Hotend temp. adjusts fan speed?

Hello guys!
I've just built a prusa i3 and I've printed a few parts. At first I thought that I had a major problem 'cause my hotend temperature dropped extremly fast when I started a print. I realized quite soon that my fans were blowing too much so I reduced the speed and the hotend temp. rised and stayed at the right temperature. However!
I still sometimes have the problem that the hotend temp. drops when I start the print so I have to lower the fan speed manually but after a while I can raise it again.
I would like to make this automatically in the marlin firmware. Something like this, If hotend temp goes below a certain value then gradually lower the fanspeed. And the otherway around, if the hotend temp goes above the desired temp then increase the fanspeed gradually. I've been working with Arduino before so I'm quite familiar with the environment and the code but I got some questions:

1. Where should I put the code that checks the temp and updates the fan speed?
2. What is the command to control the fan speed? I'm just familiar with the M106 S(0-255) command.
3. What is the command to check the hotend temp?

Any information and help is appreciated!
Thank you guys!

Edited 1 time(s). Last edit at 04/20/2014 08:01AM by Archelon.

did you run the pid command?

mabey you just need to insulate your hotend from the fan air.

Quote
jm44807
did you run the pid command?

mabey you just need to insulate your hotend from the fan air.

Thanks for your reply!
I've done the PID calibration and I'm checking right now if anything has improved.
Well, I want the fan to blow on the upper part of the hot end to prevent the hot end from clogging and jamming. But you mean that I should prevent it from blowing where the thermistor is placed?
I have now removed the second fan that is pointing down against the bed And I strongly believe that it can solve the problem.
However, do I need a fan that blows at the printed parts too?
I'll send a picture and you'll see.
Thanks for the help

yes insulate the heater block where the thermistor is from the fans.

here is a pic of mine. one heater block is simply covered with kapton tape the other i used fiber gasket material to cover it. then the fan blowing on the bed has much less effect on heater block. i also have a fan shroud to direct air over top half of hotend

I solved the problem by removing the fan that was pointing downwards and I also did a pid on the hotend so now the temperature is correct all the time.
I however have a new problem, sometimes the extruder motor just stops extruding after printing for a while. It stops and starts randomly. The gear is tight enough at the motorshaft and the bearing that pushes the filament against the hobbed bolt is tight enough. I have a fan blowing at the electronics/stepper motor driver. It's not very powerful but it blows. The motor gets quite hot, I can hold my fingers at the back of the motor for a couple of seconds but then it gets too hot. What is the problems? Is the stepper driver current too high? Or too low? Thanks for any help!

dose the motor stop or slip on filament? if it stops lower the current.

It doesn't slip, it just stops. I will try to lower the current and then see (: thank you!

here is an article on setting current for ramps if that is what you use
[www.indigent-networks.com]