Finding the short a short and sure way
Are you stuck on a short-circuit problem? Here’s a short and sure solution. Well, some might disagree about how short in terms of time the method might be; but it would be hard to argue with the sure part. In fact, the time required isn’t so long either — it just takes longer to describe it than to do it.
This method for finding a short was described many years ago by the affable and venerable Bill Detwiler, founder of Helper Instruments. Bill might have described this procedure as one of the possible applications of one of his company’s products — a dc power supply. Although I don’t use the power supply, I certainly have used this idea many times over the years and with great success. The procedure is presented here as a pictorial with text as needed. We are looking for a short circuit between ground and the A+ rail.
Usually, the procedure is implemented by first tacking (soldering) a couple of wire posts to either end of the A+ rail. It is assumed that one lead of the shorted device is connected to the A+ rail between the two posts that are temporarily tacked in place on either end of the A+ rail. (See Photo 1 at the left.) Here, a Radio Shack perforated board is used to demonstrate the technique. The A+ rail is represented by the circuit trace near the top, with a black wire tacked to the left side and a red wire tacked to the right side. The ground rail is represented by the lower trace with the green wire attached to its center.
Between the A+ rail and the ground rail are attached several simulated components. These are just twisted wires with only one of the twisted pair shorted together. (See Photo 2 at the left.) Now, the object is to determine the precise location of the short between the A+ rail and ground. The basic diagram of the setup is shown in Figure 1 on page 24.
A 2.2Ω resistor and a 1.5V “C” or “D” cell are used to produce a current flow. The amount of current should be kept to a level just great enough to produce a usable voltage indication on the digital voltmeter. The digital voltmeter is set to read dc voltage on the 200mV scale. Any printed circuit has a finite amount of resistance, and a current flow through this resistance will produce a voltage drop. This voltage drop will lead directly to the exact point of the short circuit between the A+ rail and ground. The 2.2Ω resistor is used to limit the current flow to a safe level to avoid destroying the foil on the circuit board.
In Figure 1a, the negative terminal of the “C” cell is connected to ground through the resistor. The positive side of the “C” cell is temporarily connected to the A+ rail, and the voltmeter reading is observed. If the positive side of the “C” cell is connected between the point of the short circuit and the positive side of the voltmeter, then the voltmeter reading will be positive, as shown in Figure 1a.
If the positive side of the “C” cell is connected to the A+ rail between the point of the short circuit and the negative side of the voltmeter, then the voltmeter reading will be negative, as shown in Figure 1b. The voltmeter polarity indicates the direction where the short circuit lies. The object is to find the point where the voltage reading is zero. This is shown in Figure 1c, where you will note that there is no current flowing in the A+ rail; thus, no voltage drop is produced. The current flows only through the ground rail and through the shorted capacitor.
The photos, along with Figure 1, explain how the procedure works. The procedure is certainly worth using if you are stuck on a short-circuit problem, and the equipment required is readily available in any radio shop.
Until next time — stay tuned!
Contributing editor Kinley, MRT’s technical consultant and a certified electronics technician, is regional communications manager, South Carolina Forestry Commission, Spartanburg, SC. He is the author of Standard Radio Communications Manual, with Instrumentation and Testing Techniques, which is available for direct purchase. Write to 204 Tanglewylde Drive, Spartanburg, SC 29301. His email address is [email protected].
