The origin of this design example is that I have not been able to obtain new ICs for detecting current. I need an easy-to-build discrete circuit, but still have the same precision as the new IC. This circuit seems to be able to accomplish this task.
Q2 is the first current amplifier with a gain of 6.2 (see figure). Q1 is a temperature-compensated amplifier controlled by IC1B. IC1B keeps the Q1 collector voltage constant regardless of the temperature condition of the circuit. The reference voltage of the circuit is the system 5V voltage. The voltage used is indicated on the circuit diagram.
IC1A performs differential amplification on the Q1 and Q2 collector voltages. The op amp gain is 4.9. R3 consists of two surface-mounted power resistors stacked one on top of the other. The circuit has a 25A input current range at 5V output. This structure is ideal for analog input to microprocessors.
The two protection Zener diodes isolate the circuit from the automotive circuitry. A spike of up to -90V is known in this system.
If you want to be rigorous, match R6 and R7; be more rigorous, and match R1 and R4. I did not do this step, and the mismatch does not affect the operation of the circuit. All resistors are 1% of the 0805 SMT, except for R3.
Note that your PCB trace copper foil should have sufficient width and thickness to maximize current carrying capacity and ensure Kelvin connections to the R3. This circuit is slightly hot when touched at 25A.
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