Beginner's Guide to Reading Schematics, Third Edition by Stan Gibilisco

Author:Stan Gibilisco [Gibilisco, Stan & Gibilisco, Stan]
Language: eng
Format: epub
Publisher: McGraw Hill LLC
Published: 2013-12-12T21:00:00+00:00

Go back and look again at the flashlight circuit of Fig. 4-7. Although the schematic diagram does not say so, the two batteries in series should yield a DC potential of 3 V because a typical flashlight cell provides 1.5 V, and DC voltages add up in series connections. Some schematic diagrams provide voltage test points and maximum/minimum readings that you should expect, but this simple example doesn’t.

Suppose that the flashlight has stopped working, and you decide to test the circuit with a volt-ohm-milliammeter (VOM), also called a multimeter, with the help of Fig. 4-7. First of all, you can measure the individual voltages across the cells. With the meter’s positive probe placed at the positive cell terminal and the negative probe at the negative terminal, you should get a reading of 1.5 volts across each cell. If both read zero, then you know that both cells have lost all their electrical charge. If one cell reads normal and the other one reads zero, then in theory you should only have to replace the one that reads zero. (In practice, it’s a good idea to replace entire sets of cells all at once, even if some of them still test okay). If both cells read normal, then you can test the voltage across the bulb. Here, you should expect a reading of 3 V under normal operation with the switch closed. If you do indeed observe 3 V here, then you can diagnose the problem by looking at the schematic. The bulb must have burned out! The schematic shows you that current must go through the light bulb if the bulb can conduct, so it must light up. If voltage is available at the base of the light bulb, then current will flow through the element unless it has opened up. But of course, if the bulb filament has broken apart, no current can flow through the bulb, so it won’t light up. In fact, with a burned-out bulb, no current will flow anywhere at all in the circuit.

On the other hand, let’s say that you get a normal reading at the batteries, but no reading whatsoever at the light bulb. Obviously, a break must exist in the circuit between these two circuit points. Three conductors are involved here: one between the negative terminal of the battery and one side of the bulb, another between the positive battery terminal and the switch, and another between the switch and the other side of the bulb. Obviously, one of the conductors has broken (or a contact has been lost where the conductor attaches to the battery), or maybe the switch is defective. While you keep an eye on the schematic, you can test for a defective switch by placing the negative meter probe on the negative battery terminal and the positive probe on the input to the switch. If you see a normal voltage reading, then the switch must be defective. If you still get no voltage reading, then one of the conductors has come loose or broken.

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