Electric fan wiring

[T5Sweep]

Can I just use one of the high/low feeds from the original fan relay to power my aftermarket fan? The new fan only has a live/earth whereas the original had 2 lives and an earth for high/low speeds.

I was sure whether both terminals need to be in use to make a circuit of some description, I'm not great with electrical stuff!

Ben.

[Laird Scooby]

If you use the low speed feed and and earth for your aftermarket fan then it will run when the original fan would have run at slow speed or above. If you use the high speed feed it will only run when the original was on at high speed.

Considering the high speed was designed for extra cooling when the temperature went up above where the low speed kicks in, the sensible feed to use would be the low speed.

You may also decide to consider fitting an extra fan if the new aftermarket fan doesn't cope - you can then use the high speed feed for the extra fan to give additional cooling, for example on the one day of hot summer we get each year!

[T5Sweep]

Originally Posted by Laird Scooby

If you use the low speed feed and and earth for your aftermarket fan then it will run when the original fan would have run at slow speed or above. If you use the high speed feed it will only run when the original was on at high speed.

Considering the high speed was designed for extra cooling when the temperature went up above where the low speed kicks in, the sensible feed to use would be the low speed.

You may also decide to consider fitting an extra fan if the new aftermarket fan doesn't cope - you can then use the high speed feed for the extra fan to give additional cooling, for example on the one day of hot summer we get each year!

Ideal! I was going to just wire a switch to the dash but this would be a lot easier. Thanks!

[aardvarkash10]

what is the current draw of your aftermarket fan?

[aardvarkash10]

I have a simpler solution than Dave's excellent suggestion that uses your one fan. The shockingly poorly drawn diagram is attached.

Use two BEEFY diodes in line on the LOW speed circuit. This drops the voltage by 1.2V, which drops the effective energy by around 30%.

A 30A or better bridge rectifier diode pack will do it just fine - I've linked to one here. They cost next to nothing, but should be bolted to a solid bit of steel for heat sinking.

To connect the rectifier, take the LOW SPEED feed from the relay and connect it to the terminal marked -ve on the rectifier. The +ve terminal is connected to the positive on the motor. Don't connect anything to the rectifier terminals marked ~

The HIGH SPEED feed from the relay just goes straight to the motor.

Let me know if you get stuck - Dave can probably help out cos he will see and understand whats going on as well.

Cheers

[Laird Scooby]

Nice idea Ash and i like the use of the bridge rectifier as a neat package for it.

Did you notice this bit in the product spec though?

Product overview

35A rating
Maximum load of 280 volts RMS
Max VF per diode of 1.2V@12.5A
B5 case style
RoHS compliant

A bridge rectifier with a maximum voltage drop of 1.3V per diode (model dependent). It is RoHS compliant

That's going to give a forward voltage drop of 2.6V at the most so there will be 12.4V (assuming full alternator output of 14V on a warm regulator) for the fan. Without knowing the FLC of the fan it's going to be difficult to design around but it will certainly give two speed operation and reduce the strain on the fan motor when running on slow speed.

Just for future reference, the "0.6V" drop across a diode is generally typical of small signal diodes, get into high current rectifier diodes and that voltage goes up with current so a 1.2-1.3V drop is pretty average for a high current rectifier diode.

That said, in practice because it's a bridge rectifier and the current is split across two "legs" of the bridge, it's entirely possible the voltage drop will be a bit lower but not much.

Assuming the fan draws 24A at 12V, it's resistance is going to be 0.5 ohms (it's DC so we can get away without power factors/reluctance etc), at 12V the power will be 288W, at 14V = 366W and at 12.4V (after the bridge rectifier) 307.5W so the effective input power is going to be about 84% of full speed power.

Taking all that into account, your idea to create a slower running fan on the low speed setting will actually work better than you first thought!

[T5Sweep]

Originally Posted by aardvarkash10

what is the current draw of your aftermarket fan?

I believe the maximum is 11 amps.

[Laird Scooby]

Originally Posted by T5Sweep

I believe the maximum is 11 amps.

In that case Ash's idea will work very well! You replied while i was typing my "assumed" current but at 12V (nominal battery voltage) it will be 132W, at 14V = 154W and at 12.4V (after using the rectifier to drop the voltage a little) = 136W so will drop the inputpower to about 88% of maximum.

Gives your fan an easier life on the low speed setting and ramps it up to maximum when it gets warmer.

For the sake of a quid from Maplin it has to be worth a try!