Out of Order: Low Battery in Multimeter = High Voltage Scare
very first submission for RF Cafe's new
Out of Order feature has been received from Joe Birsa. His experience
is one that many of us (including me) has been bitten by at some point
in our electronics and/or electrical pursuits. Unfortunately, this type
of situation occurs so infrequently that by the time it happens again,
we've forgotten about it and are prone to getting bitten once more.
Do you have a good work-related anecdote to share? Please
to me for consideration. Thanks.
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
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