12 volt automotive fan speed controller

Mark,

How do you want to control the speed of your fan? Is it based off of cabin heat? a user? something else?

hi, sorry , i suppose i should have said, i just need a potentiometer to control the speed , it doesnt need to be controlled by cabin heat etc...., the fan controller in my car isnt working and they havea tendency to fail , so i am trying to construct a bit more robust one . everything is working fine the heat , aircon , its just that the factory controller is broken and the fan always stays on really low,.

thanks, mark k

Sorry, I don't have a schematic for you, just a couple of ideas and pointers. 25 amps is NOT overkill by any means as most heater blowers will pull that much easily. So I would not use your 30 amp darlington and would only feel good about your 50 IF it had a real good heat sink arrangement.

Most auto heater blowers use wind cooled wire wound resistors in series with the blower motor circuit to attain low and medium speeds with high speed being a direct connection to 12+ volts. Some regulatory body specifies that a super low speed is mandated to be on all the time to guard against carbon monoxide poisoning, which is probably the low speed you've noticed.

What typically happens is the connection between the standard copper based slip on connectors corrodes with the ni-chrome wire of the wire wound resistors in the fan speed circuits. Silver soldering the connections maybe an easier solution?

If I were to build a circuit for your application I would use UA78s40 switching IC to drive the darlington that would drive a torroid step down/step up circuit. Switching in different resistors in the voltage feedback circuit would then select fan speeds. And in my old Subaru for example, I also need to hit the fan motor with some 16-17 volts in order to have me a super high speed for those really cold winter days.

CHEAP

The controller design I did for Bosch (Bühlertal, Germany) for inclusion in the Mercedes AC/heat system was a 3-phase electronically commutated motor... custom microchip plus 3 N-channel MOSFET's... continuously variable speed, but controlled acceleration (so that the chief engineer couldn't hear any whine when he stuck his head in the air duct ;-)

...Jim Thompson

to

ency

thing

er

So where is your schematic?

everything

It predates CAD, is pencil on D-sized.

I'll see if I can scan a reasonable copy.

Keep in mind, it's a CHIP... thus HUNDREDS of transistors.

...Jim Thompson