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12 volt fan, which resistor should I fit ??


trebby91

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Hi, All.

Wonder if somebody could assist me?

I have 2 X 12 volt fans and I need to reduce the speeds down by about half. 1 fan is rated at 1.8 Amps the other is rated at 2 amps.

What size resistors would I need?

Thanks, Brian.

A 3 Ohm resistor would drop about 6 volts to each fan. Depending on your fan this might slow them down more than you want. Each resistor will need to dissipate ~12 W so they will be quite chunky and hard to get. Experiment wih some lamps and find out what voltage the fans need then get the right resistor.

N

 

N

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The advantage of a bulb is that its resistance varies according to its temperature, so you would get better behaviour - ie sure of fan starting, then resistance increasing as the bulb heated up and an element of self regulation.

 

The fans do seem massive at nearly 2A each!

 

If you want to try a resistor, an exact value can't easily be calculated without knowing the characteristics of the fan, but I would start with about 4.7 ohms (4.7 is an E12 series value) but you will need BIG resistors in terms of heat dissapation - assuming the current falls to say 1A you will be dissipating around 5 watts so you will need a wirewound resistor rated well above that - 10W as a minimum and that would get too hot to touch. Maplin do them for 84p.

 

Using series resistors is a pretty nasty way to reduce the power of something, a bulb is better but of course a proper regulation circuit is best.

 

ETA the suggestion of wiring them in series seems a good one!

 

BEngo I think is forgetting that the current will reduce, so 3 ohms will drop a lot less than 6v, unless he means one resistor for both fans - I was thinking of a resistor each.

Edited by nicknorman
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A more efficient method would fitting a pulse width modultation speed control. I bought the one below which works great. Might be a bit of overkill for a couple of fans but there are other versions for less powefull applications.

 

http://www.ebay.co.uk/itm/12V-40V-10A-Pulse-Width-Modulation-PWM-DC-Motor-Speed-Control-Switch-Governor-UK-/121008731849?pt=UK_BOI_Industrial_Automation_Control_ET&hash=item1c2caeb6c9#ht_5517wt_1172

 

Just to add, if the fans are computer jobbies then they sell fan speed modules for computer cases.

Edited by bag 'o' bones
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A more efficient method would fitting a pulse width modultation speed control. I bought the one below which works great. Might be a bit of overkill for a couple of fans but there are other versions for less powefull applications.

 

http://www.ebay.co.uk/itm/12V-40V-10A-Pulse-Width-Modulation-PWM-DC-Motor-Speed-Control-Switch-Governor-UK-/121008731849?pt=UK_BOI_Industrial_Automation_Control_ET&hash=item1c2caeb6c9#ht_5517wt_1172

 

Just to add, if the fans are computer jobbies then they sell fan speed modules for computer cases.

At least that gives you variable speed so if its cold you can run them faster

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10W W/W 1R Maplin Order Code: H1R#

is a 1 ohm 10 watt ceramic body resistor from Maplin for 84pence

 

My first try would be two of these in series which may offer you full, medium and slow speed. Mount them where thay can get HOT without scorching anything maybe in the airflow..

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Hi, All.

Wonder if somebody could assist me?

I have 2 X 12 volt fans and I need to reduce the speeds down by about half. 1 fan is rated at 1.8 Amps the other is rated at 2 amps.

What size resistors would I need?

Thanks, Brian.

We use 12 V fans at 2 amps at work to keep Arc lamps from overheating, the CFM from those fans is huge as is the noise. Most 12 V fans require

milli-amps to operate, a standard 120mm fan at 12V would be .49A. I'd suggest ditching those and using something more sensible with a variable resistor.

 

Ken

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