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Printhead fan stopped working (again!)

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Hi everyone,

as the title says, the printhead fan suddenly stopped working.

I tested the fan with a second power source and is ok, as well as the blue/violet cable that connects it to the board.

I measured 0 volts at the board pins (where the blue/violet cable is plugged), even after turning the fan on with the ulticontroller or sending "M106 S255" commands. This problem persists after updating the firmware to the one released with Cura 13.06.4.

I feared I pressed the lid too tight and broke the BD679 transistor. So I had a guy check it and he said it was "open" and changed it for the most similar he could find, a BD683 which has a CE voltaje of 120V instead of 80V. But the board pins still have no voltage, and he charged me for that, of course! So I have a few questions:

  • [*]Is it really ok to use that other transistor?

[*]How can I check that the original transistor is broken using only a digital multimeter?

[*]Which pin is the collector and which is the emitter? (I guess the one in the middle is the base)

I measured 19V between the external pins in the transistor, when the fan was turned on and also when it was turned off using M106 command. And around 0.2V between the external pins and the one in the middle. But I don't know what to measure and where, as I have no knowledge in electronics (only what I could understad from the Wikpedia). I don't even know how is that transistor supposed to work (is it used to reduce the voltage or as a switch?). The problem could even be in the Arduino. So:

  • [*]What should change when the fan is turned on?

[*]Or more generally, how can I better diagnose what's wrong?

Thank you in advance.

PD: I had problems with the fan not working when I first got the printer, but it somehow started working, until now.


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I'm sure it's fine.

Do you have access to the schematic? There's one here:


and you need to download "eagle" to view it. Anyway I beleive this is the relevant area.

Um Fan


Q4 is the part that you replaced, right?


If no fan is connected, PWM2 should be at VCC2 whatever that is (19V? 12V?).


The arduino puts out a digital signal which should be around 0V or 5V leading into R20. The other side of R20 goes into the base of the transistor. Current should only flow downwards. When the arduino is "on" aka in a "high" state aka at 5V, the highest the base can get to is typically .7V for a NPN resistor but this is probably a darlington and might not be silicon so I'm not surprised if it only gets up to .2V but that's probably voltage when in the "on" state whether the fan is connected or not.


The transistor acts like a switch that switches from the collector (top connection) to the emitter (bottom connection) and not much current goes in or out the base. The base is the control of the switch. The arduino doesn't have the power to switch the fan on and off directly so it uses this transistor. If you are familiar with relays it is the same concept.


transistor off


With a fan connected, and if the transistor is off you expect 19V at PWM1 and PWM2, very close to 0V (less than 0.1V) at the base and 0V at ground.


With no fan and transistor off you expect PWM2 at 19V, and the 3 transistor pins close to 0V. PWM1 might be floating and could be anywhere.


transitor on


With transistor on you expect .2 to .7V at the base of the transistor, and a pretty low value at the collector - maybe 1V or less. 0V at the emitter (it's connected to ground so it will always be 0V by definition).


With transistor on and no fan you expect base probably around .7V (somewhere .2V to .8V I suppose) and the other 2 pins around 0V. Actually the collector might be floating around randomly.

In PWM mode the transistor switches between these 2 states many times per second. There is no flyback diode so this is kind of a bad design as you can get quite a bit of voltage and power from the fan each time the transistor is switched off. Someone should add a diode from pwm1 to pwm2 to let the current "fly back" up into pwm when q4 is switched off suddenly as fans are inductive. The diode needs to be able to handle 19V reverse voltage without conducting but otherwise can be very wimpy.

PWM mode is probably a bit of a strain on this part. I can see why it fails a lot. In 0% and 100% modes it should be fine. It's a shame because a 1 cent diode could fix this. Maybe there is already a diode built into the fan. But there probably isn't because then if you hooked up the fan backwards it would short out and destroy that diode in a second instead of simply running backwards like others have reported.


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That was a fast response. Thank you very much Gr5!

My board is version 1.5.7, but comparing both schematics there doesn't seem to be any differences in that area.

So the new transistor is ok.

And yes, I had Q4 replaced.

As soon as the current print job is finished I will start testing again, and report back.


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These are my results:

(the usb cable is alway disconnected. Only the power cable is plugged in)


  • [*]With the fan connected but turned off, I measured 19V at the power supply plug, PWM1 and PWM2, then 0V at ground (bottom in the picture) and unexpectedly -1.8V at the base.

[*]With the fan turned on (at 255) using the ulticontroller, there are no voltage changes.

[*]Disconnecting the fan lowers PWM1 to +2V, but the base is still at -1.8V.

[*]With the board power switch turned off (the rocker switch), I measured 0.03V at PWM2 (=VCC2), 0V at PWM1 and ground, but still -1.4V at the base. Unplugging the power cable increases the base to -0.05V, but is restored to -1.4V when the cable is plugged in again (and the switch is still off).


Then I measured the pins at the top of the picture (7, 6, 5...) with the rocker switch turned on. They all were at 0V, except for 6 which was at 5V.

Any clues to what is wrong? Is it the Arduino?

Thank you in advance


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Well there seems to be something wrong with your meter. I don't think there should be any voltages less than gnd. Measure the voltage at the gnd pin of the darlington. Maybe you should be measuring the base to emitter voltages (lower 2 pins) with the fan on and off. Does the fan work now? I'm a little confused.

I would expect that pin 6 coming from the arduino to be at 5V when the fan is supposed to be on and 0V when the fan is supposed to be off. Assuming 100% and 0%. Using pwm (e.g. 50%) will confuse things and make it hard to use the meter accurately.


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Problem solved!!

It was a factory problem. The ressistor R20 had a bad solder point. I would post the picture, but I don't know how to attach a picture.

Thank you Gr5! Thanks to you I could rule out the transistor and then reach R20.

And BTW, pin 8 in the schematic is actually pin 7 in the Arduino and the UM board. I guess it's a typo in the 1.5.7 schematic.

Edit; Here it is!



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You can plug the Ultimaker to your computer using the USB cable, run Cura, press the print button and in the print window, on the rightmost tab you get a console. Type any gcode on the lower box and press enter to send the instruction to the printer.

Use "M106 S255" to turn the fan completely on, and "M106 S0" to turn it completely off. If you are using a tester, you should either turn it completely on or off, but nothing in between, as it will mess up the readings.

For other codes, look at http://reprap.org/wiki/G-code


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