Ohmmeter Instructions

By J.T. Barett
Ohmmeter

For basic troubleshooting, no electronics technician should be without an ohmmeter. It's usually not a single piece of equipment, but more often part of a combination volt-ohm-milliammeter. You can use it to sort the values of resistors, test for breaks and shorts in cables, and even do basic testing on capacitors, diodes, and transistors. While digital models give clear, quick readings on values, analog needle-style meters are good for approximations.

Setup

Before you take measurements with an ohmmeter, determine if you need to set up the probe wires. Some meters have them permanently connected; with others, you have to plug them in. Choose the correct resistance range. If you're testing a low-resistance component and the meter's in a high-resistance range, the reading will appear to be zero. Testing a high-resistance part in a low range won't give you a useful number. While some digital meters have autoranging that selects the range for you, many have a selector switch you set manually. If you don't know what range to use, start low and turn the setting up until you get a good reading. If you're taking a reading on a circuit, turn its power off first. The ohmmeter has its own voltage source. If the circuit is live, its voltage will conflict with the meter's, giving a false reading or possibly damaging the meter.

Measurements

Components need to be isolated from a circuit to be measured. If you try to read the resistance of a resistor in-circuit, for example, the current from the ohmmeter will pass through other parts connected to it and throw the reading off. If possible, break the circuit by desoldering one leg of the resistor. Desoldering or removing adjacent parts will also work, provided they're connected in series. Measuring loose parts is simple. Clip or touch each probe wire to a lead of the part. Select a resistance range that gives a reasonable reading. Repeat for any parts you need to test. You can use an ohmmeter to measure continuity, to check for breaks or shorts in a circuit. Turn the power off as before, set the meter to its lowest resistance range, and begin measuring between points. Open circuits will make the needle or display go full-scale. Short circuits will read about zero ohms. Capacitors greater than a few microfarads can be checked by first shorting the part, then taking a reading with the ohmmeter. A good part will show a low intial resistance that rises to infinity with time. If the capacitor's blown, it will either read open (infinite resistance) or shorted (zero resistance). Short the part again to discharge it before use. You can also do qualitative checks on diodes and bipolar transistors. Diodes will read low resistance with the red (+) probe on the anode and the black (-) on the anode. You'll see high resistance with the probes reversed. Transistors can be considered two diodes back-to-back; the base-emitter junction being one diode, the base-collector being the other. Note: if a diode reads "good" using this method, it may still be questionable, but if it's blown, it should be obvious.

Tips

If you're having trouble getting good readings from the ohmmeter, check its battery. The meter may also have an internal fuse.

When you're done using the meter, turn it to the "off" setting. Otherwise, the battery will run down. If you go several months between uses, consider removing the battery and taping it to the outside of the meter case. Leaking batteries can damage meters left unused for long periods.

About the Author

Chicago native J.T. Barett has a Bachelor of Science in physics from Northeastern Illinois University and has been writing since 1991. He has contributed to "Foresight Update," a nanotechnology newsletter from the Foresight Institute. He also contributed to the book, "Nanotechnology: Molecular Speculations on Global Abundance."