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Low Amps Feed Off Alternator D+ ?

Dave Taylor

By Dave Taylor
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Dave Taylor Engager

Dave Taylor

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I have a 12 volt 0.2 amp computer fan which I use to pull cool air from an external engine compartment vent and direct it to the alternator. Not a lot, but I think it helps.

 

Until now, I have switched it manually but wonder whether I could feed it from the D+ terminal? I have read that the D+ can be used to trigger relays, and most relays probably have a similar current draw.

 

As I see it, if there are no reasons NOT to do it, then running it off D+ would automate its action - on with the engine, off with the engine - which seems ideal. I would fuse protect it, of course.

 

Any reasons not to do this, or snags to avoid?

 

Thanks in advance.

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Dave Taylor Engager

Dave Taylor

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That all sounds like fun

 

Of course, your 1.2watt motor is a mere weedy waft compared to the fan on the alternator which could swallow up to 1HP, but don't let that stop you playing

 

Richard

 

I agree, the fan on the alternator (2 actually) do(es) much more in terms of moving the air through the alternator. My 2.4 watt 'weedy waft' brings in some cool air via ducting from outside the otherwise quite warm engine compartment, which the alternator fans can then shift around nicely.

 

I am trying to improve what was designed as a poorly-ventilated engine compartment, especially as the alternator is just about as far as it could be from the external air vent!

 

The question I need answering is electrical, relating to the advisability of taking current off D+.

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I have a 12 volt 0.2 amp computer fan which I use to pull cool air from an external engine compartment vent and direct it to the alternator. Not a lot, but I think it helps.

 

Until now, I have switched it manually but wonder whether I could feed it from the D+ terminal? I have read that the D+ can be used to trigger relays, and most relays probably have a similar current draw.

 

As I see it, if there are no reasons NOT to do it, then running it off D+ would automate its action - on with the engine, off with the engine - which seems ideal. I would fuse protect it, of course.

 

Any reasons not to do this, or snags to avoid?

 

Thanks in advance.

 

Would it not be simpler to wire it to the ignition switch? :rolleyes:

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Dave Taylor Engager

Dave Taylor

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Yes D+ will run your little fan no bother but it MAY slightly delay the alternator cutting in.

 

THANK YOU!

 

You actually answered the question, instead of posing several other ones!

 

Thanks to all the others, too. I had already thought of the alternatives you suggested, but specifically wanted an answer to the question I actually asked.

 

Well done, Sir Nibble, and thanks again.

 

Dave

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bizzard Veteran II

bizzard

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Don't be a sucker. Its usually more efficient to suck hot air out than to suck cool air in, though in both cases you really need another engine room vent opposite somewhere. If sucking out hot air put the fan vent higher than the cool air vent one sucking in. :mellow:

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Gibbo Veteran II

Gibbo

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Don't be a sucker. Its usually more efficient to suck hot air out than to suck cool air in,

 

Exactly the wrong way round. Cool air is denser. Sucking cold air moves more molecules than moving hot air.

 

The charge warning light return being to ignition+ and the fan's to negative?

 

They are in series. Honest.

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Tony Brooks Veteran II

Tony BrooksDonate to Canal World

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Exactly the wrong way round. Cool air is denser. Sucking cold air moves more molecules than moving hot air.

 

 

 

They are in series. Honest.

 

 

Yes, but when the alternator's charging they are in effect in parallel in that both units supply are fed from one cable. Its just that the warning lamp "supply" at that time only turns the light off. However a high load on the ignition auxiliary circuit plus a resistive ignition switch contact would tend to put them both truly in parallel.

 

When the alternator is not charging then they are definitely in series.

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Dave Taylor Engager

Dave Taylor

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Wow! I didn't expect my innocent little question to start all this!!

 

One at each end of the ducting would better, perhaps run the supply cable inside ducting for furthest fan.

 

Nice idea, and worth bearing in mind if I need to increase the flow - thanks.

 

Should it still have the equivalent or a charge warning light in parallel then?

 

 

The charge warning light is in series, and yes it should still have it.

 

 

Don't be a sucker. Its usually more efficient to suck hot air out than to suck cool air in, though in both cases you really need another engine room vent opposite somewhere. If sucking out hot air put the fan vent higher than the cool air vent one sucking in. :mellow:

 

 

Instinct tells me that cool air blown directly on to the alt will be more effective than trying to cool the whole engine bay.

 

Thanks, Innisfree. My reasoning exactly! And I'm glad that Gibbo agrees about moving cool air, though the main point is that it is directed straight to the alt.

 

Exactly the wrong way round. Cool air is denser. Sucking cold air moves more molecules than moving hot air.

