neon said:
nigel you still don't get it if the diode has a low saturation voltage {reverse} to you it will breakdown and go south to mexico for a vacation and the voltage will not be .7v or whatever but a short. god you guys still learning i supose. imagine a 75 volt spike and 20ma figure it out the power and you will get the an answer fast. learn my friend.
I'm afraid it is you that does not understand.
As I said above, the back EMF is limited to about 0.7 Volt.
When the circuit is opened, the inductance wants to keep the current flowing in the same direction, so the diode is FORWARD biassed, not reverse biassed.
I suggest that you study Lenz's Law in your reference books. It states:- Induced EMFs and their resultant currents are in such a direction as to oppose the motion that produced them.
In this case, the "motion" is the decaying magnetic field in the relay's magnetic circuit.
The back EMF is therefore in such a direction as to keep the current flowing in the same direction as it was when the transistor was on. Thus the collector end of the coil becomes positive and so the diode conducts in its forward direction.
If you observed the collector voltage on a storage scope, it would rise from the saturation level to about 12.7 Volt.
You can confirm this by doing it in practice or by using a software simulation package such as Switcher CADIII.
I spent many years of my life designing complex electromechanical systems and interfacing electronics, hence the comments in my previous post.
So please don't make smug comments as they are not called for when you are right and can come back to bite you when you are wrong - none of us is the fountain of all knowledge, we all make mistakes occasionally..