Fan Speed Controller

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Assembly of this Fan Speed Control is easy enough, but getting the right capacitors requires a bit of trial & error. (Unless you enjoy complex calculations.)

The practice is actually a lot simpler than the theory. Once you understand this article, making it is easy.

You'll need:

  • multimeter or voltmeter
  • A selection of a few 600v non-polarised capacitors,
  • The switch(es)
  • The fan

Universal vs Dedicated Controllers

The type of controller described here is a dedicated controller, because its effect on fan operation depend on the characteristics of the fan. This is why capacitor values are selected to suit the fan used.

The other type of controller available is a universal fan controller. These use electronically controlled triacs instead of capacitors, and effect on fan speed does not depend much on fan characteristics, so a single controller can be used with a wide range of fans.

The Theory

Fan speed is reduced by placing a capacitor in series with the switched live feed to the fan reduce power. The capacitor acts as a dropper.

Speed depends on the value of the capacitor in circuit. The speed control switch selects a different capacitor to give different speeds.

More Components

A bleed resistor is fitted across each capacitor to prevent electric shock from a switched off circuit. An RC suppressor is fitted to avoid excessive switch arcing, as fans are inductive loads.

The Circuits

To control fan speed, we use any one of the following circuits to replace the on/off switch.

These circuits are drawn twice, first to show their essential elements so they're easy to understand, then again with full details.

Skeleton Circuits

2 speed 1 switch control:

o--------o---> o            |----------o
         3 way sw   cap

This gives 2 speeds & off.

2 speed 2 switch control:

o--------o--->              +-------o---> o--------o
         switch     cap             on/off sw

This gives 3 speeds & off.

3 speed 2 switch control:

o--------o---> o----||------+-------o---> o--------o
         3 way sw   caps            on/off sw

This gives 3 speeds & off.

Full Circuits

2 speed 1 switch control:

                  +--^^^----+            <-- bleed R
                  |         |
              o---+--||-----+            < Speed ctrl C
o--------o---> o            |----------o
         |    o-------------+
         |                  |
         +-------^^^---||---+      <-- Suppressor R & C
        3 way sw

This gives 2 speeds & off.

2 speed 2 switch control:
                  +--^^^----+            <-- bleed R
                  |         |
              o---+--||-----+               < Speed ctrl C
o--------o--->              +-------o---> o-----+--o
         |    o-------------+                   |
         |                         on/off sw    |
        switch   Suppressor R & C                  

This gives 3 speeds & off.

3 speed 2 switch control:

                  +--^^^----+     <-- bleed R
                  |         |
                            |     <-- speed ctrl C x2
o--------o---> o--+--||-----+-------o---> o---+----o
         |        +----^^^--+      on/off sw  | 
         |    o_____________|                 |
         |                                    |
     3 way sw    Suppressor R & C                  

This gives 3 speeds & off.

3 way switches

For 2 speed circuits using a 3 way switch, these can be a standard 2 way lightswitch with an extra centre off position.

For 3 speed circuits, standard lightswitches with 3 contact positions are not available. In this case another type of 3 way switch will be required. Switches come in a wide range of styles, and one rated to 230v should be picked.



The capacitors need to be 600v rated (or higher). 250v caps are inadequate for even brief use. 400v caps might or might not suffice, so are not the best.


Electrolytic capacitors are polarised, and completely unsuitable.

Safety Capacitors

Capacitors need not be class X or Y rated, since capacitors failures are inherently safe with this controller.

Capacitance Value

Capacitance values are critical in some respects, and will need to be picked partly by trial. There are 2 limits which must not be exceeded, max & min capacitance, the values of which depend on the individual fan design details.

Max Capacitance

If too large a capacitor is used, the fan will see greater than 240v, and will fry.

The following are the approx capacitance limits for 240v fans:

  • 40w fan: 2uF max
  • 60w fan: 3uF max
  • 80w fan: 4uF max

The exact values will depend on the fan's characteristics, but fortunately we don't need to know exactly.

