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What is the best technique to find out which diode is conducting and which diode is cut-off in circuits below?

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2 Answers 2

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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.

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The standard method, which doesn't rely on any tricks or circuit-specific simplification, is to make four assumptions:

  1. Both conducting
  2. D1 conducting, D2 not conducting
  3. D1 not conducting, D2 conducting
  4. 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.

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  • \$\begingroup\$ It seems that the diode with the most likely highest forward voltage would be conducting while others would be cut-off. An interesting thing to see is why only one diode is conducting when there is sufficient forward voltage for other diodes to conduct although they don't assuming there are many such diodes with different voltages across their cathode or anode which are in the same range of the voltage across the diode which is conducting. and when a voltage source is capable to switch on many diodes. \$\endgroup\$
    – Amit M
    Commented Oct 3, 2022 at 14:41
  • \$\begingroup\$ With small voltage inputs such as Va = 1mV, Vb = 2mV compared to a 5V Ref , all didoes are conducting. I tried it with 3 diodes and got the same result.When forward voltage bias across a diode is in similar range compared to forward voltage bias across other diodes which is enough to conduct, it would be conducting or else it would be cut-off. \$\endgroup\$
    – Amit M
    Commented Oct 5, 2022 at 10:20

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