Triac Lamp Dimmer Schematic

TRIAC Dimmer Avoids Snap-ON

We are all familiar with the snap-on phenomenon of inexpensive TRIAC dimmers that are often used to control incandescent lamps –turn the pot until the light comes (snaps) on and then reduce the setting to make it dimmer –but don’t turn it too low because the light extinguishes and then the process must be repeated.

However, few know that this annoying quirk can be remedied via the addition of only three inexpensive, commonly available components. Unlike the SCR Phase Control Dimmer, I cannot claim ownership of the idea, but I am surprised that it seems to be buried and rarely utilized. This makes a great experimental circuit. Note that there are a number of dimmer circuits on

Schematic of Lamp Dimmer Circuit

Triac Light Dimmer Schematic

Bill of materials

Lamp Dimmer BOM

Bill of materials file

The source of the problem

Before the DIAC triggering device fires, C1 must charge to 28 or so volts. However, if the voltage at the end of the cycle is insufficient to trigger the DIAC, the next half-cycle starts with C1 charged in the incorrect polarity so that it takes all the longer for C1 to discharge through zero and then charge to the opposite threshold.

After the DIAC initially fires, this condition changes substantially so that the phase delay angle is reduced and the lamp snaps on at a minimum brightness –sometimes brighter than desired.

The 3 added components

R4, R5 and BR1 solve the snap-on quirk. Cost is less than $1. I suggest that the experimenter try it with and without these components so the effect may be observed. Note that the resistor values are different for the two line voltages indicated (115 /230VAC).

How does the light dimmer circuit works

This is an extremely clever circuit. At the beginning of each half-cycle when the line voltage changes polarity, the voltage across C1 lags in voltage and is of the opposite polarity. To discharge C1 to zero volts as quickly as possible, the bias current through either R4 or R5 (depending upon polarity) quickly discharges the voltage across C1 to zero. At that point, it begins to charge in the proper polarity for that half-cycle. Note that this circuit can only discharge C1 –it cannot charge C1 due to the presence of the lower diodes of BR1 that short the bias current(s) to circuit common.

*Optional noise filter

L1, R6 & C2 make up an optional noise filter. Generally the values are non-critical –what ever works. It reduces the noise created by the firing of the TRIAC and also reduces the effect of line transients that may tend to turn off (commutate) the TRIAC and thus make it flicker. While a bona-fide inductor is recommended, a number of turns of AWG18 wire around a ferrite rod or even a nail may work OK. Simply listen to the noise emissions with an AM or SW radio to observe the effect.


Oscillograph Without Correction Circuit

Note the substantial voltage that is present across C1 after the DIAC fires. This voltage is a variable that is affected by DIAC turn-off properties and device temperature.

Oscillograph With Correction Circuit

Note how quickly the voltage across C1 discharges to zero shortly after the beginning of each half-cycle.

Lamp Dimmer Photos

Isolation transformer

Observe that I used an isolation transformer. This is essential if the oscilloscope is connected to a grounded computer via a serial interface –or else there will be smoke and tears… It also greatly reduces the shock hazard.


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  • brunoudi

    Is there a way do make a dimmer like this, but controlled by a microcontroler (arduino for example)? I mean a circuit with the Load in series with the input.

  • Tim

    Thank you for this amazing schematic, i have build this and it functions great!! But what i noticed is that the lamp does not switch off 100% I must tell you that this is the very first ever project i have build so chances are i have made a mistake….
    What can i have done wrong??? please help i am trying to understand this schematic.

    Thank you

    • Jim Keith

      You probably have the circuit correct, but the R-C values may not be exactly correct. The most likely problems are as follows:

      1. The R-C values in my design may be slightly off–just because it worked OK for me, does not mean it will always work.
      2. Your pot resistance may be on the high side (±20% tolerance)
      3. The value of C1 may be also on the high side (±10% tolerance)

      The easiest way to correct the problem is to reduce the capacitance of C1 slightly. Do this by connecting a 0.47uf or 1uf capacitor in series with C1 and see what happens.

      Let us know how you made out…

  • khurram

    hello dear sir,
    i need cct like this but i want to use this cct as a battery charger working: during battery charging if main voltage gose low the charging current also low this is not good for battery, this dimming effect can solve this issue, when mains low voltage of this cct gose up. load replace with charging transformer

  • Awad Mohammad

    How we may do it???
    One push-button.

    • Jim Keith

      @Awad Best solution is using a microcontroller for phase control –hardware is simple, but the firmware is not trivial. Follow my 8051 microcontroller articles and I will soon (perhaps 3months) include a dimmer –will start with a 24VAC dimmer to prevent experimenter shock hazard and then graduate to a 115 /230VAC version.

  • Awad Mohammad

    it is really great project. but i am asking for light dimmer which may controlled by a simple push bottom.

    • Jim Keith

      Push to increase and push to decrease?
      One or two push-buttons?
      Such would be rather difficult, but not impossible.

  • Jamil G

    Can I use a 220V fan instead of the light bulb?

    • Jim Keith

      Probably will not work with a permanent split capacitor motor that is used for larger fans /blowers.

    • Jim Keith

      You can achieve limited speed control of shaded pole motors –the type often used for fans. A fan load is sometimes called a “cubic” load because the power required to rotate the fan is a function of the cube of the fan speed. Therefore only a small change in fan speed causes a much greater change in air flow. At reduced voltage, the shaded pole motor does slow down, but beyond a certain point, it tends to stall. Observe motor temperature because losses become high under this condition. Some shaded pole motors are impedance limited so that stalling the motor will not cause destructive overheating.

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