To measure the current "drawn" by a 12V PTC heater element and to be able to switch it on and off from an AVR micro controller, I have come up with the circuit below.
The PTC heater element has about 2 Ohm cold and 8 Ohm at steady state and is powered by a 12V 10A switching power supply.
I tested the circuit and it works okay, I can measure the expected 600mV (cold) and 150mV (steady state) across the 0.1 Ohm 9W shunt resistor.
Now I suppose I should protect the shunt resistor from overcurrent, i.e. in case the PTC heater element has a short circuit. In code I'm switching off the relay when the measured current exceeds 9000mA and there is a 10A fuse in series with the heater and the shunt.
But what if for some reason the power supply "delivers" only, say, 8A? In that case, neither the software protection nor the fuse will kick in, and the shunt will be severely overloaded. Am I right?
If yes, how would I solve this? Would I have to add some thermal protection for the shunt resistor?