Controlling Fans

[p00t]

i posted about an Altima fan assembly i got at a local junkyard on the 240sx technical forums, link here. I have a 1993 240sx FB, and i will be hooking the fans up via a manual fan control box I will be fabricating.

my question is what is the best way to have a "lower speed" ? I was thinking of just using resistors (with alittle bit of math) which should lower the power going through them. For example: one fan has 42ohms resistance, and the power source is of course 12v, so it uses .28amps of current (12v/42ohms that right?). so all I would have to do is make a resistance inline to the fan so that its current used is less (say maybe .15amps for low speed?)

what are the best kind of resistors to use? I dont know if the local radio shack is going to stock something that can handle that kind of current, what is your suggestion? I am willing to order stuff online.

Im only 18 and i havent even started college yet so my math might be horrably wrong... if you think Im wrong dont hesitate to give me a *possibly big push* in the right direction.

Thanks for your time almighty technical gurus .

[szh]

(I am not an expert on cars, so please take these comments with a grain of salt.) You are probably on the right track, but will need to be careful of a few more practical things.

1. DC resistance of the fan coil may be 42 ohms, but that is also a function of actual fan speed (a physical thing called "back-EMF" has an effect.) So, the on-rush current when starting up may be considerably higher - would be useful to do some tests using a or fast ammeter to measure the actual current during start up. Running current may be lower too.

2. When you power off the fan, you also get a voltage spike in the opposite direction - the fan tries to become a generator for a short while. You have to take this effect into account to prevent damaging circuits.

3. 12V DC inside a car is rarely that exact. You get spikes (from the alternator and battery charging circuits), the voltage can get as high as 13.5 to 14 Volts DC when charging, etc.

4. The power dissipated in an in-line resistor is important, so simply using a regular, for example, 1/2 Watt 42ohm, resistor would be fatal to the resistor. 12V at 0.15A is almost 2 W - so if you use a resistor, make it a higher power one (probably 5W or so).

All in all, your best bet is to simply use a DC motor speed control potentiometer to reduce the voltage applied to the motor. I think Radio Shack has them.

Hope this helps ... until a real expert kicks in with better information!

Z

[p00t]

very informative

how would you propose to fix problem #2? capacitor of some sort? It will be hooked up to the ignition so the circuit will be open once the key is turned to the off position. that should prevent any chance of it generating power, but i wont know for sure until i test it.

[Q45tech]

Why not use the oem method [a oem speed controller in series with motor and controlled with a small varing voltage....Oem uses a relay to apply full voltage for max speed.

[p00t]

hmm interesting, im googlin it up for relays and resisters and stuff. you see im new to this stuff so im learning what they do. relay might be a good method for a full speed function, but i dont think i will be able to use it for smaller "steps" for low and medium speed. that would require parallel relays that switch on at different currents. maybe they make those... i guess ill find out.

[szh]

very informative

how would you propose to fix problem #2? capacitor of some sort?

That is the easiest way. Needs to be large enough (capacity and voltage) to handle decent sized spikes - the typical noisy stuff that exists in car circuits.Quote » It will be hooked up to the ignition so the circuit will be open once the key is turned to the off position. that should prevent any chance of it generating power, but i wont know for sure until i test it. [/quote]It is just a momentary reverse current as the fan spins down very quickly in general, so it is not a major big deal, but you need to allow for it. A recitifier quality (i.e., capable of higher current) zener diode might be needed to prevent that reverse current.

Z

[Q45tech]

The oem Q/J use an inexpensive module [with cooling fins ] that contains a simple high power darlington configured set of power transistors. The set up is much like the fuel pump controller only much simpler and a 0-5volts on one terminal controls the HVAC fan in tiny rpm steps............see diagrams in your service manual.

[szh]

Dennis has it right!

Like I had also mentioned in my original post, using a motor speed control (although I did not know of one for cars) - particularly one designed for a car electrical environment - is the simplest and quickest way to go here. Doing your own will mean you spend a lot of time learning about what it takes to make something work, and while that may be fun, you may not be able to take all the issues into account! So if you are looking for something reliable (assumption is that is is not a learning project for you - the intent is actually to use the fan in your car), just go the simple route.

Z