MOSFET Driver - Fan Controller problem.

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Basrad

Member
Hi Guys,

Sorry I do not have a circuit diagram, as I dont generally build from one, but I have a simple circuit that is giving me some troubles..

I have a bank of MOSFETs attached to a heat sink.. I have made a very simple PICAXE board to control the fans when up to temperature.. This all works a-ok, except at certain frequency’s (driving the mosfets) say 50 to 150kHz the fans tremble and start and stop...

The power light on the pic board stays on, + there is a 2sec 'bootup' delay I programmed so I can see if the chip is 'resetting' which I dont think it is, The power is separate from the MOSFET supply.. Can the thermistors value change when subjected to certain frequencies nearby?
 
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MikeGyver

Senior Member
How can anybody help you unless they know at least some basic stuff?
Get a pencil, draw up the schematic, take a picture of it, and host it at tinypic.com
Since I don't believe you'll do any of this I'll give it a blind shot...

Scope the rail and see if it's smooth. Try driving the pic from it's own battery, if this fixes it then your supply isn't stiff enough; the rails are probably bouncing all over the place. Decouple it with a large value electrolytic cap.
I'm curious as to what this 'bank' of mosfets is doing?
 
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inglewoodpete

Senior Member
...and some more info on these "fans" that you're using. If they're brushless computer fans then they will have a 2 or 3 phase oscillator/driver. This will run at a suitable frequency for driving the rotor at the (manufacturer's) required speed, with a DC supply. You may find that the PWM is messing with the DC-to-2[/3]-phase conversion.
 
hippy

hippy

Ex-Staff (retired)
Basrad said:
I have made a very simple PICAXE board to control the fans when up to temperature.. This all works a-ok, except at certain frequency’s (driving the mosfets) say 50 to 150kHz the fans tremble and start and stop...
Click to expand...
Perhaps the answer is not to drive them at those frequencies. Are you using variable frequency or simple PWM with fixed frequency and variable duty ?

Two things that aren't clear; is this "50Hz to 150kHz" or "50kHz to 150kHz", and do you mean the fan control works or just the software that will be controlling the fans ?

As Basrad notes, some fans don't actually work that well with PWM, are difficult to start or control, and are best controlled by a variable voltage than variable frequency or PWM.
 
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