(T7D06) Using a Multimeter

T7D06from the Technician License Course Section 8.3, Making Electrical Measurements:Which of the following might damage a multimeter?

A. Measuring a voltage too small for the chosen scaleB. Leaving the meter in the milliamps position overnightC. Attempting to measure voltage when using the resistance settingD. Not allowing it to warm up properly

A multimeter provides the capability of measuring multiple electrical characteristics such as voltage (volts), current (amps), resistance (ohms), and others. Care must be taken by the operator of a multimeter to avoid unsafe conditions that may harm the operator or damage the multimeter.

Measurement Selector Switch: A multimeter will have a selector switch, such as a rotary pointer, to select the type of measurement desired. Common selections include:

AC Voltage, with multiple scales for the maximum reading or needle deflection.

DC Voltage, with multiple scales for the maximum reading or needle deflection.

AC Amperage (current) with multiple scales for the maximum reading or needle deflection.

DC Amperage (current) with multiple scales for the maximum reading or needle deflection.

Resistance, with multiple scales for the maximum reading or needle deflection.

Other selections such as battery test and electrical continuity.

Probe Sockets: Additionally, a multimeter may have three sockets into which the probes are connected. One of the sockets will be “common” and usually color-coded black. Two color-coded red sockets will have different measurement functions. One red socket is usually designated for both voltage and resistance measurements, while the other red socket is exclusively for amperage measurements. To make an accurate and safe measurement, the operator must select the proper switch position and also connect the probes to the proper sockets for the desired measurement type.

Resistance Measurement: A multimeter is essentially a sensing and computing instrument that utilizes the relationships embodied in Ohm’s Law, where E = I x R (E in volts, I in amperes, and R in ohms). Given two of the quantities related in Ohm’s Law the third may be computed. Usually, a multimeter will contain circuits for a direct detection and conversion of voltage into a scaled value for display. Modern multimeters will use a circuit called an integrating converter for this purpose. Older needle-display type multimeters used a moving coil galvanometer to displace the needle proportionally to the current passing through the coil, or with resistors added, proportionally to a measured voltage.

When measuring resistance the ohmmeter will activate a circuit that produces a constant current along the probes, and thus, through the resistance being measured. The integrating converter (or other method) is used to detect the resulting voltage across the unknown resistance. The resistance value is then computed by the Ohm’s Law relationship, R = E / I, since the constant current (I) and the measured voltage (E) are known quantities.

The Threat: Typically, the constant current provided by the multimeter for resistance measurement will be very small and the internal resistance of the multimeter’s circuit will also be very low to afford greater sensitivity for resistance measurements. However, when measuring a voltage with the multimeter the internal resistance is usually significantly larger to prevent a large current from passing through the delicate circuits of the multimeter, such as the integrating converter circuit.

Now you are probably putting together the threat! If you are dialed up to measure a resistance the multimeter will have very low internal resistance. If you then connect the probes across a voltage, having forgotten to change the measurement switch position to voltage measurement, the low internal resistance may allow a large current to flow through the multimeter’s circuits. A large current flow can damage the circuits.

The answer to Technician Class question T7D06, “Which of the following might damage a multimeter?”is “C. Attempting to measure voltage when using the resistance setting.”