RAMBo board 12V PWM


I noticed the fan outputs on my RAMBo board produces a signal where the ground seems to wobble. 5V PWMs looks good. Pictures are for FAN2 (12V) and PWM-EXT-5 (5V). Is this 12V signal suitable for controlling a laser driver ? Am I doing something wrong with my scope ? I did the same thing for 12V and 5V : the probe is connected to signal and GND from the same pinout. For FAN2 there are only 2 pins. For PWM-EXT signal is on pin 5 and GND is on pin 2.


That definitely looks funny. There was a situation I ended up in where my fan was googling between 12 and 5V. But I can’t remember what it was now.

I suspect the floating ground is really the issue and there needs to be some load to get that to show 0V.

Try scoping between the power supply ground and the fan ground.

Don’t forget that the PWM for the fan will be switching the ground on and off with the positive being held constant at 12v. I can’t quite make out from your traces where your 0v is…the display says your min V is -4.4v on the dodgy trace!!! that isn’t right…Do you have a fan actually connected when this trace was captured? I’m thinking some inductive carp on the line?

Thanks guys !

This would make sense, I was not aware of that. Why is it different from PWM extensions ? Do you have a link to learn more about this fact ?

They do that because it is easier to sink more current than source more current through a transistor. It is called “low side drive” and it is very common on higher current DC switching.

Yes…what Jeff said! A MoSFET is usually used when you are wanting to switch higher currents…like a fan :-)…ok a fan is not particularly high current but it could be…and these controller boards do have high current outputs for the heated bed and extruder so it makes sense to the designer to use a MoSFET for the fan too. It is just the way a MoSFET works, have a look at this site for an explanation of high side and low side switches. A PWM signal source could very well be just the output from the uP (for example, there are many IC’s that can generate a variable mark/space output), which is fine for a PWM low current output signal but no good to drive a fan directly.