Transistorized fan control circuit

Discussion in 'The Projects Forum' started by BradSk88, May 11, 2011.

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  1. BradSk88

    Thread Starter New Member

    May 11, 2011
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    I've retrofitted an automatic HVAC system from a Kia Optima into my vehicle and everything has been going great, but I can't figure out how to get the fan speed control to work.

    Here is the circuit diagram not sure how much it will help.
    http://dl.dropbox.com/u/800458/Untitled-1%20%282%29.jpg

    I don't have the actual Kia transistor, and the people who work for Kia don't even know what I'm talking about when I try to buy one, so I am left to my own devices (literally).

    I have a 2n3055 NPN connected to my fan. Collector is connected to the ground side of the fan motor, Emitter was connected directly to ground of the vehicle.

    The base was connected to a 1W, 25Ω to 1kΩ potentiometer and I was able to get the fan to turn on and modulate speed. Great, but the output from the auto HVAC unit is too small to turn the 2n3055 on.

    So I picked up a 2n2926 and hooked it to the 2n3055 in a darlinton pair configuration. (The emitter of the 2n2926 connected to the base of the 2n3055)

    I connected the pot to the 2n2926 in the same way as previously and it SMOKED my pot. So I am a little hesitant to connect it to a very expensive control unit.

    Can anyone advise what I should do here? I can test the values of the two wires (gray/blk and blk), but I don't know what to measure for.

    How can I protect the control unit from smoking when I finally hook this stuff up?

    Thanks.
     
  2. #12

    Expert

    Nov 30, 2010
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    Definitely measure voltage on gray/black to black while telling the controller to change speeds. If it just stays high, you might have a resistor in series with the output, but no load. That doesn't mean it's broken, just harder to measure.

    Consider that 12V into 25 ohms is 5.76 watts, and that will smoke a 1 watt pot, depending on where you turned the knob (shaft). I think you just made a simple mistake, but you didn't say exactly how the pot was wired. A bigger pot AND a resistor in series with the base of the first transistor? You have to make sure the pot isn't the only thing in series with the base at 25 ohms.

    I'd start by estimating the gain of the transistors and put a resistor in series from +12 to the first base and demonstrate that you can get the fan to run at almost maximun current. Then leave that resistor in series with the base and calculate a pot that will carry enough current to feed the newly found base resistor.

    That's a start.
     
  3. BradSk88

    Thread Starter New Member

    May 11, 2011
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    Interesting.

    For the record, until I can go look at the system again, the pot was wired:

    (Looking at top (knob side) with pins facing down.)

    Ground - 12v - To Base
     
  4. #12

    Expert

    Nov 30, 2010
    16,278
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    As a disclaimer, I've never heard of a 25 ohm to 1k pot. The pots I know about go from very close to zero ohms to whatever the label says, in this case, 1k. Assuming your pot is like the ones I know, the wiper is the center pin. I would wire it Ground-base-12V and connect the center pin to a resistor limiting the current to the transistor base. Dialing 12 volts directly to the base will cause unlimited current through the base-emitter junction.

    If you have made the mistake I suspect, you dialed 12 volts directly to ground at one end of rotation and directly to the base at the other end of rotation.

    In the middle of rotation, you would have the 12 volt supply about 500 ohms from each end, limiting the current to 48 milliamps entering the 12v pin and going 24ma in both directions, toward ground and toward base. That's a bit over 1/2 watt in the pot. When you dial the knob in either direction, the current increases toward 2 unlimited current sinks, ground and the base-emitter junction.

    Of course, this is theory. As you gather more information you will find out if I guessed correctly.
     
  5. debe

    Well-Known Member

    Sep 21, 2010
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    Try ground.base.12v. The way you wired it if the pot was down near ground it would be a short to 12V.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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