What is the best technique to find out which diode is conducting and which diode is cut-off in circuits below?
Look for simplifications. For instance, in your first circuit Vc is -3 volts and, quite simply this can be made to be 0 volts if 3 volts is added to Va and Vb. Then, whichever of Va and Vb is the highest, will be connected to the diode that conducts.
You can do a similar trick with your 2nd circuit and make Vc = 0 volts. Hence Vb will be -1 volt and Va will be -2 volts and, D1 will conduct.
What is the best technique to find out which diode is conducting and which diode is cut-off in the circuit?
I'm not saying my method is the best because "best" is purely subjective and has no "across-the-board" meaning.
The standard method, which doesn't rely on any tricks or circuit-specific simplification, is to make four assumptions:
- Both conducting
- D1 conducting, D2 not conducting
- D1 not conducting, D2 conducting
- Neither conducting
Analyze each of the four cases. If your analysis shows that the voltage is greater than the minimum forward voltage for any diode assumed to be non-conducting or that the forward current is negative for any diode assumed to be conducting, then the assumptions for that case are incorrect. Your assumptions will be valid for one and only one of the four cases.