages to avoid electric shock.

Measuring Current
Current is measured in series (in line) with a circuit and the positive lead of the multimeter must be plunged into the current socket of the multimeter to measure current.  Failing to do so could damage the multimeter. To measure the current through the resistor R in the diagram to the right, the amp meter can be connected in series as indicated where the + represents the positive red lead of the multimeter and the – the common black lead of the multimeter. If the multimeter doesn’t have auto ranging you will need to select the correct current range to ensure that the meter doesn’t overflow.

Measuring Resistance
Measuring resistance is done out of circuit to ensure that only the resistance of the component is measured. If the component is measured in circuit the total resistance between the two points will be measured, giving a false reading of the component. The polarity of the meter doesn’t play a role in the measurement of resistance. If the multimeter doesn’t have auto ranging you will need to select the correct resistance range to ensure that the meter doesn’t overflow.