Electronics Question: Problem controlling Fan Switch with relays...

**Dexter** · 12-26-2006, 02:37 PM

hey all,
i was using my carputer to control the stock fanswitch via 4 relays, but it seems this is causing a TON of current to flow and i've melted the plastic harness over time. Not good. I'm wondering why this is happening and what I could do to fix the problem.

Code:

Fanswitch Harness
X12
G34

X = not used
G = ground
1,2,3,4 = fan speed contacts.

The stock fanswitch connects 1,2,3 or 4 through the resistor pack to Ground. Thats how different speeds are obtained.

Code:

+-------MOTOR--------RESISTOR PACK
                  |4 |3 | 2 | 1
                  |  |  |   |
GND-------SWITCH--|  |  |   |

I replaced the switch with 4 relays, one for each contact on the stock fanswitch harness, and all sharing the same ground.

Is there a better way to do this? I'm thinking the added resistance from the relay is causing power output to go up, but im not really sure. I tried to do some ohms law calculations but I'm not really sure what i figured out, because i havent measured the resistance of the blower motor itself. Plus, its just a hunch that the increased resistance is screwing things up.

Any ideas? Thank you.

**Jahntassa** · 12-26-2006, 08:22 PM

Increased resistance would make the resisters heat up, not the wire. When you replaced the resistor pack with the relays, did you match the resistance values of those resistors? As well as the current requirements? It sounds like something in those relays is causing an open circuit, which is letting too MUCH current flow, thus melting the wires.

**GoHybrid** · 12-26-2006, 08:56 PM

well a fan in a car is a DC brushed motor. The best way to control the speed of a DC motor, particularly one that requires a fair amount of current is to rapidly switch the power, rather than vary the voltage (as with resistors or potentiometers/rheostats)

Here's how I'd get started: I'd google plans for a 555 timer circuit, and when you understand that, research the term "astable multivibrator".

After that, look into simple transistors and MOSFETs. A transistor is what's going to allow your timer IC with it's tiny output signal to control many many times more power to the fan.

When you understand how to build a simple timer circuit with a transistor output, rather than resisting tons of power, you can switch it very rapidly, like 1ms ON and 1ms OFF gives you fan an apparent 50% of your supply voltage which makes it spin slower, but does not suffer any of the ill effects of insufficient voltage which can make the fan burn out. 3ms ON 1ms OFF gives you 75% power.

The advantage of this is not only that you have a more safe and reliable way to regulate motor speed, but you could use your computer to vary the resistors in the timing circuit to a nearly infinite degree.... or you could stick with your resistor pack and use the relays to engage the appropriate one.

**Dexter** · 12-27-2006, 03:59 AM

Thanks for the responses!

Jahntassa: The resistor pack is stock and was not replaced or modified in any way. Per the factory manual, the resistance between the various contacts is from .17 to 2.7 Ohm.

GoHybrid: Good suggestion on the 555. I've thought about using PWM to control the motor, but I wasn't sure if its really that good of an idea to be switching a high current (40A) motor like that. I don't know if it would put extra strain on the car's electrical system or not.

http://www.picotech.com/applications...dex.html#chap7
How do the MOSFET circuits here look?

I am using a Crystalfontz 633 in my setup which has PWM configurable outputs, but they can only go to 100Hz, so I guess I'll need to build a dedicated PWM circuit.

**GoHybrid** · 12-27-2006, 11:23 AM

dexter, next time you use a power drill press the trigger only partway to get the motor to turn over. when you hear that whine, that's a mosfet switching power at probably 16-20Khz. it's really an efficient way to manage power to a motor. some of those tools will take 40A+. Another good example is to check out a golf car. Those motors take about 180A peak and then settle down to about 70A when they get up to speed. So I don't think your car will have any trouble switching high current if it's done properly.