Simple PC Fan Diver With Discrete Components

This is a circuit capable of driving an inductive load, specifically a PC fan at 9V, made from simple components. Although I designed it towards a simple fume extractor when soldering, the circuit may be used elsewhere.

Components:

1N4001 Diode

2N2222 NPN transistor

1 K ohm resistor (x2)

Wire (I used solid core 22 AWG with the insulation stripped off)

Perf board/ proto board (here I am using the type where the lanes are connected to reduce amount of wire and soldering)

2 way switch

PC fan (this one is a 3 pin, rated at 12 V 0.7 A)

Headers

5.5 mm power socket (although this is optional depending how you want to power the fan)

Tools:

Wire cutters

Tweezers

Soldering iron

Solder

Flux/ rosin (with the cheaper copper perf boards like I am using here, this makes a big difference)

Craft knife

Super glue (optional)

Here I have drew up some schematics of the circuit. The fan is a motor so has the M symbol, although in the last diagram I have drew it as an inductor.

As the fan is an inductive load, the diode is to prevent any 'back emf' generated by the fan from damaging the transistor. When power is removed, the fan is still spinning, generating a current and not only this but the copper coil in the fan also stores current. For this reason the switch not only removes power from the circuit but also creates a RLC circuit which I have drawn separately in the final diagram. The idea was to essentially isolate the fan using the diode and RLC circuit, allowing any stored energy to dissipate safely without damaging the transistor circuit. (In this circuit diagram I have represented the fan as an inductor).

The capacitor value can probably vary (I tested a few in the 10-100 nF range) with the 10 nF working well for me.

On the breadboard, this was probably the 4th or 5th design. I tested the currents and voltages of each component in the circuit when using a 9V supply and all are well within the rated values. (My tests gave 8V 230 mA across the fan)

This design was chosen as the final with it being the simplest and most effective I found.

I chose to use a 5.5 mm power receptacle because it is more universal and can be used with either a battery or a power supply.

In the picture above is a small switch-mode power supply which allows different voltages to be selected. I highly recommend them and they also tend to come with various adaptors, for various projects and applications.

I designed it to be used at 9V, although it was effective at 7.5V and 6V. Note there is a considerable decrease in speed when going as low as 6v or 5V and a smaller fan and adjusted circuit may be better suited at lower voltages.

Before soldering I placed all the components so an effective layout was found.

Other than soldering I also glued the 5.5 mm power input with a small amount of super glue to reinforce the structure, as this will have some force applied. Also, using a craft knife, I removed small amounts of copper from the perf board to prevent overlap of some components. (This wasn't necessary and could have been avoided by placing the switch in a different place and just using a little more wire)

As an extra note, which may not be worth mentioning. My fan already had a plug attached which was compatible with the standard sized headers. So I just cut off a section of three and used them. Although the fan is a 3 pin, I found the two power leads and only used them in the design. The third pin is soldered to the board but not attached to anything in the circuit.

I may make a more advanced version in the future including the third pin (which can give a readout of the speed). I may make a variable speed circuit if anyone is interested.

I have ran the circuit on for several minutes at 9V and the circuit is fine, with the fan running smoothly. I made this for use as a fume extractor when soldering, however, may use it in the future for other purposes as it seems pretty efficient.

Attached is a quick video of the fan working.

Enjoy!