how to boost power from lcd

[hekke]

I'm using a Matrix Orbital LCD but it can only provide 1A @12V (5V also available). I started searching for a circuit which could supply the fans, but with the 12V (or 5V) from the LCD as a signal. This way the lcd doesn't have to provide the power for the fans, but the psu has to do it.
I searched for amplifiers, opamps, etc. but haven't found anything useful.
Most circuits i've found use a variable resistor to control the speed of the fan.
I'm thinking of a transistor/mosfet circuit, but i don't know what type nor the other parts i need

in short:
needed :
variable 0-12V @3-4amps (for fans)

available:
fixed 12V (from PSU)
variable 5V and 12V from LCD (but low current)

thanks
Hekke

 

[evandude]

sounds like what you need is a voltage-to-PWM circuit, driving a power transistor to run the fans.

 

[Dr.EM]

I have this breadboarded and find it works great. To drive those larger fans, you will want to replace the output transistor with a power MOSFET (or possibly just add the MOSFET after the transistor, using that as a driver). I managed to drive over half an amp with this and the transistor only warms slightly (it was my power supply that struggled)

**broken link removed**

 

[hekke]

@Dr.EM
isn't the fan speed in that circuit controlled by R1 ?
Thats what i want to avoid, I want the LCD itself (with software) to control the fan speed. So the input shoul be a 0-5V signal.
If i could replace the R1 with some other small control signal it would be great.

p.s. i'm currently using such a circuit to drive the fans.

 

[Dr.EM]

Oops, didn't notice that bit. I expect there is a way to modify it to accept a control voltage signal, mabye something involving an optoisolator, but a different circuit is probably better.