Nuts and Volts - September 2010

■ 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.

Construction

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).

Non-Linear Resistance

Ohm Meter

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.)