problem in diode circuits

[bhuvaneshnick]

i dont know how to ensure that two diode are conducting .my professor said to remove the diodes and find the current direction ,if the current direction of branch matches with reference current direction of diode then it is on else it is off.Is that way to ensure the conduction of diode or any other way we have

 

[MikeMl]

In the circuit you posted, the current in the two diodes is the same (except for a slight difference due to non-matching of the diodes).

Diodes always conduct if the voltage between anode and cathode is non-zero.

 

[bhuvaneshnick]

yes it is same

 

[KeepItSimpleStupid]

Another way to check conduction is to measure the voltage ACROSS the diode. Rds(ON) is the major contributer below Vf (or the forward voltage), so the diode as mike says conducts at very low currents (leakage currents).

If the diode has the forward voltage shown in the datasheet it's conducting. Vf is dependent on temperature.

 

[MikeMl]

Actually, my point was that diodes conduct when the anode to cathode voltage is negative, just not as much...

Do you see anyplace on these two plots where D1 or D2 is not conducting?

 

[MrAl]

i dont know how to ensure that two diode are conducting .my professor said to remove the diodes and find the current direction ,if the current direction of branch matches with reference current direction of diode then it is on else it is off.Is that way to ensure the conduction of diode or any other way we have

Hi,

I'd like to see you verify the voltages you have written on the left side of the page you posted.

Are they both 10 volts, or is the top 10 volts and the bottom 16 volts?
If they are both the same and the two diodes are exactly the same then they are both conducting.
If the top is 10v and the bottom 16v then only the bottom diode conducts.

There are several basic models for diodes that are often used in studies in classrooms.
The first is the ideal diode which is looked at as a switch that turns on when the anode voltage gets even a tiny bit higher than the cathode voltage. You could then add a little series resistance to make the model a little more realistic.
The second is the ideal diode with voltage drop. This turns on when the anode to cathode voltage exceeds a certain small but greater than zero voltage, like 0.4v to 0.7v depending on the diode. You can also add a little series resistance to this model.
The third is also called an ideal diode but it actually uses an exponential curve to calculate the diode voltage drop which is a function of the forward current. You can also add series resistance to this one. There is some conduction for any voltage level forward or reverse, but the current in low voltage forward mode is going to be much lower than the normal forward current, and in reverse it will be quite low.
The fourth is the Spice model diode. This takes into consideration a bunch of parameters that characterize a real life diode.

So you can see there is a lot of variation in the diode models used, so you usually have to get this info from your professor or take it from the context of the course work so far that you have done.
Because of your professors remarks however, it sounds like he is using the simplest of all diodes and that's the first one above.

For an alternate method and this is useful for any model, we can draw up an equation for a diode in series with two resistors:
Vs-Vd-i*(R1+R2)=0
where Vs is the source voltage, Vd is the diode assumed voltage, and i is the current.

For two resistors R1 and R2 and the third R3 the bottom resistor in your drawing, we could draw up a set of equations:
Vs1-Vd1-i1*R1-vR3=0
Vs2-Vd2-i2*R2-vR3=0
vR3=(i1+i2)*R3
For simplicity you might specify the voltages of the two diodes Vd1 and Vd2 and then calculate i1, i2, and maybe vR3 knowing all the resistor values and using simultaneous equations. For these static problems it's usually that simple.