Out of Order: Low Battery in Multimeter = High Voltage Scare

Low Battery in Multimeter = High Voltage Scare

By Joe Birsa  N3TTE

Last year when I was adding a new accessory to my ham radio station at home, I noticed that the power supply I use for accessories was putting out 16 VDC instead of the nominal 12 volts I expected.

So, I turned on my soldering iron and grabbed a 7812 (a much-used linear voltage regulator), a scrap of project board, and a heatsink from my stockpile of parts and made a little voltage regulator.

When I then checked my voltage regulator, its output was also reading 16 volts! Suspecting a malfunctioning meter, I then used a new 9 volt battery to check my multimeter's accuracy. Rather than measuring a little over 9 volts as it should, it read 12 volts instead.

I replaced the 9 volt supply battery in my multimeter and the power supply then read the expected 12 VDC and the battery read 9 VDC. I then measured the battery I took out and it read low, about 7 volts.

Apparently the voltage reference in the multimeter required more than 7 volts in order to function properly. Seven volts wasn't low enough to prevent the meter from working, but it made every voltage reading display as being higher than it really was. A simple "LO BATT" indicator on the display would have save me a lot of trouble.

In retrospect, I should have grabbed my backup multimeter and verified the reading; but it was at night after a long day at work. Hopefully my experience will come to mind if you ever find yourself getting nonsensical readings during a measurement - do not just assume your test equipment is functioning properly.

Posted February 12, 2014