# How will the diode act after its junction is penetrated?

Above is a circuit diagram with 2 sources V1 and V2, a switch SW, a diode D1 and a resistor R1. The points X,Y,A,B,C,D,E,F are terminals of the components indicated with red dots.

If the switch is OPEN as in the figure, the diode D1 is reverse biased and no current will flow.

Lets say the switch is closed(SW CLOSED) and opened in a very short amount of time which is enough to penetrate the p-n junction so that V1-V2 = 10V is applied in that duration to the ends of diode and the diode is forward biased for that short time.

And then lets assume suddenly the switch is opened(SW OPEN) again and kept always opened from then on. Here is my question:

Will the p-n junction remain penetrated and will a current flow through the points C,B,A,D,E,F in order? Or will it block the current since it looks like it is conencted as reverse biased?

• If the switch is closed you will instantly blow the diode. The diode will let current flow trying to reduce the 15V to 0.7V (a bit higher). Unless you add a resistance in series with the PSU you will just see smoke. – Requist Oct 26 '14 at 18:39
• thnx for the input i made an edit. how would be ur answer now? – user16307 Oct 26 '14 at 18:43
• Initially, current will flow. After a short time (actually quite long for a 1N4007) known as its reverse recovery time, it will block the current. (Assuming you include some current limiting in series with V1. Otherwise, ... blown diode) – Brian Drummond Oct 26 '14 at 18:55
• i ask this question because it is similar to pwm controlled transistor switching a DC motor like here: i.stack.imgur.com/Mcmfg.png some guys wrote that during the switch is switching from ON to OFF a large inverse voltage will be applied due to inductor so it will bias the diode forward. i observe that back emf of the dc motor doesn't change its polarity but the they say the current will flow through diode in OFF mode. i was wondering if diode is forward biased with great voltage for a very short time will back emf cause a current even though it is still reverse biased. – user16307 Oct 26 '14 at 19:03