Proportional fan control. I suck at math!

[Byte_Butcher]

Ahh, that's right, the minimum on speed is 50%... (duty = 128)

So then you've got to increase duty by about 4.2 for every degree between 90 and 120 ...

How about something along these lines:

If temp < 90 then duty = 0

If temp > 120 then duty = 255

If temp > 89 AND temp < 120 then duty = 128 + ((temp - 90) * 42 / 10)

steve

[polymer52]

Perfect!
I knew you guys were brainie.
I'll buy you a virtual beer.. Cheers!

[malc-c]

Nice one Steve !

Good teamwork

[polymer52]

And thanks to alec also. Somehow I missed his post earlier but he did a lot of the work for me and between Steve and Alec I think we've got it..
I have a solar hot air furnace which I'm building this control for. It is just a relay controlled fan right now which, early in the morning, kicks on at 110 degrees then cools the collector down too much and shuts off at 90 deg then cycles again. It also outputs data to a serial LCD module such as ambient temp, collector temp, fan relay on/off and vent servo position.
With the proportional control I can have it to come on at 90 degrees and only move the amount of air that it can heat without shutting off and cycling increasing the heating time.
As the sun rises and it gets hotter in the collector the fan can move ever increasing amounts of air until it gets to full speed. Full collector temperature ranges from 90 at cut in to around 140-170 deg at noon depending on outside temperature.