12 V Fan Motor, Which Resistor Do I Need?

TenaciousP · 4 Feb 2016

Hi

Newbie here and struggling with the maths so your assistance is greatly appreciated.

I have rigged up a 12v DC 80w (9" car radiator) fan in my garage as an extractor.

I am driving it from a mains LED/Fan transformer outputting 12v DC and rated for 100W.

I want to slow the fan down and experiments with a graphite pencil as a resistor suggest I need about 5 Volts at the motor.

80w/12v is 6.6 Amps. So I guess this is what I am working with.

Keeping it simple, if I want a resistor to give me 6 volts then is it 6v/6.6a = 0.9 ohms or do I need to allow for the fact that the current will decrease at 6v so it it 6v/3.3a = 1.8 ohms?

And what watt rating do I need the resistor to be? Is it 6v x 6.6a or 3.3a (40 or 20 watts)?

Then how do I apply this to 5v output, I need a 7v drop so do i stick 5v or 7v into the formula?

I know there are other methods of achieving what I am after, I have tried a PWM unit rated for 100w but found there was excessive buzzing at the motor. I believe I can cure this with some sort of low pass filter but that gets even more complicated with capacitor and diode ratings to work out and to be honest, it's too advanced for me.

I have tried a 100W variable resistor but its min rating turned out to be about 4 ohms which was too slow.

The other method I thought of was an automotive heater blower resistor (on ebay for about £6) but I'm worried they won't have a high enough watt rating (this info isn't readily available).

I look forward to your advice, I'm sure this will turn out to be much more simple than I am making it.

Cheers

JohnW2 · 4 Feb 2016

TenaciousP said:
I have rigged up a 12v DC 80w (9" car radiator) fan in my garage as an extractor. I am driving it from a mains LED/Fan transformer outputting 12v DC and rated for 100W. I want to slow the fan down and experiments with a graphite pencil as a resistor suggest I need about 5 Volts at the motor.

You'd probably be far better off considering trying some sort of 'dimmer' (but you'd have to experiment, as you already have) - as well as being more efficient, you'd need a large (power rating) resistor.

TenaciousP said:
80w/12v is 6.6 Amps. So I guess this is what I am working with. Keeping it simple, if I want a resistor to give me 6 volts then is it 6v/6.6a = 0.9 ohms or do I need to allow for the fact that the current will decrease at 6v so it it 6v/3.3a = 1.8 ohms?

If you're aiming for 5V, you really need to determine what current it would take at that voltage. You could guess that it was linear, hence 6.6A x 5/12, hence about 2.75A, but that might be way off.

If it were 2.75A, then to drop 7V would require a resistor of 7/2.75 = about 2.5Ω, and it would dissipate about 7 x 2.75 = 17.5 Watts. The nearest preferred value you'd be likely to find would probably be 2.7Ω, and it would have to have a power rating of at least 20W. That might not be all that easy to find.

Kind Regards, John

flameport · 4 Feb 2016

TenaciousP said:
I need about 5 Volts at the motor.

Get a 5v power supply, rather than waste power heating a resistor.

danechip · 5 Feb 2016

I did put 'USB charger', but at that current, you'd be pushed to find anything reliable.

ericmark · 5 Feb 2016

In the car the resistor is built into the ducting so the fan cools the resistor. So using one from a car is not much good. As said the motor current draw may not be linear so calculations may be out anyway. What I have used in the past is a spring. Spring steel has a relatively high resistance so some heat resistant lump to connect it across I used an old fuse holder and you can start with full spring and shorten until speed required is found.

The standard ready built was to use a voltage dropper for an ignition coil the coil was rated at 9 volt and the current was around 5 amp the resistor was shorted out when starting. The carbon buttons were designed either stand alone or bolt onto coil except ford you used special wire in the harness.

The resistor used for the series heaters on a Landrover are likely too low of a resistance but the metal unit would take a lighter spring and be ideal to safely radiate the heat. (Note late Landrovers used parallel engine start heaters so no resistor) The other simple resistor is a bulb. The BA15s bulb holder can take a 6, 15, 21, or 36W bulb old lamp holder in series and swap bulb until you get right speed also acts as indicator that fan is running and will last ages as so little voltage at bulb. Note may need diode across bulb to stop back EMF.

stillp · 5 Feb 2016

Try a car headlamp is series with the fan.
Even simpler, get a mains fan!

ban-all-sheds · 6 Feb 2016

stillp said:
Try a car headlamp is series with the fan.

