freewheel diodes ?

[turbinef14]

would someone please explain the term freewheeling diodes

 

[Skogsgurra]

A Free-wheeling diode is connected parallel to an inductive load (a solenoid, relay coil, valve coil etc). It is only used with DC actuated coils and it is connected in such a way that it does not conduct in normal operation.

It comes into play when the switch (mechanical or transistor) opens. Since current in an inductor cannot be interrupted immediatley, a very high voltage spike would occur and probably arcing across the switch if the diode were not there.

The diode presents a path for the coil current when the switch opens and the coil current decays (slowly) to zero. The use of a Free-wheeling diode can delay the release of the magnetic system from a few milliseconds up to seconds (especially large valves in hydraulics can be a problem). There are ways to decrease the delay. There are also other means than diodes to avoid overvoltage and arcing.

 

[turbinef14]

thanks skogsgurra for your reply and the help you give to this forum

 

[sreid]

I usually think of a freewheeling diode as the diode across a switching power supply output inductor-capacitor filter which provides the current path whhen the PWM transistor is off.

 

[blacksea]

That maybe also diode across power switch (MOSFET, IGBT) for PWM power stage for brush/brushless motors which provide current path upon back EMF when swich is off. The diode in this case must have low recovery time, must be ultra fast one.

 

[TheApprentice]

Another neat analogy....
Imagine a thousand horses running through your farm gate, half way through, you decide to slam the gate shut? the horses will destroy it!!!
Now imagine using those circular doors you go through at the hotel that's divided into 4 pieces, when you close off the entrance, the horse run half a circle and come back out.=)

 

[baltic]

When bi-directional energy flow capability is required from power converter, each transistor switch must have an anti-parallel free-wheeling diode.

 

[frankeng9]

It should also be noted as skogsgurra has alluded to; that a suppression resistor placed in series with the free-wheeling diode will improve the drop-out time of the magnetic circuit.

 

[Senselessticker]

Here is a link to simple buck converter using a freewheeling diode. Its actually a rather cool applet.

http://www.ipes.ethz.ch/ipes/dcdc/e_Buck_3.html

 

[Skogsgurra]

Sorry Senseless, this is really off off topic. But couldn't you go to Pat's Pub and answer the question about handles? I can't even make out if you are a ticker or a sticker.

Nice applet, BTW. (Added to come back on topic again)

 

[turbinef14]

thanks lads for your replies you have restored my faith in human nature