from the Instrumentation department...
free wheeling diode
Engineering and workplace issues. topic
Posted by kannan.b on 15 June, 2003 - 9:11 pm
what is a free wheeling diode.
In one of the DG set circuit a diode is kept parallel to the contactor. what is the necessity and use of the same. please mail me the reply to

Posted by Thomas Hergenhahn on 17 June, 2003 - 11:31 pm
When switching of an inductive load, the current cannot go to zero in zero time. There is some energy stored in the magnetic field. The coil produces a high voltage large enough to let the current continue to flow over the contact gap, producing a spark. The spark can damage the contact and radiates unwanted radio waves.

The free wheeling diode is connected antiparallel
(anode on minus) to the coil. Hence it doesn't conduct normally. If the coil is switched off, the voltage across the coil reverses to maintain the current's direction. Now the diode carries the current until the energy is consumed by the coil's inner resistance.

This can be used for DC only. For AC use resistor/capacitor combinations or varistors.

Posted by Robert Scott on 18 June, 2003 - 1:00 pm
In an inductive circuit, such as the coil in a contactor, a free-wheeling diode is used to provide a current path for the inductor when it is turned off. Current will try to keep flowing in an inductor, and if no free-wheeling diode is used, then the voltage from the inductor may rise to levels that will damage the switching circuitry. With a free-wheeling diode, the inductor current decays gradually, according to the L/R time constant of the circuit.

Posted by Mickey on 18 June, 2003 - 10:49 pm
Free wheeling diode is used to absorb inductive kick when
coil of contactor is deenergized. It helps to prevent arcing of contacts upstream of coil.


Posted by Miguel on 19 June, 2003 - 12:08 am
The function of the diode is to let the current of an inductive load continue flowing after the contact is opened, allowing the discharge of the coil without making sparks.

Posted by Chris Hart on 19 June, 2003 - 1:27 pm
I don't know what a DG set is but if a diode is in parallel with a contactor I know why. This is a dc circuit and the coil of a contactor is inductive. An inductance does not like change. When you switch an inductance ON the current has to build slowly (millisecond) because a voltage is developed in the coil which opposes the incoming current. The same thing happens in reverse when you switch OFF. When you interrupt the current in an inductor it develops a voltage that tries to keep the current going. If you are using mechanical contacts (switch, relay) to control this current the voltage at switch-off will cause arcing at the contacts, shortening their life. If you are using a transistor to control an inductive current then the transient voltage at switch-off could destroy the transistor. To prevent either event a 'flywheel' diode or 'freewheeling' diode (same thing) is fitted across the load. In normal service this diode is reverse biassed and does nothing, but when the coil is switched off and tries to keep the current going the current flows in a closed loop around the coil/diode circuit. This dissipates the inductive energy harmlessly, preventing a dangerous rise in voltage.

With an ac circuit a flywheel diode cannot be used as the diode would represent a short circuit for half of the waveform. In these cases a 'snubber' is used. The snubber is a capacitor and resistor in series. This may be connected across the load or the switch.

Hope this helps.

Posted by salman on 19 June, 2003 - 2:08 pm

it is used for back emf protection given to other circuit .as contactors coils produced back emf and these diodes gives a discharging path when contactor get off. (back emf is not dangerous to contactor coils)


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Posted by Phil Corso on 20 June, 2003 - 2:02 pm
Search List archive for origin of the term "Free Wheeling"

Phil Corso, PE
Boca Raton, FL
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