I have a pc power supply for my reprap (prusa mendel) printer. I have attached a led (bright white) to the 5V rail. But when i attach a 5V fan to the same connection, neither the led or the fan work. The led (without the fan connected) lights up but not very bright. It has been ok until now because i had enough light to see but with the 5v fan it really is a problem.
Is there something i need to adjust (like a dummy load) to the psu?
Haphazard guess, but if the LED is already not that bright, I wonder if the full load is just browning out the circuit. Was thinking that the pins on an Arduino board only had a small output, less than 100mA.
You mean that the supply is not sufficiënt enough? Because than the led’s intensity would change depending on how much the printer uses. That is not the case.
My guess is that the powersupply is not giving it’s full potential. If i attach the fan and the led together and the printer is not running i get nothing… while there should be enough current to fry both.
The led is not connected to the arduino/sanguinololu but connected directly to the powersupply’s 5V rail.
And even though i have a 550W (30A on 12V) psu, the heated bed won’t go over 60 degrees celsius.
Assuming a RAMPS controller, I believe the 5V rails are primarily used for controlling the logic of the board through the Arduino Mega. the Arduino only outputs a few milli-Amps of power to each pin. Should only be enough to blink LEDs or power a small servo.
The printer on the otherhand, as well as the powersupply going into the control board, is normally 12V and on its own power rail. So the 5V logic of the control board tells the motor driver when to turn on and off the 12V power to the motors.
I am only guessing here though. Hopefully someone more knowledgeable will weigh in soon. Have you tried just powering the fan? Are you sure it is a 5V fan and not a 12V fan, which I assume would draw more power from a 5V source?
Nevermind, I was thinking you were connecting the LED and fan to your RAMPS board, not directly to the 5V rail of a PC supply.
What voltage and current do you measure on the 5v rail with and without load? Sounds almost like you switched positive and negative leads?
@Peter_Hertel except for the led, there is no load on the 5V rail. Maybe the led is a to weak load?
I will have to measure that though. I will post it as soon as i did that.
Try adding a 12v 30w spotlight bulb or car bulb to the 5v side (psu not arduino )
Measure voltage on 12v side before and after adding bulb to 5v. My 12v read 10.5v whitout bulb and 11.8 with it.
look like you will have to do some modifications to the PSU. Search Google for “ATX Lab PSU”. There’s some stuff need to be done to work right!
@Markus_Granberg i tried the halogen bulb but it made no difference. Not that i measured the voltage, but it didn’t heat up any faster.
Even though, there is no harm in trying again. 
Simply just add a 25watt 10 ohm power resistor between one red and one black cable, group all the other reds together and heat shrink them.
Heads up the resistor will get very hot, so tie wrap it to the psu fan grill.
