Digital voltmeters, often known as multimeters, can be used to obtain many useful readings from an electronic circuit. The name "voltmeter" is perhaps misleading, because it is very uncommon for modern meters to measure volts alone. Current, resistance and other readings can be obtained using a multimeter, and the digital readout will automatically adjust to provide the most accurate units.
Connect the black probe to the common terminal, which is usually both colored black to match the probe and labeled "COM," or something similar. Connect the red probe to the terminal labeled for volts, ohms and diodes when wanting to measure one of these particular aspects of a circuit or component. Connect the red probe to the jack labeled for amps when wanting to take a measurement of current. Often, a digital multimeter will have a second jack for amps that will read lower currents rated in milliamps.
Select the proper measuring units using the dial on the face of the multimeter. For volts you will choose either "VAC" or "VDC" or a similarly labeled position. Current will be labeled with an "A" for amps, resistance with an ohm sign, and diode testing with the diode schematic symbol.
Measure AC and DC voltage by touching the probes to any point in a circuit or to a battery's terminals. The red probe will go to the positive connection, and the black probe to the negative connection. The multimeter will auto-adjust to the appropriate units based on the voltage, be it "mV" for millivolts or "V" for volts, and will give you a reading of the voltage from a battery or across a portion of a circuit.
Measure resistance by placing one probe on each side of a component you wish to know the resistance of. Be sure to first discharge all capacitors in the circuit by jumping their leads of the caps with wire to ensure the reading is accurate. The display will show the resistance of the component in ohms, which can be used to calculate voltage drop across a circuit.
Test diodes by touching one probe to each lead of the diode. Diodes are directional components, so the reading may vary if you touch the positive probe to the negative terminal. When connected to measure forward bias, the display will indicate a number if it is working and will read ".0L" if bad. If the diode is connected to measure reverse bias, a good diode will read as ".0L" and a bad diode as ".000."
Test current by first breaking the circuit at the point you wish to measure current flow. Connect the multimeter leads in series with the broken circuit to measure the current flowing in amps. For circuits with expected low currents, using the "mA" jack when connecting the probes will ensure accurate readings.