free wheel diodes for coil driving?

[Suresh R]

Hi,
Can any one clear me on the exact use of free wheel diodes in coil driving?
Iam not clear with the direction of reverse current flow. does the diode used to protect the transistor or the supply voltage from getting spoiled?

Please give some light on this.

Suresh.

 

[pinoy]

It protects the transistor. It prevents the inductor kickback voltage. inductors have the property V = L di/dt, it is not possible to turn off the current suddenly, since that would imply an infinite voltage across the inductor's terminals. What happens instead is that the voltage across the inductor suddenly rises and keeps rising until it forces current to flow. When the transistor switch is opened, the inductor "tries" to keep current flowing. The best solution is to put a diode across the inductor. The diode must be able to handle the initial diode current, which equals the steady current that had been flowing through the inductor. When the switch is on, the diode is reversed-biased and during turn-off the diode goes into conduction, putting the transistor switch terminal a diode drop above the positive supply voltage.

 

[A.Anand Srinivasan]

actually when we turn off the power across a inductor a voltage spike results due to the di/dt factor and is called inductive kickback.... the free wheel diode is connected across the inductor such that it is reverse biased during normal operation... when supply is turned off the voltage spikes up in the opposite direction due to the direction of reduction of di/dt and when this happens the diode gets forward biased and prevents the spike by holding the voltage across it constant...

 

[Suresh R]

thank you. the replies were useful.