Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

free wheel diodes for coil driving?

Status
Not open for further replies.

Suresh R

Member level 1
Joined
Nov 16, 2006
Messages
34
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,286
Activity points
1,535
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

Member level 1
Joined
May 25, 2007
Messages
40
Helped
2
Reputation
4
Reaction score
0
Trophy points
1,286
Activity points
1,508
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.
 

    Suresh R

    points: 2
    Helpful Answer Positive Rating

A.Anand Srinivasan

Advanced Member level 5
Joined
Oct 15, 2005
Messages
1,804
Helped
257
Reputation
514
Reaction score
39
Trophy points
1,328
Location
India
Activity points
10,680
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

    points: 2
    Helpful Answer Positive Rating

Suresh R

Member level 1
Joined
Nov 16, 2006
Messages
34
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,286
Activity points
1,535
thank you. the replies were useful.
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top