Using Diodes for Fan Control.




This in alternative to using rheostats and chips for controlling fan speed.

The idea for this came from NOW and I wanted to explain it a little more and build a few of them. Basically a diode type 1N4001 drops around 0.75 volts with a typical fan. When you link them together then the voltage can be varied.

This is taken from the site
Using a chain of diodes mounted on a rotary switch, a series of voltage steps can be produced giving adequate control for fans.

This control method has several advantages;

  • Cost is less than that of a half-decent rheostat, and much less than the price of a good one.
  • number of fans wired in parallel (or to a fanbus) can be controlled from the unit without danger of overload. Cheap rotary switches will handle 1amp at low voltage, rheostats to handle such currents are big and expensive.
  • The full 12V can be used if required. Basic semi-conductor voltage regulators lose about 2V and would need a by-passing switch to run the fans at full speed.
  • The heat produced is spread over a number of diodes. Each one will only generate about a watt at the maximum 1amp load, and less than 0.1W per diode with a typical single fan.
  • Unlike resistors, voltage drop is about the same at all fan currents, so fan start-up at low voltages is more assured and a wide range of fans can be used on the same unit.
  • Last but not least, adding display lights to form a voltage bargraph or show switch position is easy.
This is my adaption to the original idea. I will use the pictures from the site to help explain.

I did one of the switches as a speed up video.

Step 1: Options for LEDs

Their are several ways that LEDs can be used for this to show what setting the switch is on.

  • Bar graph
  • single Indicator
  • No LEDs

The pictures help explain.

Step 2: Things You Will Need

You will need

  • soldering iron
  • solder
  • wire
  • 2P6P rotary switch. (a 1P12P will allow for more control but will not allow for LEDs
  • 6(or more)Diodes type 1N4001

Step 3: Wires for Leds (optional)

The wireing positions for the LEDs on a 2P6P rotary switch are 7-12.

You can do like what I did for the LEDs or do them like the diodes in Steep 4 if you want to do a bar graph.

I added a wire to each position (7-12) Then did the diodes type1N5817 Schottky diodes on a PC board.

I will show how to do the LEDs later on another Instructable.

Step 4: The Diodes

The are also options for doing the diodes for the fan control.

This is from the site

If you don't want lights, just use the left-hand section of the schematic, for pole 'A'. Using a 12-way 1-pole switch. This would allow the range to be extended, using single diodes between each position. You could then use 1A Schottky diodes, they drop under 0.5V per diode so would give smaller steps, and/or have an unconnected "off" position at one end.

For my switches I used 6 Diodes(Type1N4001). I did my wiring like this.

  • no diode for full 12v
  • 2 diodes
  • 2 diodes
  • 1 diode
  • 1 diode
  • No diode for off (later updated to 2 diodes for a very low speed)

Step 5: Hack the Old Fan Plug for New Fan Controller Power.

I used the 2 pin power wire that came with the fan,

I removed the yellow and the black wire from the plug and sniped the old 2 pin power wire off at that end. Then stripped a red and a black wire for power and ground. Red wire was pushed into the hole in the molex pin and soldered the same thing for the black wire fro ground. Afterward they where heat shrinked and pushed back into the molex plug.

Step 6: Attaching Power to Fan Controller and Ground.

connect the ground on the molex to the ground to the fans 2 pin ground wire. Then the red wire from the plug goes to the wire on position 6 of the rotary switch. Position A gets attached to the power wire on the 2pin fan power wire. Then heat shrink.

Step 7: Testing

Make sure all solder connection have heat shrink. Plug the fan in and then plug the molex in.
You should now be able to control the fan with the rotary switch.

I will make a new instructible for doing the LEDs or add to this one later on.This is what id did with mine. I forgot to take pictures when i made it, but I am making one more.

Pleas Rate and Comment.

Thank You CPEMMA for letting me use pictures and text from your website.



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    11 Discussions

    Rob Kdemsony

    Reply 9 years ago on Introduction

    With the rotary switches that I used, it should be able to do 3 of the low power 120 MM fans. Just add up the fans current draw and then look for the switches current ratting.


    9 years ago on Introduction

    What are those diodes just before the LEDs in the positions 11 and 12 of the SW1b in the first schematic? do you need to drop the voltage or something like that?

    3 replies
    Rob Kvinacarv

    Reply 9 years ago on Introduction

    Those are Schottky diodes. Each position puts out 12 volts but to have every LED on at the highest setting would require a higher voltage to keep it as a bar graph. So instead when a higher position is reached it turns on a second series of LEDs.

    vinacarvRob K

    Reply 9 years ago on Introduction

    Ah, so the diodes are there just to avoid the current to kinda make a "loop" and running the LEDs of the first chain in reverse-biased? (i think i got it now... the thing of the parallel connection for lowering the power requirements of the bargraph i've got from start =D)

    btw, is nice to see speed controlers that dont require some IC. those are pretty hard to find for me D=

    Rob Kvinacarv

    Reply 9 years ago on Introduction

    I have not tried to bar graph the LEDs that way. I used the option with the Schottky diode per LED. I was also using 3.6 fv aqua LEDs and another circuit that was suppose to fade them that messed up.

    Rob Ksdallesasse

    Reply 10 years ago on Introduction

    I asked your question Cpemma responded with.

    In theory you could use zeners, but it's hard to find low values. A 2.4V 1.3W zener in place of the first pair of 1N4001 would be worth doing to give a bigger starting step (12V to 9.6V) and extend the bottom of the range to 6.6V

    Phil B

    10 years ago on Introduction

    I assume you are utilizing the property of a diode that drops the voltage across it by about 0.6 volts per diode.

    1 reply
    Rob KPhil B

    Reply 10 years ago on Introduction

    Yes it is, I forgot to add that in there. I will update the instructible.