■ FIGURE 3. Batteries, battery clips,
and resistor needed for the project.
voltage internal battery. In
the case of the meter
assembly I had come up
with, the voltage of all the
batteries combined was
about 27 volts: the three
volts of the battery inside
my ohmmeter, plus the 24
volts from the two added
12 volt batteries.
I really needed to have
one of these testers to carry around with me for this kind
of troubleshooting, but nothing was available that I could
afford. I would have to craft my own. At first, I tried the
digital ohmmeter that I normally used for troubleshooting,
and put batteries in series with its leads. When I tested the
resistance of the same type of trouble I had encountered
— water on the surface of the insulator — the numbers on
the display kept changing and were hard to read. It turned
out that water is transient so the resistance of water keeps
changing slightly up and down. I am a troubleshooter and
I wanted to concentrate on troubleshooting, not spend
time trying to read the meter, so using the digital meter
wasn’t going to work for me.
Next, I tried an old fashioned analog meter that I had
lying around. With the extra batteries, the numbers on the
ohm scale weren’t accurate anymore, but I could deal
with that. At least the needle on the meter didn’t move
back and forth very much, and I could get a somewhat
stable reading. Then, I thought about voltage. In general,
fire alarm systems use 24 volts and I needed to test the
wiring at voltages that were higher than that. The 36 volts
from four nine-volt batteries seemed to be a reasonable
voltage, and the four batteries could be strapped to the
■ FIGURE 2. Inexpensive multimeter.
back of the meter, so that’s what I used.
The one real worry was that the meter movement
would be damaged with excessive current when the leads
of the meter — along with the added batteries — were
shorted together. To prevent this meltdown, a limiting
resistor of a yet unknown value would have to be inserted
in series with the added batteries (see Figures 7 and 8).
Just to make it easier to calibrate the meter, the value of
this resistor would have to be chosen so the meter would
read zero ohms when the leads were shorted together —
like a normal ohmmeter.
Start out with the battery clips. Solder the battery clip
wires red to black so the batteries are in series. Just don’t
cut the leads short. The extra wire will allow for flexing
when the batteries wear out and need to be replaced.
Individually tape or heat shrink the connections.
For neatness and organization, tape all four batteries
together. Connect the clips to the batteries and use tape
to make a pigtail of the loose battery clip leads (see
Figure 7 again).
Wire insulation doesn’t have to be smoking
before it’s considered bad; it can be slowly
breaking down. According to Megger — one of
the many manufacturers of insulation testers —
causes of insulation breakdown can be electrical
stress, mechanical stress, chemical attacks,
thermal stress, and environmental contaminations
(water condensing on the insulated side of a
switch, for instance).
All of these irritants cause voltage-based
non-linear resistance — at least at voltages below
the smoking stage. The low voltages common to
digital ohmmeters show high resistance because
the low voltage isn’t enough to drive the current
through the insulation. The higher voltage in an
insulation tester, on the other hand, is enough to
drive the current through the partially broken-down insulation, and the measured resistance is
lower. (Refer to Figure 1.)
September 2010 43