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I am planing to upgrade the radiator fans on my car and I would like to do some modifications to reduce the noise and have them run at lower speed when a curtain temperature is reached and run at full speed only when it reaches the maximum temperature. I have already figured out the relay setup to trigger each circuit but I need a way to reduce the speed. I have already made a circuit with a voltage regulate that was intended for car LED lights but then I have found out that dropping voltage can damage LEDs and I cannot afford damaging them so I replaced it with a PMW circuit. Now I am planing to use that circuit for the fans instead. Here is the schematic for the voltage regulator (12v to 8v)


simulate this circuit – Schematic created using CircuitLab

Can I use this for the fans? I am going to install two fans and each one draws maximum of 10Amps so 20Amps in total. The reason why I have chosen this over a PWM circuit is for the simplicity of this one and the availability of those parts in my aria.

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You are better off getting fans that have a lower noise output, but then again, its a car, so there is going to be plenty of noise anyway. – Passerby Sep 1 '14 at 23:29
up vote 4 down vote accepted

Some cars slow the fans in a variety of ways, but a 7808 regulator is never going to be one of them, they can manage 1 amp flat out and even then they'll get hot. A car fan will draw 20A, but will potentially take a much bigger hit (closer to 40A) at startup. It's not unknown for 15-20A fans to blow 30A fuses.

Anyway, on to what you can practically do to control fan speed:

1: A sodding big resistor. Don't laugh, I've seen it done on Saabs. It's simple and effective, and if you bolt said resistor to a large lump of metal cooled by a large fan (both of which are handily available in an engine bay near your fan) you know that all that wasted heat is getting dissipated. You do need a REALLY powerful* resistor, although I've also seen (from the factory) a bundle of parallel ceramic 5-10w resistors sharing the load.

*= Powerful as in rated for the number of watts (amps * volts dropped) it will be trying to dissipate as therms, preferably +25% as it'll be in a hot engine bay and you want it to live long & be reliable. Headlamp bulbs may also work as a cheap alternative, but obviously more fragile.

2: Series/Parallel fans. I actually run this setup using a 4-relay circuit designed by a friend (who is in the business of high reliability fail-safe design). The basic idea is that, when the temperature is only "quite hot", the fans are switched in series, effectively running at half-speed. Usually this is good enough, but if the temperature rises higher (the fan switch is a 2-stage one) the fans are switched in parallel so both run at full speed.

Downside: 4 relays, 2 fans, and you've got to think about the circuit so the relays can't cause a short-circuit during switching, the right way round they automatically delay each other, the wrong way one switches before the other and shorts +12v to ground. However, I and several others run this circuit in Land Rovers and it works very well.

3: PWM control. Power MOSFETS etc. are cheap these days, a simple 555 timer circuit can provide a PWM pulse train with adjustable duty cycle. Just beware a few things: fans are a massive inductive load, so you get a BIG startup spike, back EMF, interference on your radio, etc. that will kill delicate FETs. The motors in your fans will not enjoy running very slowly, much less starting off from insufficient voltage (a good way to burn out electric motors). It could work though.

4: Replace sodding big resistor from #1 with sodding big transistor / FET bolted to big heatsink. Advantages: Might be cheaper, could be made adjustable.

Have a look on www.LR4x4.com technical archive for dual-speed / twin-fan installs & Intermotor fan switch part numbers.

Edit: I found the circuit diagram. Twin-speed twin-fan circuit

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Thank you so much my friend, your setup is exactly what I am looking for, run at full speed when hot and low speed at lower temp. I did not think of that series/parallel setup, thats a great idea and I really do not care about having 4 relays since it is a restomod project and I even still can hide them. Your great help is really appreciated. – 3madi Sep 2 '14 at 10:35
In that case, may I be so bold as to suggest an upvote and/or accepting it as an answer? – John U Sep 2 '14 at 10:36
Sorry I forgot about it :p. I cannot upvote due to my low rep. – 3madi Sep 2 '14 at 11:10


Well yes... for about a second. But it'll only be able to do that trick once.

The 7808 is a linear voltage regulator. The means it takes a higher voltage and reduces it to a lower voltage by shedding the difference in power directly to heat. To figure out how much heat, you can use the simplistic electric power equation:$$P=VI$$

In this case, V is the voltage difference from your car's 12V system to the 8V you want for the fans. Actually, the alternator in your car will hold the voltage at about 14.5 volts. So really it's $$14.5V - 8V = 6.5V$$

And at 20Amps draw: $$P = V*I = 6.5V * 20A = 130 Watts$$

A typical TO-220 package enter image description herewill raise about 65°C for every Watt of power dissipation (with no heat sink). Most silicon devices can handle up to 150-175°C before they start to melt. 130 Watts times 65°C per Watt raises the temperature over 8000°C. Even if you put a huge heat sink on the TO-220, the best you can get is 5° per Watt. At 130 Watts of dissipation, you're still exceeding 650°C, which will still make it melt very quickly.

To do what you're trying to do probably deserves a question in of itself, so I won't get into details here.

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Thanks for the reply. I will try other methods and see the results. Some OEM fans come with two speeds control module which allow them to run as I am trying to do. – 3madi Sep 2 '14 at 8:45

In simple words, no. With appropriate cooling, 7808 can handle 1A, some versions perhaps 1.5A. 20A is simply too much. And with any linear regulator, you would have to dissipate 20*(12-8)=80W.

In any case, slowing down engine cooling fan is not a good idea. It is designed for specific airflow for a reason. Even on full 12V it is possible to overheat the engine, so at 8V it could very easily result in a disaster.

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Thanks for the reply. Slowing down the fans when not needed is an OEM setup for some cars, cars with single large fan stays off until it reaches a certain temperature and the kick off at full speed or when the AC is on. Some cars have 2 speed fans and thats what I am trying to achieve for my classic car. – 3madi Sep 2 '14 at 8:43

Certainly not. The 7808 is limited to ~1A continuous (and that is usually not achievable because of power dissipation considerations). Also the 1N4001 is only good for about 1A.

At 15A you'd be dissipating 60W total, which is very very high- you'd have to cool the case to roughly liquid nitrogen temperature.

Usual method to slow a car fan is with a big power-wasting wirewound resistor or with PWM.

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In this case, I should forget about this circuit :p. What about using two or three 100W 1 ohm resistors in parallel? Can I achieve the same result? – 3madi Sep 2 '14 at 8:38
You will waste the electrical power (which comes from gasoline) and create heat which has to be gotten rid of, but it should work. It reduces the fan torque so you can't go too slow or the fan may stall from friction. You may have to experiment to get the value right as the fan current will drop with the voltage. PWM does not reduce the torque so much, but it's more complex (especially at 15A). I recently replaced an OEM part in a Nissan A/C blower PWM with this MOSFET: FDH5500_F085. – Spehro Pefhany Sep 2 '14 at 9:47

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