Fan switches on with heated bed (ramps 1.4 setup)

Hi all,

I have a small but annoying problem with my prusa setup, running ramps 1.4. I have a small cooling fan attached to the x carriage, the pwm cable of which I have connected to pin 9. It's working perfectly, and I can control the speed of the fan with g-code commands as expected. However, for some reason whenever the heated bed power switches on, the fan also starts up at low speed (about 10% power or so). It's still completely controllable while this is going on (ie, I can dial it up to say 20% and it will respond). However, It will refuse to switch back down to zero until power is no longer flowing to the heated bed.

Anyone have any ideas on what could be causing this? It's a bit annoying, as quite often I don't want the fan running at all early in prints to prevent warping. I have tried moving the fan to other pins on the arduino, and have tried other fans, but the problem still occurs...

Do you mean that your fan is connected to D9 + and -? Also, where is your heated bed connected?

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-David

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The fan is a 4-connection PWM controllable type. I have connected PWM to pin D9, positive to +12V, and negative to ground. The heated bed is connected to D8 via a MOSFET as per the standard RAMPS 1.4 setup. I'm running the latest version of Marlin.

Edited 1 time(s). Last edit at 01/21/2015 05:23AM by beentheunseen.

Hi,
You should just connect the + and - wires to the D9 pins, and not use the PWM wires. To control the speed of the fan, your ramps just send more or less current to the fan, and do not use the PWM feedback (also a simple two wires works too).

Agreed. The D9 outputs are already PWM controlled. The positive terminal is constant 12 volts and the negative terminal is PWM through the MOSFET.

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-David

Find me online at:
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Check out my FolgerTech Prusa i3 (plexi) at MindRealm.net

Oooh, I had connected the fans PWM directly to arduino pin 9, skipping the mosfet. I will try connecting +/- to the D9 screw terminal on the ramps instead. When you say the D9 outputs are already PWM controlled, does that mean that the MOSFET somehow converts the PWM signal to a continuous voltage with varying current limit?

D9+- are controled by the software, you don't need the PWM cable connected.

Edited 1 time(s). Last edit at 01/21/2015 11:11AM by Zavashier.

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Collective intelligence emerges when a group of people work together effectively. Prusa i3 Folger (A lot of the parts are wrong, boring !)

Hello,
I've had the same problem, and it turns out the mosfets near D8 and D9 we're touching, I've bent them a little so they don't touch and the problem was solved. Hope is helps somebody as well.
Best regards,
ahito