Help investigating BJT configuration in DIMMER - Trap for young players - Page 1
Forum section: electronics / projects

author: Houseman
author_rank: Regular Contributor
timestamp: September 28, 2018, 12:46:48 am
content: Hi forum.I am asking your valuable help since I wanted to study an old "analog" electronic resistive dimmer for a 12V lamp I had to repair.The circuit is basically around a TRIAC that dims the halogen lamp 12V in AC.But the question is behind the 4 complementary 337 /327 BJT's that are controlling in a unknown way the TRIAC.Now the questions I cannot answer myself:Do the BJT configuration form an oscillator?What is the 470nF Capacitor there for?How do they handle ac voltage?How is the TRIAC driven by them? using 50Hz frequency?Why are the 6,2V Zener between every Collector and emitter? to keep them in the active direct /inverse region?On the sheet there is a question mark for the part controlling the DIMMER POT.Does this part go against Voltage Kirchoff's laws?Any help would be very appreciated.RegardsSteve

author: Houseman
author_rank: Regular Contributor
timestamp: September 28, 2018, 08:04:46 am
content: Will post some waveforms later maybe

author: glarsson
author_rank: Frequent Contributor
timestamp: September 28, 2018, 08:12:57 am
content: The four transistors implements a DIAC-like function.

author: Houseman
author_rank: Regular Contributor
timestamp: September 28, 2018, 09:06:02 am
content: So why also use the TRIAC?How is the dimmer feature implemented?

author: glarsson
author_rank: Frequent Contributor
timestamp: September 28, 2018, 09:23:30 am
content: Also? A TRIAC is commonly used for dimming AC. A DIAC, on its own, can not be used for dimming. It is only used to trigger the TRIAC at the correct phase angle of the AC.

author: capt bullshot
author_rank: Super Contributor
timestamp: September 28, 2018, 10:07:11 am
content: As others told, these transistors implement a DIAC alike funktion.A Diac is a trigger device that discharges the 470nF cap right of the transistors into the Triacs gate (left of the transistors). Because it is AC, it has to work in both directions (say the cap gets charged to a negative voltage in one half-wave and to a positive voltage in the other half-wave).As soon as the voltage across the capacitor gets high enough (no matter of polarity), the diac "fires" (makes a short between its two pins) and discharges the capacitor into the Triacs gate. This turns the triac on. The charging rate of the capacitor (set by the 47k pot) determines when the triac gets triggered by the diac. The 9.1V zeners do a bit of voltage regulation for the dimmer set pot, probably to reduce "pumping" effects due to the transformers output impedance. The other resistor and trimmer are used to set a "minimum brightness" of the halogen bulb, to pre-heat the filament, even if the brightness is set to zero by the main pot.One cannot buy DIACs suitable for 12V AC operation, so the creators of this circuit decided to replicate its functions with transistors. Otherwise, this circuit works the same way as any other primitive light dimmer intended to use at 120V / 230V.One should find "better" explanations than mine by googling for the standard AC dimmer circuit, as I said, this circuit works the same way.