This Design Idea presents a low power short circuit finder that capitalizes on the ear’s sensitivity to changes in frequency. The heart of the circuit is a voltage-to-frequency converter (based on Ref. 1, Figure 13), which converts millivolt-level DC voltages to a wide range audio frequency output (Figure 1).
|Figure 1.||Low power short-circuit tracer/finder.|
The excitation signal is created by low power DC-DC regulator U1. This circuit provides a 100 mV output, current limited to 100 mA by the 1 Ω output resistor. This voltage should be low enough that it won’t hurt or bias sensitive circuits.
Four-wire probes with Kelvin connections near the tips are used because of the low voltages. The bias current flows through the probes and the shorted part of the circuit, creating a very low voltage drop between the probe tips.
The Kelvin-connected sense wires apply that low voltage to the voltage-to-frequency converter (see LT AN45 for a description of that circuit). The frequency increases as the circuit resistance increases, and can be tuned to your preference using potentiometer VR1.
The output of the voltage-to-frequency converter is applied to flip-flop U4 to produce nice squarewave output signals to drive audio transducer SP1.
When the probes are removed from the circuit, the output frequency goes up to about 6 kHz. To prevent insanity, the output of the error amplifier, U2, is used to disable the audio.
To use the device, you first null the probes by probing the circuit that’s shorted, and adjusting VR1 to a frequency that you like. Then, moving the probes along the conductor, the frequency will increase as you move away from the short, or decrease as you get closer to it. The pot provides a lot of frequency range so that it can be adjusted as desired. With the probes shorted together, the frequency can be nulled to zero. In that case, the sensitivity is about 100 Hz per milliohm. If the 0 Ω frequency is adjusted to 1 kHz, the sensitivity is about 1 kHz per milliohm.
Operating current from a 9 V battery is less than 10 mA when the audio is disabled (probes open), but with a reasonable audio level, the current is still less than 20 mA.