Same here, back fan always on...
Sebastian
The fan on the back of the print head is supposed to be on all the time. The two side fans should be on as determined by the gcode/slicer - usually they come on full after the first few layers, and stay on to the end of the print.
You may have had a loose wire in your original printer, or perhaps the rear fan was accidentally wired into the side-fans circuit.
Ok thanks for the reply’s, I think that the fan on my first U2 was indeed not working properly than.
It should be always on. If it was not, then that was most likely what caused your problems on the first machine.
Some information for everyone:
We're working on an update that can toggle the fan on/off from the firmware. But this will only be in a new revision of the electronics board. So if at some point in the future you'll notice people that say that their fan is not always on but only during printing, then that's normal then. But it will most likely take a few months before this is actually implemented. New board revision still needs to go trough testing and we still have stock of the older revision.
Thanks very very much for this info Daid. Not knowing this could have made me give bad advice on some future post.
- 2 weeks later...
I've just noticed that my rear fan is not on at all.
Should there be something I should do about this?
I am sure it used to be on, but not always from the start, I thought it was like the side fans and came on when it 'felt like it'!
No, the fan should be on all the time. Pull back the mesh above the print head, and find the connectors. There will be three red/black pairs. Two slightly shorter ones connect to connectors with green wires, and go to the side fans. The third red/black pair is slightly longer and goes to the rear fan. Check that connector to see if the wires are loose, or the connector is unfastened.
If it's ok there, you'll need to look under the printer at the electronics board, although that gets a bit more complicated as there are two places the fan can plug in. And they're both either hard to get to, or hard to identify. But usually the problems are down to the connectors above the head.
I've checked the connections and they all seem secure. So I guess the next stage is having a look at the electronics board , not sure about this bit, but do I remove the large rectangular box under the printer to find it?
Unknown waters here for me!
Yes, just remove the two screws and nuts that hold the larger cover on. And then locate the fan wire. It can go to one of two places - either a socket in the middle of the board, on the underside (impossible to get to without taking the board off too), or a couple of pins on the back of the short side of the board (tricky to get to, but not impossible).
On further investigation; I tried a long print again and observed the fans. The left side fan is on all the time, as soon as I turn on the printer. The back (central) fan and the right side fan both came on at the same times as soon as the print started the second layer. So now all 3 are going fast. The print is going ok except that the first layer was not binding to the build plate as well as usual, but it may be because of needing more glue on it, or possibly connected to the wrong fans being on?
Any observations or advice on this please?
Thanks
That means the fans aren't connected correctly. The back fan should always be on whenever the printer is on. Unfortunately this isn't the first time this has happened. I haven't personally had to mess with those fans so I can't give you specific colours and such but it should be fairly obvious which wire goes where once you move the wire loom out of the way.
Two of the fan connectors are joined by a short green wire up at the print head. The two side fans go to those two connectors. (currently, the right side and back fans are connected there, instead).
The other connector that is currently connected to a red/black fan wire is for the back fan (but is currently connected to the left side fan).
If you look carefully you may see that of the three red/black pairs, two are the same shade of red, and one is different. The two that are the same are most likely the side fans. The other one is the back fan.
The back fan cable I believe is also slightly longer. When I got my brand new UM2 I had this exact same issue. I just unplugged the two connectors and swapped them and got lucky on the first try.
Can someone explain the purpose of the back fan on the printhead.
By the looks of it keeps the main printhead frame on the right (low) temperature ?
Why is your font small?
The back fan keeps the upper portion of the head at a cooler temperature. There are parts there that can't withstand temps of 260C. But the more important reason is if PLA gets over 50 or 60C it gets soft and can expand under the very high pressures (around 3-5kg) and will fill the area and cause a clog or jam.
Every time you retract, you move heat upwards a little bit - each time you retract filament it moves the previously moved heat a little higher. Plus hot air rises. Plus everything conducts heat somewhat. So you have to keep the upper part of the head below 50C.
Not as important for ABS which has a glass temp of around 100C.
Why is your font small?
The back fan keeps the upper portion of the head at a cooler temperature. There are parts there that can't withstand temps of 260C. But the more important reason is if PLA gets over 50 or 60C it gets soft and can expand under the very high pressures (around 3-5kg) and will fill the area and cause a clog or jam.
