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 electroschematics.com
Bill of materials
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.
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.
Note how quickly the voltage across C1 discharges to zero shortly after the beginning of each half-cycle.
Lamp Dimmer Photos
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.