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. in short: needed : variable 0-12V (for fans) available: fixed 12V (from PSU) variable 5V and 12V from LCD (but low current) thanks Hekke
Well, you don't aactually want anything less than 5V, the fans can stall, but the easiest way is to use a high current npn transistor. This will give you about 5V-11.75V, depending on the transistor. Go to any electronics shop and ask for a high gain, high current npn. (High gain will bring the max voltage closer to 12V) Then set the transistor up as a voltage follower: 12v(from PSU)--->Collector of transistor-->fan +v line connected to emitter of transistor-->Fan ground to PSU ground. Then just connect the lcd output to the base of the transistor. That would be the cheapest and simplest way to do it.
The gain will affect the ratio of base current (drawn from the LCD) and load current (what the fans need) so with 1A max available most any NPN power transistor will do. TIP31 is a borderline 3A rating, but TIP41 is 6A and common.
Could you tell me if a fan with a 3 pin connection, connected straight in to the matrix orbital [i have an mx4...] draws all its power from those 3 pins? would this also be the case if the fan had 4 led's on it? i'm not sure how those are wired up as i don't have any yet, but i thought someone on here might know. thanks!
