Diode Models problems

[paulmdrdo]

determine the voltage across each diode. assuming practical model.
determine whether each silicon diode is reverse-biased or forward biased.

can you help me with this circuit. I can't solve it because of the extra voltage sources and resistors.

sure I can solve circuit A if there's no extra 8v source attached to it. and also circuit B if there's no extra 10v source and 10k-ohm resistor attached to it.

how do i deal with those extra voltage sources and resistors?

 

[pradhan.rachit]

Just an assumption for (a)

The cathode of the diode is at a higher potential(8V) than the anode (5V minus drop across 10 ohm resistor).
Hence, the diode seems to be reverse biased which will ideally state that no current flows through the circuit.

Since current = 0, there will be no voltage drop across the resistor and the voltage across the diode will be the algebraic difference between the potentials of the 2 voltage sources which will result in +/- 3V.

 

[Audioguru]

You should see the reverse-biased diodes by simply looking at the schematic.

 

[paulmdrdo]

how did you know it is reversed bias?

 

[Audioguru]

how did you know it is reversed bias?

Because the battery voltages and polarities obviously make the diodes reverse-biased.

 

[paulmdrdo]

In the first figure the diode is forward biased with respect to the 5v source and reverse biased with respect to 8v source. how do I determine which source is my reference in deciding whether it's forward or reverse biased? the extra voltage source make me confuse. please bear with me i'm new here.

 

[Audioguru]

In the first figure the diode is forward biased with respect to the 5v source and reverse biased with respect to 8v source. how do I determine which source is my reference in deciding whether it's forward or reverse biased? the extra voltage source make me confuse. please bear with me i'm new here.

It is VERY obvious that the cathode has +8V and the anode has only +5V so the reverse-bias is 8V - 5V= 3V.
Maybe you do not know the polarity of a battery on the schematic? The long bar is positive and the short bar is guess what.

 

[paulmdrdo]

why do you subtract the 8v to 5v? why is the 3v became the reverse-bias voltage? aren't they suppose to add-up because they are both acting on the same diode? what i'm most confuse about is when there's an extra voltage source connected to the diode, can you tell how should I deal with them?

 

[Audioguru]

I explained in post #7 that the cathode is +8V and the anode is only +5V. DUHH! Then the diode has reverse-bias. Why can't you see that? It is obvious without even thinking about it.

The batteries DO NOT ADD because they are not in series.