 

 

They are in series. Honest.

 

 

Yes, but when the alternator's charging they are in effect in parallel in that both units supply are fed from one cable. Its just that the warning lamp "supply" at that time only turns the light off. However a high load on the ignition auxiliary circuit plus a resistive ignition switch contact would tend to put them both truly in parallel.

 

When the alternator is not charging then they are definitely in series.

 

I am no electrical expert, but this is how I understand the proposed setup:- As far as I can see, when I turn on the ignition key, but before I start the engine and the alternator 'fires up', the warning lamp and the fan will definitely be in series, current flowing through one and then to the other, though some of it will also flow through the field coils. They will be in parallel with the fan, and both of them in series with the lamp. As soon as the alternator is producing current, the current to the fan and the field coils (still in parallel) will be fed from D+, and there will be no current flow through the lamp as there will be no potential difference across it. At that point, the series / parallel question (as regards the lamp) seems hardly relevant, as no (or very little?) current is flowing through that part of the circuit, except perhaps in the scenario Tony describes.

 

However, if the alternator were to fail, the situation would revert to the 'ignition-on-but-alt-not-firing' state, and the lamp would light. Hopefully it would still light with my fan in series!!

 

When I initially posed the question, it was not with these sort of questions in mind. I was just trying to make sure that I would not be overloading the D+ supply, designed primarily to supply the field current to the rotor coils. However, having thought this through a little more, I don't think that the function of the warning lamp will be impeded by the presence of the fan, will it?

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Tony Brooks Veteran II

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Wow! I didn't expect my innocent little question to start all this!!

 

 

snip

 

 

When I initially posed the question, it was not with these sort of questions in mind. I was just trying to make sure that I would not be overloading the D+ supply, designed primarily to supply the field current to the rotor coils. However, having thought this through a little more, I don't think that the function of the warning lamp will be impeded by the presence of the fan, will it?

 

 

As far as illuminating then no it will not but as far as exiting the rotor/alternator then the question is not as clear cut because the bulb will only allow a given current to flow at battery volatge so if some of this is diverted to the fan there will be less to make the magnetic field in the alternator so (as I think has already been said) you may have to rev the engine a bit more than you did to get it charging or perhaps fit a higher wattage bulb or a resistor in parallel with the bulb. Just try it and see, any problem that might occour should be easily solved.

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nb Innisfree Veteran II

nb InnisfreeDonate to Canal World

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Ventilation is the important thing and directing ventilation at the back of the alternator is not a bad idea but regardless of little fans the mass of air will be moved by being sucked into the air filter and blown out of the exhaust.

 

Actually this may have the effect of assisting any fan ventilation, but it would really need the ducting outlet to be close to the alternator to minimise any cool air bypassing it.

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Dave Taylor Engager

Dave Taylor

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As far as illuminating then no it will not but as far as exiting the rotor/alternator then the question is not as clear cut because the bulb will only allow a given current to flow at battery volatge so if some of this is diverted to the fan there will be less to make the magnetic field in the alternator so (as I think has already been said) you may have to rev the engine a bit more than you did to get it charging or perhaps fit a higher wattage bulb or a resistor in parallel with the bulb. Just try it and see, any problem that might occour should be easily solved.

 

Thanks, Tony! Have now tried it, no problems at all! Not even the merest suspicion of delay in the alternator 'firing up'. Thanks for the technical backup - my understanding has been growing apace!

 

Actually this may have the effect of assisting any fan ventilation, but it would really need the ducting outlet to be close to the alternator to minimise any cool air bypassing it.

 

Agreed. And it IS!

 

Maybe see how hot the alt gets at max output at lowest revs, one of those cheap £6 IR thermometers off Ebay will do it. Have you fitted a shunt ammeter yet?

 

cheers, Pete.

~smpt~

 

The fan and ducting are already in place, so I shan't bother with a thermometer. As you will have gathered, my main objective is to direct some cooler air to the alternator in an otherwise not very well ventilated engine 'room'. In theory it shouldn't need it, especially now that I am not running the Sterling A to B unit which worked the alternator hard. But it's there now, and cooling the alternator a little may help to prolong its life.

 

I wasn't planning to fit a shunt ammeter, but MIGHT get a clamp one. Not so essential now, as I have looked at the current output graphs for the Iskra alternator in question, and the amps at normal engine revs look OK to me. (I assume you're picking up the thought from my post about high amps or high volts.) Might be worth getting the clamp ammeter to check that Iskra are not telling porkies!

 

Thanks, all, for your help and reassurance. The fan is now connected on D+ and continuing to work, but now automatically when the engine starts. Objective achieved!

 

Dave

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