Heated bed interferes with fan pwm

Hi All,

I just hooked up a fan to my ramps, which i have done using pwm control. I have hooked up the ground and 12v wires from the fan directly to the printer's power supply, and am then sending the fan's PWM feed directly to pin D9, which is set up in my ramps as the fan output pin. I am skipping the mosfet alltogether as this does not allow PWM control.

At first this set-up seemed to work fine - when I type M106 S50 or similar in pronterface I am able to precisely control the fan speed, which is pretty cool. However, as soon as the heated bed (hooked up to pin D8) switches on the fan seems to be picking up some kind of phantom PWM signal from somewhere, and starts to spin mysteriously at low speed (I would guess about 10%) every time the red heated bed light is on. I am still able to control the speed with M106 commands, though I cant switch it off entirely while the heated bed is on.

Anybody experienced anything like this? Strange that the fan should be coming on at low speed, as if there was some kind of short between the heated bed pin and the fan pin, surely the fan would come on at full power?

I don't understand your description of how you've hooked up the fan, but that's ok.

What I'd like to suggest is that you measure the voltage on your 12V lines both with and without the heated bed and nozzle heater running. I'm guessing that you'll find the voltage drops quite a lot when the bed turns on, due to the high current draw from the bed and the cheap and nasty voltage regulation in ATX power supplies.

If that is the case, you need to look into installing some sort of load resistor on the 5V line on the power supply. I wrote a post about doing this on my blog, after reading up about it on the reprap wiki. The extra voltage also reduces heat-up times for the bed and nozzle.

Thanks for your reply, I have a load power resistor on my 5v line already. Forgive me but I dont quite follow how a voltage drop could cause the fan to switch on?

Sorry, I misread your initial description of the problem - I did say I didn't understand it ...

My only other suggestion would be to try re-routing the fan connections as far from the heated bed as possible, in case its some sort of cross-talk problem.