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03-26-2015

Sense automobile high-side current with discrete components

Larry Beaty, Beatys Mills

EDN

This Design Idea came about as a result of my not having access to those wonderful new ICs that sense current. I needed a discrete circuit that I could build easily but that would still be as accurate as the new ICs. This circuit seems to do the job.

Q2 is the first current amplifier; it has a gain of 6.2 (Figure 1). Q1 is the temperature-compensation amplifier controlled by IC1B, which keeps the Q1 collector voltage constant no matter what the temperature does to the circuit. The reference voltage for the circuit is the 5 V system supply. The voltages noted on the schematic are as built.

Sense automobile high-side current with discrete components
Figure 1. Q1 and Q2 convert the high-side voltage drop across current-sense resistor R3 into a voltage that is within the common-mode range of IC2.

IC1A differentially amplifies the Q1 and Q2 collector voltages. The op amp has a gain of 4.9.

R3 comprises two surface-mount power resistors, stacked one atop the other. The circuit has a range of 25 A in for 5 V out. This setup works nicely with an analog input to the microprocessor.

The two zener diodes protect the circuit from the automobile electrical system. Such systems have been known to spike to −90 V.

If you want to get critical, match R6 and R7; more critical again, also match R1 and R4. I didn’t do this step, and the mismatch did not seem to affect the operation. All resistors except R3 are 1% 0805 SMT.

Observe sufficient copper weight and width on your PCB traces for maximum current-carrying capacity, and be sure to use Kelvin connections to R3. This circuit ran slightly warm to the touch at 25 A.

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