wiring twin fans with parallel resistors?

Hi everyone,

I have recently purchased some twin 12" fans for my car and having a little trouble trying to work out how to wire them up.

Basically what I want to achieve is to have the fans permanently running at a low speed whilst the engine is running, then kick up to full power once the car reaches a set temperature.

I have a fan controller so I can output the 12v for high speed at a set temperature so that parts fine. The bit I am struggling with is how to make the fans run at a low speed, and how to switch from the low to high speed.


What I was thinking is i could either connect the fans in series for low speed, and switch to parallel for high speed. Or use a seperate source 12v and 2 parallel resistors to give me the low speed.


The part I am confused about is that if I connect them in series so each fan has 6v for the low setting, when the temp reaches 100oC and the fan controller kicks in. will sending a further 12v to the fans in parallel cause damage? or will i need another relay to switch the series circuit off, when the parallel circuit switches on.


Any help would be greatly appreciated.

Cheers
Matt

Using resistors means you will need a handy fire extinguisher. Those fans are probably drawing in excess of 40 amps each on start-up.PM Oldspark for the answer which will probably be a variable voltage device.

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Don't PM me - I'm not sure....

It seems many use resistors for slow speed - a low temp relay with series resistor for slow, and a high temp relay direct to the fans effectively bypassing or shorting out the resistor for fast speed.

I'd expect 6-7V when in series is too low a voltage, but a test should confirm.

The Wattage of the resistor is VV/R or IIR where V is the voltage drop, I is the current, R is the resistance.
EG - if 40A and a 5V drop is required, the resistor is 5V/40A = 0.125 Ohms (from V=IR) and its Wattage = 40 x 40 x .125 = 200W. (Maybe ten 1 Ohm 20W resistors in parallel? Mind you, 200W is a lot of heat...)


I'd expect a MOSFET PWM circuit is used.   MOSFETs that handle 100A are cheap (a $few) and PWM isn't too hard - either my elsewhere posted 555 PWM circuit (0.1% - 99.9% duty cycle) or others, or a modern vehicle's dashlight dimmer (usually good for ~10A) that drives a MOSFET.

But if going to that trouble, why not a proportional system - eg, off when not needed (normal cruising etc), then some minimum speed when warm enough top be required (maybe 40-60%?), rising to 100% for hot?

And the easiest way - a PIC or PICAXE08M (8 pins). It has an analog to digital converter (ADC) to read the temp sensor, and PWM output(s), and space for ~80 lines of code. The newer superseding 08M2 has more - like space for over 2,000 lines of code; parallel tasking, more pin functions, etc. (They have done the clever thing - the M2's share the same guts so the 8, 18, 26 (or whatever) pin packages have the same program space... (Not that I've used one yet, but with that coding space, I think the time is right....)

I see what you're getting at Peter, Bosch type relays activate on 7.5 volts, probably need a 6 volt coil relay.
Why would you need 2 fans on all the time, especially in Cheshire?
I'm in North West London, more stop starting than anywhere else in the UK and I don't need my fans on, the only time I ever hear them is when I turn on the AC.

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Connect the fans in series.  Does this give you the desired LOW speed?  If so, you can use 2 relays and a diagram here in the relay section.  The diagram is labeled something about running speakers in series and parallel.

Voice Coil one = Fan one.  Voice Coil Two = Fan Two.    Amplifier output one channel = +12 V and Ground.

Voice coil? Is someone thinking of disk drives...?

It was an ANALOGY; have a cup of coffee Mr. I simply said substitute fans motors for speakers.

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.