Tuesday, June 22, 2010

I’ve just bought a new digital multimeter (DMM) ...

Q Charlie Fortner, KF4GJQ, asks, “I’ve just bought a new digital multimeter (DMM), but I notice that it doesn’t measure capacitance or inductance. Is there any way to measure these quantities with a standard DMM?”

A A DMM cannot tell you the inductance of a coil, which is the coil’s most important characteristic. If you attempt to measure the coil’s resistance, you’re likely to discover that it is so low that your DMM will read a dead short (zero ohms or something close to it).

By the same token, a DMM cannot divine capacitance directly. If you attempt to measure the resistance of a nonpolarized capacitor, you may see a very quick “bump” in the meter reading as the capacitor charges up to the voltage available across the meter’s probes. The DMM should very quickly indicate close to infinite resistance for a good nonpolarized capacitor.

In the case of a polarized capacitor such as an electrolytic, you will probably see more pronounced and prolonged charging activity when you first put the probes across the capacitor (evidenced by a low-resistance reading while the cap charges). The resistance will gradually rise to several hundred thousand ohms when fully charged.

The best approach may be to build outboard adapters that will allow your DMM to measure inductance or capacitance. See page 26.22 in either the 1997 or 1998 editions of The ARRL Handbook. If you want a commercial device that’s specifically designed to measure inductance and capacitance, you’ll need to purchase an LCR meter. LCR meters offer a fair degree of accuracy, but good ones will set you back $200 to $300. That makes the home-brew Handbook adapters look pretty attractive!

From QST January 1999