Simple protection logic is needed

Thread Starter


Joined Oct 1, 2009
Hello everyone, i need simple protection for my active load device. Maybe it can be made with triggers. I draw simple schematics. Normaly input IN1 is high (state 1), output OUT is high. If current through my device will exceed 10A, input IN1 will be set to low. Output then must immediatly go low and stay in this state even if IN1 will be again set to high. Only when input IN2 will be temporary set low manually with pushbutton (RESET) and INPUT1 is high (no overload), output must go to high again. Maybe somebody can help to do protection?



Joined Jun 7, 2009
your logic is a simple flipflop, but you've lost me on the 'triggers'. Overcurrent, once occuring, typical sets a mechanical latch that requires resetting/replacing, ie: fuse. Thermal buildup from high currents may sometimes need a time delay to dissipate heat before restarting. Circuit breakers often have auxiliray contacts which can be used for indication/circuit lockout.


Joined Apr 24, 2011
I've done things like this more times without a fuse then with a fuse. It's an electronic protection that doesn't need a pile of fuses (that you can run out of) to restart after a problem.

Some sort of flip flop is required to "remember" the trip. At power on, it is SET, and the fault conditions (there may be several) use a logic "OR" to combine them into a single RESET signal for IN2.

The Q output of the flip flop is then uses a logic "AND" to combine with IN1 to provide the turn on signal OUT.

You also need a logic OR gate with the power on reset signal and IN2 to accomplish the reset.

Depending on your specific hardware there is probably no real need for 3 discrete gates here (AND, OR, FlipFlop). The F/F is probably easiet to buy as a chip, the AND may have another way to activate the power area, and the power on reset may just be a resistor/capacitor short pulse maker, and you can usually put the switch on that to make the pulse.

I had some fun once doing the spec design on a power switch that needed both instant trip (very large over current) and an I-squared-t (I^2*t) (integral of current over time) to sense smaller yet significant loads that need protection if they continue for some time. I^2*t is essentially an energy term; since if you multiplty it by ohms you get joules so it is already scaled for any specific system.


Joined Sep 20, 2005
Doesn´t a 555 do exactly what you need?
It´s been a long time since I last used it, but i recall it has active-low inputs and a latch.
You would use threshold and trigger as your active-low inputs and either ouput or discharge pins as outptus (output is active high, discharge is active low).

I assume you have the ovecurrent signal sorted and need just the latch logic.