240v to power 2 12v computer fans what trany I need

Hello dogb8t;

You can't directly use 240V alternating current (AC) to power 12V direct current (DC) PC LED case fans. You need some type of converter or transformer.

That's what a computer's power supply does. It converts AC input to DC output for your computer parts.

What you need is a computer power supply -> probably the one you already have?

Cooler Master R4-L4S-10AB-GP 140mm Blue LED Case Fan - 4 LEDs.
Uses 1.68W of DC power.
Current (Ampere) = 0.14A


Apr 23, 2012
The full story ! I have a TV cabinet with my new amp in an enclosed area which generates a lot off heat the shelf cavity is H130mm / W500mm / D600mm, my amp is H110mm / W480mm / D500mm and made from solid jarrah.
I want to install 2 computer fans with 4 led's 1 to push air in and 1 to suck air in.

I tried to use a trany from a 12v halogen light while it turned on the led's but didn't make the fan rotate.

I would like to know what transformer to use that will operate the 2 computer fans with 8 led's in total I have 2 120mm computer fans with 4 led's on each with no spec's written any were on the fan's.

I have worked as a TA for a Electrician for 9 years doing rewiring house's and making wiring loom's from plan's for shopfitting. This in more Electronics and not my area of experience.

Any help would much appreciated cause were there's a will there's a way, so just call me WILL !!!!!
You should be able to rig any cheap PC power supply as the transformer and someone with your background shouldn't have too much trouble doing it safely.

You'll probably need +5VDC for the LEDs and +12VDC for the fan motor.

Pin # Color Function
1 Yellow +12 V
2 Black Ground
3 Black Ground
4 Red +5 V
OK, happier now.

well you need a 20:1 steo down transformer, if you are talking about 'transformers' as opposed to transformers.

http://www.maplin.co.uk/chassis-mounting-transformers-454504 N64JB would do you, 2 Amps max at 12V and then build a rectifier on some breadboard.

I've just brought a noctua which claims to be 0.6W at 12V = 0.05A and it claims that it really low power, so lets say that normal is 2W, = 0.16A. so for your 4 fans you need about 0.64A total.

what I don't understand is why the halogen transformer couldn't rotate them, you were connecting the right pins weren't you, or are we talking about big molex connectors and not little 3pins? the 3 pins one of them is for a sensing circuit.

I don't see why this wouldn't work http://www.maplin.co.uk/low-cost-6w-multi-voltage-power-supply-226 not sure if it is rated for 24x7. Unless the 'chopper' type systems being used in cheaper lighter power supplies don't allow fan to function, probably the electro magnetics.
Probably, BUT until we understand why your halogen tranny didn't work its difficult to tell. Also note that that is passively cooled itself and is probably simple compared to a PC PSU, i.e. it might have the majority of it working at full power regardles of what you are drawing.

You could try looking for a flex TFX or a SFF power supply.
They are smaller and will definately provide rectified and not chopped DC power


Apr 23, 2012
I've since found out that the halogen tranny didn't work cause of the amp's output was not high enough.

The AC100~240V To DC 12V 6A 72W Power Supply Adapter Transformer work fantastic with 2 CoolerMaster 120mm LONG LIFE Fan Blue LED.

I have installed the 2 fan's into the back of the TV cabinet, I've added some fine stainless steel mesh so no wires get court in the fan's and also a hood over the fan's to get better reflection from the Led's. All wiring joint's are soldered and re-insulated with heat shrink insulation. And every thing is recessed so the TV unit is sits almost flat against the wall.


Electronic 12V halogen ballasts/transformers are often designed as resonant circuits with narrow operating ranges, they will fail to operate properly when under-loaded or when connected to anything other than a purely resistive load. Fans are highly non-linear loads and as such, would have very poor compatibility with such transformers.