DC Fan Transistor Switch and Amplifier

Discussion in 'General Electronics Discussion' started by painstream, Feb 6, 2013.

1. painstream

3
0
Feb 6, 2013
Hey All,

I'm new to electronics and just learning. I have been experimenting with NPN Transistors and finally wanted to use one as a switch and one as an amplifier to control a small DC Fan. The fan speed will also be controlled by a Potentiometer. I've attached my current circuit diagram. Is this proper usage of doing what i'm trying to accomplish? Basically just trying to apply a voltage to one transistor to switch on the other circuit to turn on the fan. Everything works\tests fine on my end. I'm looking to learn from mistake and learn best practices!

1) The Pot controls the speed of the fan
2) The 15K resistor helps limit the bottom end speed
3) Flyback Diode For Reverse Protection
5) Transistor gets switched on by its own 5volt power supply
6) Both Share a common ground

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2. duke37

5,364
772
Jan 9, 2011
Q1 just dumps current and wastes power.Turn it up and you could blow it up. Get rid of this and use the pot to control the current to Q2.

3. painstream

3
0
Feb 6, 2013
I guess I was thinking i'd need the pot on the 12 volt side to control min and max speed of the fan. However I did try as you suggested and it works fine. Looks like I have more reading to do about transistors

4. duke37

5,364
772
Jan 9, 2011
You could put a variable resistance between the power supply and the fan but this would need a big resistor and would dissipate a lot of heat.
If the current into the base of the transistor is controlled, the collector current will be the base current times the transistor gain, a guess of this would be 100. Therefore the current in the base will be low and can be adjusted with a diddy resistor.

Make sure that you always have a resistance in the base feed to limit the base current to a safe value. Also, the 2N3904 should not be asked to take more than 200mA through its collector. What is the fan current?

5. painstream

3
0
Feb 6, 2013
I reworked my circuit. I am now controlling just the base current. I had the wrong transistor value inputted previously. The one i'm using will handle up to 3 amps. Limits the current well within specs.

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