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As a learning exercise, I am trying to control a 12VDC/0.18A computer fan with a thermistor. As I am planning the circuit (and I certainly can be wrong in my calculations), I plan to have a 12V power source (8AA batteries in series). I am trying to use a PN2222 transistor to control the fan's speed. According to my calculations, I would need 6mA into the base of the transistor to get the 180mA needed to run the fan at (or close to) full speed. So, then to control the speed from there, i.e. clamp down the current going to the base to reduce the current into the fan, I have placed a 5-100K thermistor in line with a 2K resistor.

Connections: Negative rail connected to Emitter and 2K Resistor which also connects through thermistor to base. Collector is connected to negative side of fan. Positive rail connects to positive terminal of fan. enter image description here

I believe I have included all the pertinent information, but if I left something out, I'll be happy to add more.

So, the problem I am having is that the fan doesn't even turn. I have connected the fan directly to the battery source, and the fan turns. I have started some basic troubleshooting, but found that I am reading 0V across the 2KR (1.9K pictured). So, I guess my question is what am I missing here? Why can I not get the fan to turn in this circuit, and I am sure it is probably something stupid.

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  • \$\begingroup\$ A picture (or in this case a circuit diagram) is better than a 1000 words. BTW you haven't actually asked your question. \$\endgroup\$
    – JIm Dearden
    Aug 4, 2017 at 13:10
  • \$\begingroup\$ We need a circuit diagram to see how you've connected this. You're probably going to want to have your transistor as an emitter follower and be thinking voltage control rather than current limiting as you seem to be as that is dependent on the (highly variable) gain of the transistor. Also, a 2n2222 is going to get quite hot, you might want something beefier in a TO220 package. \$\endgroup\$
    – Ian Bland
    Aug 4, 2017 at 13:12
  • \$\begingroup\$ I have added a circuit diagram about my idea. I also added some additional information of what I have tried so far to troubleshoot. As for the transistor, I believe it is a TO-92 package, according to the datasheet that I have for it. Also, this is not going to be running long-term. I don't know how quickly the transistor will heat up, but I am only trying to get this running to prove the concept. This is not for a real-world application...just learning. \$\endgroup\$
    – Jonathan
    Aug 4, 2017 at 13:23
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    \$\begingroup\$ I might have missed something in your question, but the transistor base needs to be positive with respect to the emitter to turn on. The transistor base is currently connected to the emitter. As a heuristic solution, you can use a potentiometer to see at what point the fan will turn on, then size your resistor and thermistor accordingly. \$\endgroup\$
    – PICyPICyPICy
    Aug 4, 2017 at 13:32
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    \$\begingroup\$ @Jonathan You can control the speed using PWM from a 555 timer. No need to shy away from micro controllers with the Arduino platform. \$\endgroup\$
    – PICyPICyPICy
    Aug 6, 2017 at 4:16

2 Answers 2

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I might have missed something in your question, but the transistor base needs to be positive with respect to the emitter to turn on. The transistor base is currently connected to the emitter. As a heuristic solution, you can use a potentiometer to see at what point the fan will turn on, then size your resistor and thermistor accordingly.

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enter image description here

I am thinking you want something like this, using a mosfet it can handle more current and using the "pot" makes it temperature variable.

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  • \$\begingroup\$ 1: MOSFET vs BJT has nothing to do with current handling capability. // 2: Why not use a real schematic? // 3: Could you tell us a little about the "characteristics" of this circuit? The transistor has a Vgs threshold voltage and non-linear transconductance. Oh and the NTC is also non-linear. A curve of (temperature, current~RPM) would be nice. // 4: How come there is a copyright notice in the image? \$\endgroup\$
    – Oskar Skog
    Sep 26, 2017 at 5:03
  • \$\begingroup\$ I copied straight from the source .... heatsink-guide.com and have built several like this . \$\endgroup\$
    – Dustin W B
    Sep 26, 2017 at 7:26

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