dc fan motor speed control

This circuit cant meet the spec. With 5v in, youve got 4.4v at the e then 3.8v at the next e. Allow 1.5v Vce and that leaves you only V_battery - 5.3v on the fan. Also any circuit needs to cope with a very dirty supply.

I'd likely use an auto relay with 2 speed setting. Cheap, simple, and crap proof.

NT

Reply to
meow2222
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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

Reply to
Rich Grise

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.

Reply to
leon

12v----------o-----o---------------. | | | [22k] | | | | s o-----)--------------g | | d | | | c c | in---[470]- b b--o---[1.2k]--o---. e e | | | | | | F | '--o--' | A - | [1k] N ^ [1k] | | | | | | | GND-------------o------o-----------o---'

This might work. This is an amplifying buffer, with a gain of 2.2. As in increases, the output is near in*2.2.

The output won't start tracking the input until about the NPNs start to conduct, so it doesn't really meet the spec. Note that at 2.5V, the P-MOSFET will be dissipating quite a bit, so it'll need a big heat sink.

Do an ac loop analysis, and watch for oscillations. One possible issue would be the non-constant fan load. I wouldn't trust it without a lot of testing.

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Regards,
  Bob Monsen
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Reply to
Bob Monsen

I haven't tried it, but it might be this simple:

+12 -----------------------+ | [FAN] aircon 0-5 v ---+ | | | / | \\ /c 500 /
Reply to
ehsjr

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