Summary:
This is a functional overcurrent protection circuit. It measures the voltage drop across the 220 mOhm resistor on the far left supply. That voltage is boosted and compared against a reference voltage in the second opamp (comparator mode) and an SR latch is used to latch the overcurrent result.
If the output of the first opamp is lower than the inverting input (-) voltage the output of the second opamp is low, otherwise the output goes high.
The output of the second opamp is fed to an SR latch (2 NOR gates). When the opamp output and the reset switch are low the output Q is low and the relay is off (i.e. the circuit operates normally).
When the opamp goes high (due to sensing an overcurrent event across the 0.22 ohm resistor) Q latches, the red LED turns on and the circuit deactivates nearly instantly protecting your supply. You can simulate overcurrent by reducing the 8 ohm resistor resistance.
To reset the system properly, flip the switch to high and back to low. This resets the latch (and green LED protected indicator). Note the circuit will only continue operating again when you increase resistance of the primary load (>= 8 ohm).
For advanced users:
if you desire to change the maximum current level this system will pass, you can tune the circuit behavior with the central potentiometer.
Side notes:
The 10k Ohm resistors between each opamp stage are pulldown resistors to keep noise out of the lines in the real life circuit.
The NOR gate was used in bridged configuration (effectively a NOT gate). This was done because I built the SR latch from a quad NOR gate (74HCT02N). Since NOR gates are universal, and I had extras laying around, I could easily create a NOT gate (green LED) to indicate the circuit is in protected mode when the switch was low.
The entire SR latch & NOR gate can be powered in the real life circuit with < 50 mA provided by the 5V zener diode.
Closing:
I assure you this system works in real life with a dead short at 16V on my supply. I hope this saves you as many ICs as it will save me :).
