Sense automobile high-side current with discrete components

Posted on Feb 5, 2014

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-compensa

Sense automobile high-side current with discrete components
Click here to download the full size of the above Circuit.

tion 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 5V system supply. The voltages noted on the schematic are as built. R3 comprises two surface-mount power resistors, stacked one atop the other. The circuit has a range of 25A in for 5V out. This setup works nicely with an analog input to the microprocessor. 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 25A. If you liked this feature, and would like to see a weekly collection of related features delivered directly to your inbox, sign up for the Design Ideas newsletter here:

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