More waste.

ban-all-sheds · 6 Feb 2016

stillp said:
get a mains fan!

Tenacious - what volumetric rate does that radiator fan manage? Particularly if it's got to push air down ducting, something it was never designed to do? Is it really any use as an extractor for a garage?

Why do you want an extractor in a garage anyway?

bernardgreen · 6 Feb 2016

Depending on what you do in the garage you may need to use an intrinsically safe motor, if you are paint spraying or otherwise producing inflammable fumes then and instrinsically safe motor will have to be used. ( no sparks to ignite the fumes ) A 12 volt DC motor from a car is very unlikely to be instrinsically safe.

ericmark said:
In the car the resistor is built into the ducting so the fan cools the resistor.

And when the fan stalls the resistor get hot enough to ignite the plastic ducting.

ericmark · 6 Feb 2016

bernardgreen said:
Depending on what you do in the garage you may need to use an intrinsically safe motor, if you are paint spraying or otherwise producing inflammable fumes then and instrinsically safe motor will have to be used. ( no sparks to ignite the fumes ) A 12 volt DC motor from a car is very unlikely to be instrinsically safe.

ericmark said:

In the car the resistor is built into the ducting so the fan cools the resistor.

Click to expand...

And when the fan stalls the resistor get hot enough to ignite the plastic ducting.

Yes there has been a resent recall over that, can't remember the make of car off hand? It was the common fault with fan speed control. However I wonder now what fan is being used, I had assumed from the cab heater radiator but it could also be from the main radiator like those made by Kenlow.

As to explosive mixtures the fan is normally there to ensure the ratio required is not arrived at and I know I have wired fan controls into the PLC so should the fan stop the second one is auto started and if that stops the machine which was producing the fumes auto stopped. Using fans to remove dangerous fumes needs a lot more than instrinsically safe motors, you would need a full method statement and risk assessment and likely alarms.

I remember working in a garage and saw all the new health and safety stuff come in. Removing exhaust fumes was a problem as clearly the extractor can only be used on static vehicles, rolling roads and the like mean vehicles move.

It will not be long before we have auto sprinkler systems in our houses and a load more and really it is getting OTT. I would say a large fan was more likely to remove fumes so they could not ignite than a small instrinsically safe fan and would make the air better for the people in the garage, our aim is not to have the fumes rather than to ensure they don't ignite.

theprinceofdarkness · 7 Feb 2016

I wanted to drop the voltage to my string of halogens bulbs, which were rated at 100W total, so I used about 1m of thin soft iron gardening wire. In my case as it was buried under the kitchen units, I sleeved it in silicone heat proof sleeving.
Frank

JohnW2 · 7 Feb 2016

theprinceofdarkness said:
I wanted to drop the voltage to my string of halogens bulbs, which were rated at 100W total, so I used about 1m of thin soft iron gardening wire. In my case as it was buried under the kitchen units, I sleeved it in silicone heat proof sleeving.

Interesting

How much voltage drop did you want/achieve?

Kind Regards, John

TenaciousP · 12 Feb 2016

Thanks for all the responses. I've managed to find a variable resistor with a more suitable range, 0.1 to 10 ohms, rated for 100 watts so will give that a try. Couldn't belive how hard it is to find such a thing, had to get it from the states in the end.

For those who have asked, it's not for paint fumes or anything, I am trying to encourage air flow through the garage as it suffers from damp. Crappy pre fab Hanson garage which has always let rain in around the Base, then gets into the air and makes my tools all go rusty!

With fan running at 12v there is too much air flow to get out of the hole I made resulting in back flow and it is very noisy.

ericmark · 12 Feb 2016

In my day the Lincon welding set was still around and I had a few large pops from that.

ban-all-sheds · 12 Feb 2016

TenaciousP said:
Thanks for all the responses. I've managed to find a variable resistor with a more suitable range, 0.1 to 10 ohms, rated for 100 watts so will give that a try. Couldn't belive how hard it is to find such a thing, had to get it from the states in the end.

Why are you doing all this rather than just installing a normal extractor fan, designed to do the job, not be very noisy, etc?

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12 V Fan Motor, Which Resistor Do I Need?

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