# Determining the state (on/off) of a diode

#### Circuit111

Joined Jul 24, 2013
1
I found this question online (see attachment) and I'm not sure how the voltage drops across the 'off' diodes, 7V and -3V, were found.

Also, is the current in part (c) 10V/(4k+6k) = 1 mA?

#### studiot

Joined Nov 9, 2007
4,998
OK,

The question states "start by...

D1 is off, D2 is on and the diodes are ideal diodes.

What does that mean about the diodes?

If D1 is off, What is the current through it?
What is the voltage at each end of the 4k resistor?
So what is the voltage at the anode of D1?

If D2 is on what is the voltage across it?
So what is the voltage at the cathode of D2?
So what is the voltage at the cathode of D1?

So what is the voltage drop across the off diode?

phew!!!!!!!!!!!!!!!!!!!!!

Last edited:

#### WBahn

Joined Mar 31, 2012
25,266
I found this question online (see attachment) and I'm not sure how the voltage drops across the 'off' diodes, 7V and -3V, were found.
In general, you make assumptions about the states of the diodes, analyze the circuit under those assumptions, and then verify that the resulting answers are consistent with all of the assumptions made. When you assume a diode is off, then what do you know about the constraints on the current through it and the voltage across it. When a diode is on, what do you know about the constraints on the current through it and the voltage across it?

Also, is the current in part (c) 10V/(4k+6k) = 1 mA?
Yes, and thank you for using units (though you should include the Ω).