Every time you retract, you move heat upwards a little bit - each time you retract filament it moves the previously moved heat a little higher. Plus hot air rises. Plus everything conducts heat somewhat. So you have to keep the upper part of the head below 50C.
Not as important for ABS which has a glass temp of around 100C.
Thanks for your quick reply!
I ask this because I'm considering making a independent temp sensor driven back fan.
The idea is to add a NTC thermistor (temp sensor) to the printhead frame that will adjust the fan speed according to the temperature and also turns the fan off if the temperature is low, like when there is no print job running.
Can you tell me if the 3mm (M3) screw hole between the print nozzles wil be used in the future (for example dual extrusion kit) because that would be a perfect spot for a M3 screw-type NTC thermistor?
Thanks!
I really doubt that hole will be used for dual extrusion but I don't know. Maybe someone else who knows can answer but I think you should definitely go for it.
Why would you want to go to the trouble of doing this? Daid already mentioned that UM is likely to change the fan to be controlled at some point in the future... but what's the advantage to not just leaving the fan on full time? Especially as a non-standard modifcation?
Why would you want to go to the trouble of doing this? Daid already mentioned that UM is likely to change the fan to be controlled at some point in the future... but what's the advantage to not just leaving the fan on full time? Especially as a non-standard modifcation?
Yes but as Daid mentioned you need to upgrade your electronics board for the newer revised version, and I guess that electronics board will not be cheap.
Also I want to do this upgrade because my fan is making a annoying high pitched sound (I'm in the same room as my printer). During printing this is not so much a problem but when a printed object is cooling down it is some what annoying. I can shut off the printer but most of the time I want to run another print job and I'n not a big van of setting devices on, of, on, of all the time. The fan probably needs some lubrication or I can simply replace it, but I also want to experiment with some Arduino based microcontroller boards so I find this a nice project for that.
The idea is to program a Trinket microcontroller that reads the temp from a NTC thermistor and translates this into an appropriate fan speed or even completely shuts of the fan (when your not printing). And this will be much cheaper and more fun than replacing the main electronics board 
I really doubt that hole will be used for dual extrusion but I don't know. Maybe someone else who knows can answer but I think you should definitely go for it.
It's a leftover from development. It has no use right now. (We tried to see if we could fit an over-temperature protection sensor on there, but experiments showed that it triggered too late)
Why would you want to go to the trouble of doing this? Daid already mentioned that UM is likely to change the fan to be controlled at some point in the future... but what's the advantage to not just leaving the fan on full time? Especially as a non-standard modifcation?
The fan could wear out, it also makes noise.
The future modification will only be in new printers. (Current fan is wired straight to the 5V, no way to control it) But the update is a bit delayed due to some problems with a tiny layout change causing changes on the motor current.
Funny how small things can have so much effect.
The fan control is already in the firmware now:
https://github.com/Ultimaker/Ultimaker2Marlin/commit/d85409f22204c606a331f49a83d31745b3d961ba
The future modification will only be in new printers. (Current fan is wired straight to the 5V, no way to control it) But the update is a bit delayed due to some problems with a tiny layout change causing changes on the motor current.
Funny how small things can have so much effect.
The fan control is already in the firmware now:
https://github.com/Ultimaker/Ultimaker2Marlin/commit/d85409f22204c606a331f49a83d31745b3d961ba
I understand the fan is a 2pin 5v fan ?
Can you provide me with the dimension of the fan, 25mm x 25mm x7mm ?
The easiest way to control the fan is with a PWM signal so I probably need to replace the fan for a 3pin fan.
Thanks
The easiest way to control the fan is with a PWM signal so I probably need to replace the fan for a 3pin fan.
The other fans on the UM2 are PWM and they only have 2 pins. PWM turns the power on and off many times per second. No need for a 3rd pin. The 3rd pin is usually a tachometer. You would need somewhat fancy hardware/software to be able to measure the tachometer. But I suppose you could do it. Instead I would consider using a PID controller. Or just a P controller. With a P controller you measure the temperature error and multiply by a constant and that is your PWM % setting. I'd fake the I setting with a 50% constant. For example if goal temp is 60C and actual temp is 61C and multiplier is 10 then you set the fan speed to 60%. If temp is 55C or lower fan is off. If temp is 65C or higher than set fan to 100%. You can experiment with other constants and multipliers.
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ian 32
my ultimaker2 back fan goes on the second you turn on the printer... never had any problems with that :-)
Ian :-)
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