1. Turn dial to . Some digital multimeters (DMMs) also include . If uncertain of which to choose, start with , which handles higher voltage.
2. First insert the black probe into the COM jack.
3. Then insert the red probes into the V ? jack. When finished, remove the probes in reverse order: red first, then black.
4. Connect the test probes to the circuit: black to the negative polarity test point (circuit ground), red to positive test point.
Note: Most modern DMMs automatically detect polarity. When measuring dc voltage, it is not critical for the red lead to contact a positive terminal or black to touch negative. Just recognize if the probes touch opposite terminals, a negative symbol will appear in the display. With an analog multimeter, red leads should always touch a positive terminal and black a negative terminal. Otherwise, damage to the meter will occur.
5. Read the measurement in the display.
Other useful functions when measuring dc voltage
6. Modern DMMs default to Autorange based on the function selected on the dial. To select a specific fixed measurement range, press the RANGE button multiple times until the desired range is selected. If the voltage measurement falls within the range of a lower setting, follow these steps:
- Disconnect the test probes.
- Change the dial setting to [mV dc symbol].
- Reconnect the test probes and read the measurement.
7. Press the HOLD button to capture a stable measurement. It can be viewed after the measurement is complete.
8. Press the MIN/MAX button to capture the lowest and highest measurement. The DMM beeps each time a new reading is recorded.
9. Press the relative (REL) or delta (?) button to set the DMM to a specific reference value. Measurements above and below the reference value are displayed.
Note: Avoid this common technician mistake: inserting test probes into incorrect input jacks. If measuring dc voltage, be certain to insert the red probe into the input jack marked V, not A. The display should show the dcV symbol. Placing test probes in A or mA inputs and then measuring voltage will create a short in the measurement circuit.
Voltage Measurement Analysis
- Voltage measurements are normally taken to a) establish that voltage exists at a given point and b) ensure that the voltage is at the proper level.
- AC voltages can vary widely (between -10% and +5% of the power source rating) and cause no problems in a circuit. Yet with dc voltages, even small variations may indicate trouble.
- The exact amount of acceptable dc voltage variation depends on the application. See chart below for an example.
- In some dc applications, large dc variations are not only acceptable, but intentional.
- Example: The speed of dc motors can be adjusted by varying the amount of dc voltage supplied. In this application, the measurement of dc motor voltage depends on the setting of the voltage regulator.
- When taking and comparing dc voltage measurements, refer to manufacturer’s specifications for specific values in the circuit.
As shown in the chart above, a fully charged auto battery rated for 12 volts may have an open-circuit voltage ranging from 11.9 V to 12.6 V (typically 2.2 V per cell).
- A measurement of 11.9 V indicates a dead battery.
- A measurement of 12.6 V indicates a battery with a 100% charge. In-between measurements indicate a charge of less than 100%.
- A battery with a slightly higher voltage measurement (3% to 5%) is much better than a battery with a lower measurement. A dc voltage variation below the normal rated voltage indicates a problem.
AC and DC Voltage Measurements
- In some applications, dc voltage measurements may be taken in circuits that include ac voltage.
- To ensure maximum accuracy of a dc voltage measurement, first measure and record the ac voltage. Then measure dc voltage by selecting a dc voltage range (using the RANGE button) that is the same or higher than the ac voltage range.
- Some DMMs can simultaneously measure and display the ac and dc components of a signal. The DMM display can show results three ways (see illustration below):
- The ac portion of the signal appears in the primary display and the dc portion in the smaller secondary display.
- The dc reading can be switched to the primary display while the ac drops to the secondary (on most DMMs).
- The combined AC+DC value – the signal’s equivalent rms signal value.