Electronic Switch

Those fans draw very little power. Check around the console for something 12v, and splice into that line.

You can use a solid state relay.

Can you sketch a circuit of what you're trying to do?

This is basically what I am trying. I would like the 5v from the USB port to power the relay so the 12v power supply is connected to the fans. My only thoughts are that using a relay, it will always be using some power? I also tried with an Op amp, linked straight from a USB port, using a 741 op amp. This still requires the 12v to power it though, and the op amp will only provide 10v to the fans. Am I going along the right tracks?

Cheers.

Here's a smaller, cheaper solid state relay than the one RDGRNR mentioned. A lot cheaper still would be an electromech. relay, e.g. this one. The current drawn from the USB 5V line by either would be pretty negligible.

The advantage of either approach is isolation - virtually nothing the fan supply could do would affect the games console (but you do not need and should not use a common connection between the USB 0V and the fan supply 0V).

A transistor would be even cheaper, and use even less power, but you'd lose the advantage of isolation.

If you use a buck/boost switchmode regulator, like a LM2574, there is a ttl input on/off switch.

There are other Nat Semi simple switcher regulators which may be closer to your needs, use the "webench" to aid selection.

If you like you could add an optocoupler to isolate the USB port signals from the regulator.

You probably forgot to provide a current path for the back EMF from the fans in your transistor circuit. Its a common mistake. You will get much better cooling if you find a way to provide 12V instead of 5V to power these fans. I suspect that it will be more than 5 times better because likely the power provides the magnetic field for both the rotor and the stator fields. If you have to use the USB port to power these fans then a 5V to 12V boost switching supply will be what you need.

Thank You. I am trying to use the 5v rail to "Switch on" the 12v supply to the fans, to aid the cooling.

This relay would work. There are board mount models available. I use these for low batt alarm circuits. Cheap and bulletproof.

Yahlasit, I like this , simple, inexpensive..

Thank you very much Yahlasit. The only modification I have made is to increase R1 to 20k, because on this model the transistor kept blowing. Shall set to work on making it now.

Thanks to everyone else too, lets just hope I don't blow the house

Most motherboards made for gaming have a thermal sensor port that turns a 12 volt connector on/off. Some may be proportional.

dig here. Your motherboard may be already equipped?

http://www.google.ca/#sclient=psy&hl=en&q=motherboard+%2Bthermistor&aq=&aqi=&aql=&oq=&gs_rfai=&pbx=1&psj=1&fp=2c0b2ef780091f9e

Hello

You could use an Optoisolators - Transistor, Photovoltaic Output, that can turn the fans on, and also provide isolation between the 5V and the 12V supplies.
use a resistor to bias the led in the unit, and find one that can safety handle to current and the voltage being used, I would recommend that the handling voltage be at lest twice the supply voltage to handle any induction kick back

Use a relay with a 5vdc coil.

Add a 12 volt power supply for the fans and wire it Normally Open to the relay.

When the relay gets the 5vdc signal from the usb power it will close sending 12vdc to the fan motors.

do not run your 12v fans from the 5vdc usb.

Cheers,

Tom

Regards.

Already good replies there which are quite practical, but may I ask the purpose of controlling fans On/Off or something like speed of fans or or duty cycle of fans through USB port.

Of cource seems that you intend to use USB only for +5V for control, there will be some more power supplies in console.

Regards,

The purpose is to turn the fans on when the console is turned on. The fans would have a supply going to them which is only turned on when the console is on

Check the current rating of the Transistor. I hope the peak current of the transistor selected is low than the required to drive the fan. Use a boost circuit to pickup the voltage & required current.

______________________________________________