Cabinet Temperature Controller

K

kool-lites

Full time elf
Joined
Jun 26, 2010
Messages
179
Location
Baulkham Hills
With the changes to the layout this year, I have several new 350W SMPS out in the garden. Most have been in fairly large cabinets with some form of passive cooling. This year things have got smaller so I am using some fan forced cooling.​
But I don't want the fan running all the time.
So I have set out to make a simple fan controller.​
Design Requirement:​
  • Operate of 12 Volts
  • Keep fan OFF until needed
  • At 40degree ( this is my first guess) turn the fan ON slowly)
  • At 60degree and above make the fan go flat tack.
  • Between these two set point, the fan speed is to be set proportionally
  • Capable of driving 2x 80mm or 120mm fans
The Temperature measurement uses the Dallas DS18S20. I have a number left over from a previous project.
Tiny13FanPWM_PCB.png


Tiny13FanPWM_Schem.png

So the Prototype is built, the code is finished and the PCB design complete. I hope to get them out to the fab house this week.​
 
Looks good.
I'd be highly tempted to run with lower temps like 30C and 50C. Ideally you don't want the electronics to be climbing too much above ambient temperature and not turning on the the fan until the air temp inside the enclosure hits 40C means that some of the stuff inside the boxes is likely to be well above 40C. Simply moving the air around when the temp is above 30 should prevent too much of a temperature rise.
 
I'd be putting a diode in reverse polarity across the FET to protect it from the back emf of that fan. Its eventually going to spike.
 
QFK,
Good find. Diode added.

AAH,
That is an issue I wondered about. The inside temp will always for above ambient. During the day when the ambient is 30 and a above when the load is supper low also (I don't run a sequence during the day), there is no need for the fan to on as convection should assist. I also place my controllers out of direct after noon sunlight. Tropical lids are also another solution to play with, but that is another story for another time.

I may wind it back to 35 and see how it pans out.
In hindsight 60degree is probably too high. 50 it is.

Thanks for your input guys. Much appreciated.
 
Following on from what David implied these shouldn't be working, I got a couple of beefier server fans out and tried them. And yes they do whine on start up.

The PWM clock is 9.6/8 MHz.
Lifting the start duty cycle to 50%, lifting the lower threshold to 35 and lowering the upper threshold to 50 degrees makes them turn on faster appears to be a good compromise.
 
Also, Do you really need such a chunky FET? That FET is super sized for this task?


The energy required to drive that FET at 1Mhz, is quite high, and the associated losses will be high, To make it cooler, i'd drop the PWM frequency to 1kHz or so, theres no point in it being so high


Whats the purpose of the 470R Resistor on the gate of the FET?
 
You must log in or register to reply here.
Back
Top