The 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
email it
to me for consideration. Thanks.
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
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