flak monkey
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| posted on 18/3/07 at 02:43 PM |
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Radiator supply diode...
Ok, so has anyone found it necessary to fit a diode into their radiator feed wire?
I have a switch with a light on, and if the fan is not on and you are driving along the light stays on, because the fan is spinning obviously. Also if
you forget to turn the fan off before you kill the ignition the engine runs on a bit (not particularly good for any of the ignition system).
So what sort of diode should I fit in the rad fan feed? Dont know anything about speccing electronic components, give me a bundle of wires any day!
Cheers,
David
Sera
http://www.motosera.com
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omega 24 v6
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| posted on 18/3/07 at 02:51 PM |
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Your gonna need a pretty big diode for this job. I'm no electronics expert though.
I'd imagine you've not got this fan wired through a relay at the moment??? It would cure the problem if it was (IMHO)
If it looks wrong it probably is wrong.
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flak monkey
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| posted on 18/3/07 at 02:55 PM |
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Thats what I was thinking. Theres no relay at the moment. Just switched on a 10amp switch. Could add in a relay if its easier than fitting a diode
though. Got to pull my loom apart again to fit my immobiliser anyway.
David
Sera
http://www.motosera.com
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David Jenkins
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| posted on 18/3/07 at 03:06 PM |
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I'm confused...
Radiator fan wiring isn't rocket science. It's a good idea to wire it through a relay, as for any other electric motor of any size.
It's also a good idea to set it up on the non-switched side of the electrics so that it does run on when you switch off the ignition, until the
thermo switch decides that the water is cool enough. This is generally regarded as A Good Thing.
Or have I got hold of the wrong end of the story (again)? 
David
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flak monkey
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| posted on 18/3/07 at 03:09 PM |
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I know its not rocket science... 
The fan is just on a manual switch on the dash, no thermo switch in there at all at the moment. Switched as normal. The wiring in the loom was heavy
enough to do it without a relay, so I didnt bother with one.
I meant that the engine runs on for a second or so when you kill the ignition while the fan is running. Obviously the fan motor takes a while to stop
and so is working as a generator and providing some power to the ignition circuit. By putting a diode in the supply wire you could stop that
happening.
Sounds like I will just fit a relay and be done with it.
David
Sera
http://www.motosera.com
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Macbeast
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| posted on 18/3/07 at 03:43 PM |
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I would think 10A, 125V PIV (peak inverse voltage) would do it. I threw my Maplin catalogue away just the other day but they should have it. There
will be an 0.7 V drop through the diode but that shouldn't worry the fan.
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tegwin
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| posted on 18/3/07 at 03:48 PM |
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Is it not possible to put the diode on the lower current feed to the ingition circuit on the engine and leave the fan circuit un-restricted...
Im just concerned about relying on a diode when there is so much current involved...at the very least its going to get warm...
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Macbeast
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| posted on 18/3/07 at 03:53 PM |
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Putting a diode in the ignition circuit won't work cos the spurious current from the fan will be flowing in the same direction as the ignition
current.
Just looked at Maplin on line. Diode P600D, code UK60Q would be worth a try at 69P each.
[Edited on 18/3/07 by Macbeast]
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ReMan
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| posted on 18/3/07 at 04:40 PM |
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If this is indeed the cause of the run on, then I would have thoght the relay would be a better method than a diode.
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flak monkey
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| posted on 18/3/07 at 04:42 PM |
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quote:
Originally posted by ReMan
If this is indeed the cause of the run on, then I would have thoght the relay would be a better method than a diode.
Well it only does it when the rad fans running, so thats my best guess
Sounds like i might as well just fit a relay. (Another job to add to the growing list!)
David
Sera
http://www.motosera.com
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02GF74
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| posted on 19/3/07 at 11:07 AM |
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quote:
Originally posted by Macbeast
Putting a diode in the ignition circuit won't work cos the spurious current from the fan will be flowing in the same direction as the ignition
current.
Just looked at Maplin on line. Diode P600D, code UK60Q would be worth a try at 69P each.
I would agree with the diagnosis that the motor is acting as a generetor but would not agree with the above ^^^^^.
Once you turn of igntion, the motor is spinning so acts as a generator, the emf (output voltage, red, is shown below).
Since the igntion switch is not connected to anything when off, the ignition module (assuming it is wired like in diagram) will see the emf from the
motor. Current would flow but for the diode which prevents current flow so the igniton module should not work.
Take a lok at P600A UK59P 494 0.69 , 6 A diode.
May be enough for the motor - or you can try putting two or more in parallel. (not sure if diodes like that or will share currently equally?)
Alternativley look at maplins, bridge rectifiers; they do some up to 35 A for under £ 2; you only need to connect one of the diodes.
You will lose a bit of power to the motor due to the voltage drop.
A relay, a bit more wiring, would be another solution.
[Edited on 19/3/07 by 02GF74]
 
Rescued attachment motor diode.JPG
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Macbeast
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| posted on 19/3/07 at 06:50 PM |
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But if you read the previous post, the suggestion was to put the diode in the ignition feed, not the fan feed as you show.
P600D has higher PIV than 600A and this may be important on switching the inductive motor. Diodes can be paralleled but there is no guarantee of equal
current sharing so usiual procedure would be to put in series resistors but then you get more voltage drop.
[Edited on 19/3/07 by Macbeast]
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