tegwin
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| posted on 22/9/08 at 09:25 PM |
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Relay diode on fan?
Looking at a modernish mondeo fuse box the fan relay has a fairly large diode across it...(I assume to stop electromagnetic feedback into the control
system...
But it got me thinking...
Do people usually fit diodes on their fan relays?....
Im guessing if its being opperated by a non delicate switch then it wont really be needed.... But on a modern car the electromagnetic force might
destroy the ecu?...
What sort of current do a pair of smallish rad fans really need anyway?....the cables in the monedo box look like you could hook them up to a nuclear
powestation!
thoughts?
[Edited on 22/9/08 by tegwin]
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Dusty
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| posted on 22/9/08 at 09:42 PM |
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Might be there to stop the fan motors feeding current back in to the system when you switch off and the fans are still running.
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Macbeast
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| posted on 22/9/08 at 10:32 PM |
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If it's across the relay coil it's to stop back EMF generated by the magnetic field in the relay coil collapsing when it's switched
off. The resulting pulse might well damage control circuitry.
Fans do seem to take a fair current.
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Rosco
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| posted on 23/9/08 at 11:44 AM |
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If it's across the contacts I'd guess it's what's called a fly-wheel diode (named after the application not the type of
diode). The purpose being to permit current in the motor windings to decay after being switched off (hence the analogy with a mechanical fly-wheel)
and prevent a voltage spike which could damage the relay contacts and possibly other circuits.
It's common in industial circuits where where a high-current DC circuit is has on-off control of a high inductance load and it's pretty
much essential to have some form of protective if you're using semiconductor switches since these are much more fragile that a relay.
I wasn't aware of the techniquie being used on car electrics but I guess it makes sense on some circuits.
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02GF74
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| posted on 24/9/08 at 09:51 AM |
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If the diode is fitted across the relay contacts (as opposed to the relay coil) then this is a snubber
diode
It kills the negative emf that results when current to an inductive load is removed i.e. the fan is turned off.
This can be quite substantial so can damage sensitive electrickery.
Likewise the same principle can be applied to the relay coil but that is much lower power so we normally do not worry about the back emf.
There was a case where the spinning fan acted as a generator that continued to feed the ecu that ran the engine that kept the alternator running that
supplied voltage to the ecu that kept the engine running so that the ignition off switch did not work.
I seem to recall a diode in between the supply and fan motor stopped that.
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02GF74
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| posted on 24/9/08 at 09:54 AM |
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quote:
Originally posted by tegwin
What sort of current do a pair of smallish rad fans really need anyway?....the cables in the monedo box look like you could hook them up to a nuclear
powestation!
how small is small? 4 inch? 13 inch? 18 inch? 
typical twin radiator fans will be around 13 inch; I estimate about 10 A each.
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