Hot Transistors controlling computer fans

[qbasicer]

Hello!

I'm currently building a controller for some CPU fans so that they only turn on once a command is sent to an Arudino microcontroller. The Arduino is powered by 5V via USB, and I have a 12V power supply connected to a breadboard. In addition, I have a 3 way switch to switch between off, automatic (arduino controlled), and manual on. Manual on bypasses the transistors and provides direct power to the other power rails on my breadboard.

I have a pin on my arduino connected to the base of a transistor with a 680 ohm resistor in series. I have the +12V connected to the collector of the transistor and then a line going from the emitter to the power rail of my breadboard.

The first version of this project worked completely off the USB 5V, however, there's insufficient current available via USB (500mA) for both the arduino and the fan to allow it to run at a reasonably high speed. I then connected up a 12V power supply and grounded the negative terminal the ground of the board and detatched the 5V+ from the board and replaced it with the 12V+.

I didn't have a heat issue with the transistors with the 5V version, but when I switched to the 12V, they get almost untouchable in seconds. They're 2N2222 NPN transistors. I've attached a schematic to the post, and if you want to see my circuit on the breadboard I'll attach that too.

It's also to note that in the initial 5V+ version, I unable to get enough current to drive the board, and when I added more transistors (up to 3) all sharing the same leads, I got better speeds. I continued this (thinking that it would allow me to transfer more current), but I still have massive heat issues.

Is a relay the best option here? I'd like to stay solid state but I don't know if I could design the circuit better and it'd work or I need to order some beefier transistors.

Any help or comments is very much appreciated!!!

 

[Boncuk]

ํYou have selected the wrong type of transistor.

A PC fan normally takes up about 250 to 400mA, some even more (depending on fan diameter, hence power demand).

I suggest using an N-channel MosFet and connect it via a 100Ω to the Arduino output, which has to go H if fan operation is required.

R2 takes care of gate discharge if the fan is switched off.

You also won't need a three position switch for auto/man/off.

Leaving the switch on auto and disabling the MCU output the fan won't run either.

Regards

Boncuk

 

[Diver300]

You problem is that you are running 5V fans from 12 V. Your 2N2222 circuit is a voltage follower circuit that will output about 4.3 V (5 V less a diode drop of about 0.7V). That means that you will generate a voltage drop of about 7.7 V. Whatever does that will get hot, so just about whatever you use to reduce the voltage will get hot and need a heatsink.

Your circuit will also put 12 V directly on the fan when you turn the switch on, and the fans won't like that. You would also be exceeding the Emitter-Base voltage of the 2N2222 in those conditions and so you could get some current back into the output of the Arduino.

Boncuk's circuit is a better way of doing things, but that will always power the fans at 12 V. You should probably add a diode in parallel with the fan as there might be a back-emf.

Alternatively, run the fans in series. You should add 100 uF or so across each one when doing that. The fans won't mind 6 V.

 

[qbasicer]

Thanks for your input guys, really appreciated. I don't have the ability to make the modifications right now but I'll see what I can do.

I have one question for you Diver300, why the capacitor across the fans?

 

[Dragon Tamer]

The capacitor across the fan will help reduce feedback into the circuit that causes noise.

 

[Diver300]

Also, the current taken by the fans may not be continuous. If the fan current does turn on and off, the capacitors will absorb the current and keep the voltage fairly steady.

 

[Dragon Tamer]

Yes, but it would have to be a pretty large capacitor depending on the frequency.