They allow a higher current circuit to flow on the relay than a normal toggle switch could allow. Most the fans I have seen will pull 30 amps at startup (this is going by todays cars with fans). When you do flip the switch on, or the thermosistor switches on, it only has to turn on the relay, which only draws about 1/2 an amp or less. A higher current circuit usually runs from either the battery lead, or a ignition feed through the switched relay to your fan. It does the heavy work for the switch. Hope that helps alittle. :Smarty:
Opps, Mike did a better job, so... EDIT: For example, the switch for the heater fan does not need a lot of voltage. Too much and it could overheat and melt, causing problems for your dashboard. Instead, it controls a relay under the hood that can handle the demands of the fan. Completing a circuit (fan switch, for example) merely powers an electromagnet in the relay. This magnet pulls a piece of metal to complete the high voltage side of the circuit - acting as a switch.
So uh Mike, I just bought a Perma-cool thermostatic switch for the ol' 'lectric fan. The kit does not come with a relay. Do you think I can run down to the electronics store and just get any old 30 amp relay and wire it into my system?
Any "fog lamp" type relay will work. Some will have 4 prongs and some 5. You only need the 4 prong....or just dont use the 5th prong. Let me know if you need a pin out of the relay and what to wire where.
Tim, I bought the switch through Jegs.com since they had it in stock. Actually kind of pricey at $50 but is adjustable and includes everything necessary for hook-up. Summit also carries but was back-ordered. Mike, thanks. I'll check around for such a relay - probably can find at the local white-trash auto parts joint.
Electric fan power source So Mike, I'm about to make all this new fan stuff work. But, the directions from Perma-cool want me to connect this fan directly to the battery with no relay and no switch other than the thermostatic control. If I connect directly to the battery then the fan will run with the ignition off (assuming the radiator is hot enough). I don't want this!! My car is very sensitive about how much juice is left in the battery. Next choice (which Perma-cool does not offer in its directions) is to wire the fan to a 12 volt, ignition-switched source on the fuse block. Would this be an acceptable solution? Do I want to run all this current through the fuse block? Or should I run a relay? If I run a relay, I need to figure out how to wire the fan controller to the relay, the relay to the ignition-switched power source, and (I guess) the battery power through the relay to the fan. Can you offer some knowledge or advice?
I've never dealt with a vehicle's electric fan before, but let me take a shot at this. It basically depends on the amperage draw of the fan and what your car's wiring is rated to handle. However, I'd just use a relay so I'd not have to worry about high voltage through small, old wires. Personally, I'd wire the fan side of the relay to the battery. That way, when the relay is activated, you'll get the juice right from the battery. I'd run heavy guage wires and put a fuse in that circuit. Its simple. There should be only two terminals on the high voltage side of the relay, so imagine wiring the fan directly to the battery. Then hook the relay in series with the fan (cut one wire and insert relay). On the low voltage side of the relay I'd wire the thermostatic switch in series with the ignition-switched source. That way it will only activate the relay when the ignition is on AND the thermostatic switch is activated. A fuse probably wouldn't be a bad idea on this side either (had an electrical fire once, so I'm fuse-happy). For this side of the relay connect one lead to the negative battery cable (or ground) and the other to the thermostatic switch. Then wire the other lead of the thermostatic switch to an ignition-switched source of your choice. If the relay uses its mounting as ground, you will have fewer terminals, but wiring will basically be the same. Hope this helps.
Why Relay? The simpliest explaination is this. High current draw devices suffer badly from voltage drop, caused by excessive wire length i.e. 12 ft of wire to and from the cooling fan to the dash, 20ft or so from rear mounted fuel pump to a dash switch.. You can get around this two ways: Increase the wire size dramatically, and use very heavy duty, expensive and hard to find switches,-- and then figure out how to deal with the bulk of the wire.. Or Install a relay, run 1 main power wire to the load side of the relay, and 1 to the pump (in the case of the fuel pump example) And then use the relay to turn it on and off. Now you are working with current flow of less than 1 amp in the switching circuits, and can use much more readily available switch gear, and work with much smaller wiring, going in the car, and to the dash or wherever. Ideally, the relay will be close to the device, as will the power supply, and this will allow you to reduce the wire size. It's really all about wire length, and ease of installation. I would much rather work with 18 ga wire than 10 or 8 ga..ou: JW
As much as I hate to admit it, the son-of-a-<fill in the blank> thermostatic control burned up in a cloud of smoke. Of course, without the controller in place and the relay wired exactly as before with a dash board kill-switch, the fan works fine. After the car was on for about 30 seconds the fan came on. I said "what the hell!" I got out, popped the hood only to see the fan running, then turn off, then come back on. Then smoke rising from the controller, then a pin-whole burst out the side of the controller with a smoke plume/jet, and the fan is still running! Run back, pull the key, get the extinguisher just in case. SHEEEEEET! Guess I gotta call Perma-cool and see if they'll send me a new controller. Yeah Right!!!!!! :af:
Ouch!:eek2: Sounds like it couldn't handle the voltage and fried and fused shut for a permanent "on." You originally said "But, the directions from Perma-cool want me to connect this fan directly to the battery with no relay and no switch other than the thermostatic control." This makes me think the unit was built to take that kind of voltage and maybe you got a bad one. I doubt polarity (which lead of the swtich goes to positive, which to negative) is an issue, but it might be. You may need an inline resistor. For example, if the thermostatic switch requres 4 volts, and your system has 12, that is too much. You can put a resistor in to "soak up" the extra 8 volts. There is a formula to calculate what resistor you need, but I can't find it at the moment. Do you have the specs on the voltage the switch requires? If the directions you originally had did not say you needed a resistor (or low voltage source), then I'd think you just got a bad unit. On the other hand, if it was not designed for that voltage and the directions don't make it extremely clear, then I'd point that out to the company.
Well Truzi, This thing would normally be connected directly to the battery - it has a 10 guage lead with a 30 amp fuse AND a fusible link. A separate lead routes power from the controller to the fan. The fan and the controller both have their own grounds. The controller is not supposed to close any circuit until the temperature probe reads the proper heat. Since it obviously closed the circuit before it ever saw any heat, I'd have to argue it was defective. I'll talk with them on Friday.