Min Capacitance

If too small a capacitance is used, the fan will have inadequate starting torque and risks stalling. Stalled fans are liable to catch fire.

The minimum C values depend on fan characteristics and are non-trivial to calculate. We guard against too low a capacitance by measuring the voltage on the fan during operation, and avoiding it falling too low.

Picking Capacitors

The Explanation

Start with a selection of perhaps 4 capacitors. For a 40w fan one might try a selection of values from 1.5uF down to maybe 0.7uF. There is no reason to use the maximum 2uF, as this would give approx full speed, whereas we only need capacitors to provide the reduced speeds.

A multimeter is connected across the fan to measure fan voltage, each capacitor is connected in series with the fan in turn, and the fan is powered up to monitor starting performance and fan voltage.

Since this means working on mains electrics which will at some times be live, it hardly need be said that some basic skills for this work are required. If you don't possess them, find something else to do, and buy a ready made universal fan controller. Fans are inductive devices, and can produce very high voltage spikes when switched off, far above mains voltage. I'm not going to give safety advice on this, if you need to ask then this is not a project for you.

The Practice

Starting Behaviour

When the fan is first switched on, the first thing to do is watch how the fan starts. If it starts immediately, all is well. If it hesitates before starting, or struggles to start picking up speed, switch off, the capacitance value is too low for safe continuous use. Running it like this for 10 seconds is harmless, but continuous operation would be a fire risk.

Fan Voltage When the fan is starting ok, then we turn attention to the voltage across the fan. Meter readings can be ignored while the fan is starting, what we need to know is the readings when the fan is upto speed.

If the voltage is 240v or more, switch off, the capacitor is too large.

Stalling voltage is in the region of 80-100v for a 240v fan, so the fan must run at well above 100v once it has picked up speed. 130v is ok as a minimum safe value. If the fan voltage fails to reach this value, the capacitor is too small for safe continuous use.

Take Your Pick These tests will eliminate capacitors that are too large and too small, and you can pick capacitors from the ones within the safe range.

Completing the Circuit

A few more bits are needed to complete the circuit and ensure reliability.

Bleed Resistors

Each speed control capacitor needs a bleed resistor connecting across it. Bleed resistors need to be rated at 400v or greater, which means using either one 0.5w resistor or 2x 1/4w or 1/3w resistors in series.

Bleed resistors can be either

  • a single 0.5watt 270k to 470k resistor
  • or 2x 150k to 270k 0.25watt or 0.3watt resistors in series (both Rs must be the same resistance).

Switch Suppressor

A 0.1uF 400v capacitor and 100 ohm resistor are connected in series, then connected across the ends of the speed control. This greatly reduces switch arcing, which would otherwise burn out the switch contacts prematurely. These suppressors can be bought as a single R+C component if wished. See Snubber for more information.


Fans should always have an overheat cutout incorporated.

Wiring should be sufficiently sturdy, and if any risk of wires touching anything metallic exists, the wire concerned should be sleeved.

Where metal switch plates or pattresses are used, all wires should be sleeved

Where a slim pattress is used, and the capacitors placed in a separate box to the switch, that box should be fire resistant like any other mains wiring accessory.

Using a controller designed for a different fan is sometimes unsafe. When using a controller made for another model, whether a commerical one or this one, one should check that fan voltage and starting performance are both ok.

Sourcing Capacitors

  • Electronic component suppliers
  • Used parts
    • Mains motor capacitors
    • PFC capacitors from 2' or smaller linear fluorescent lights
    • TVs
    • Various scrap electronic equipment

Comparison with Commercial Controllers

  • Commercial controllers are unavailable for most fan models.
  • A household 1 or 2 gang switch may be used, of any style to fit the rest of your electrical accessories. Commercial fan controllers are plastic, rarely nicely designed, and unstylable.
  • £5 versus £35. (More if fancy switches are used)
  • Starting can occur at any speed setting, so very low fan speeds can not be used
  • Requires basic electrical skills

See Also