Can anybody help with a simple 12volt auto fan speed controller. The signal
from the aircon controller is zero to 5Volts positive and the desired output
to the fan zero to 10V or higher at about 7 Amps. Plenty of heatsink area
available, so preferably not PWM due to complexity.
Leon.
Well, lessee if I can come up with something sane:
+12V +12V
| |
C [M] Fan Motor
0-5V ---B Q1 |
E |
| C
+----------------B Q2
E
|
[R]
|
12V ret.
For the transistors, you could use a 2N3055 although there are probably
better transistors on the market for Q2, and Q1 has to be able to handle
whatever base current Q2 needs to get 7 amps. The R value should be
whatever resistance passes 7 amps when it's got (5 - Vbe1 - Vbe2) volts
across it. I leave the arithmetic as an exercise for the reader.
This is a "constant-current sink", even though I like to call it a
"source", since it's sourcing electrons and I'm "only" a tech. ;-)
Just rechecked the 2N3055 data sheet, and holy crap! Vbe can go up to 1.8V
at 4A Ic on the 2N3055! Find a better transistor - maybe a darlington,
then you'll only need the one.
And, yes, since it's analog, you'll need quite a substantial heat
sink. I'd even go for putting some of the big ones in parallel,
and beefing up the driver some - just remember that equation,
and if you use three in a "trilington", you'll have to subtract
Vbe3 as well.
Good Luck!
Rich
