Radiator Fan Switching

Ian Anderson · 08-14-07, 04:02 AM

This came up on another forum I frequent.

http://www.x-eng.co.uk/X-Therm.asp?MID=38

It is said to be made to switch fans without relays etc. so a lot easier on the wiring.

It sounds like an ideal thing for a 40

Ian

JohnC · 08-14-07, 12:16 PM

Ian, FWIW I wouldn't trust a snap switch for fan duty. I know the advert says it will "drive the most chunky fans available." however fan duty is really tough on switch/relay contacts. Anything that pulls lots of current, and cycles on/off, I'd wire through a quality relay.

I have a 16" SPAL fan in another hotrod, and it pulls 22 amps continuously, but around 60 amps for a half second or so when the motor starts. That 60 amp peak has wiped out a number of components in this circuit, including two 30/40 amp Bosch relays, two 40a automotive fuse holders, several connectors, and other misc. items. I now have this circuit wired with a 40 ampere large bodied glass fuse, #10 AWG (or about 4-5mm2 European size) automotive stereo power cable, and a 75 amp Bosch relay, and it's been running OK for two years.

So for fan circuits, I'd take the continuous current rating of the motor, multiply by 3 and size the relays or switches accordingly.

Big-Foot · 08-14-07, 02:47 PM

Another bad thing about "some" fans is the Back-EMF that they generate when you shut the master switch off to kill the engine - etc...
I've seen numerous nuked components in the Tech barn when the scrutineers have done this..
Diodes and Relays.. The only safe way..

Paul Thompson 'Hooligan' · 08-14-07, 05:03 PM

I hear what you're saying chaps - but with diode protection and careful design/selection of components, there's no reason why a solid state circuit cannot achieve the same result. No moving parts - encapsulated it should prove more reliable long term... IMHO

Sandy · 08-14-07, 09:27 PM

As Paul said a well designed circuit will do this no problem and without the failures of the mechanical contacts. The other thing you can get is soft start and a host of other things for little additional cost. It is hard to beat the cheep relay/switch combo but you can definitely do better now. This discussion sounds familiar...

Sandy

JohnC · 08-15-07, 12:35 AM

Completely agree on the solid state relay, however these things require judicious heat sinking & mounting to optimise their performance don't they. I'd used them years ago with mixed results, as I failed to follow their advise on heat sinking, and smoked one.

I'm now using an SSR in my fuel pump circuit, along with a Kenne Bell BAP and PWM control from my ECU to vary the voltage to my fuel pump from 8.5v at idle to 16.5v for heavy boost. Works a treat and minimises the amount of hot recirculated fuel to the tank. I plan to convert my radiator fan circuit to a strategy like this.

These guys: Solid State Relay (SSR) for Switching of DC currents or loads up to 300 amps IMO, are the last word in "packaged" SSR's. Their relays "start" at 50 amps and go up to 250 amps. Then, if you need to switch more than that, you can parallel them.

Ian Anderson · 08-15-07, 04:52 PM

John

I hear what you say and accept relays etc should prove more reliable but look at the simplicity

It has built in redundancy with 2 switches - one for each fan - if one switch breaks you still have one fan running until you can short it out with a paper clip!

Also at the price if you are that worried a small unit to carry as a spare!

They are used on 4x4 beasts where the cooling is done by fan and not movement air flow so should have been reasonably well specified.

That said I do not have a unit like this on my car and co cannot comment on the reliability.

Ian