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G8MNY  > TECH     17.11.23 08:21z 123 Lines 5900 Bytes #12 (0) @ WW
BID : 60492_GB7CIP
Subj: TRIAC Light Dimmer
Sent: 231117/0820Z @:GB7CIP.#32.GBR.EURO #:60492 [Caterham Surrey GBR]

By G8MNY                                         (Updated Aug 10)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

Here is a simple circuit used in most dimmers.
Live  LAMPo/o()))1KĿ - - - - 
110-250V     FUSE     CHOKE  A''     1W   VARIABLE   1M
50-60Hz                       ____         100k      __ LOW BRILLIANCE
                       TRIAC  \/ /\   Diac   (2W)    \ / DC OPTION
           Filter ===              >            400V PIV
            cap     47n       A' \Ĵ Ĵ         
         "X" rated  250V         Gate <    - - - - 
                    AC                     ===

The circuit is shown here on the Neutral but in practice may be in the LIVE
side. It is not really suited to Transformer loads as there is a risk of DC
output, but some makers do use it into transformers!

Filtering must be provided to reduce the high frequency content of Anode A''
from getting out of the dimmer. The Choke is often 20 turns on a small ferrite
rod or ring.

The DIAC is a 20-30V (20V on 110V) device, when the 0.1uF cap charges up over
its' strike voltage, the 0.1uF charge is dumped into the TRAIC gate. I have
seen 2 identical low voltage transistors used as instead of a diac, when they
avalanche they produce a similar action, provided the current is not excessive.

                        +30V      Breakdown
 Bases not connected         _.-'
   _____   ____            
____/ \e____e/ \____                 '-._

When the pot is set to minimum, the 0.1uF cap charges up through the 1K almost
instantly. The CR provides some false spike mis-trigger protection. So the
TRIAC is triggered when the Live is about 30V, & stays on, lighting the lamp
until the mains is gone to Zero volts at the end of the half cycle.
  __               __               _
 /  \  MAINS      /  \   FULL       \   HALF         MIN             DC
      IN             POWER         POWER        POWER           MOD
             _     _        __   ___      __\_____  __   _\_________
        \__/             \__/             _/
                                         Max QRM!

When the pot is about half way around, the 0.1uF capacitor reaches 30V around
half way along the half cycle when triggering occurs. This still produces the
full peak voltage, & generally causes the most QRM, but only half the energy is
delivered to the lamp.

When the pot is at maximum the 0.1uF cap only just charges up 30V near the end
of the half cycle. The Traic is then triggered & provides the very low voltage
mains at the end of the half cycle to the lamp, until Zero volts releases the
traic ready for re-triggering.

Due to the uncertainty of triggering the TRIAC when the mains is low etc, as
the 0.1uF may not reach the 30V required, a modification to the circuit is
sometimes incorporated. This applies a small amount of DC to the 0.1uF cap as
the POT resistance increases. This actually puts out unequal portions of the
+/- mains phases causing the non triggering effect to be reduced. But the DC or
half frequency (50Hz not 100Hz) powering can cause visible flicker in small
power lamps! If used on Transformer excited lamps, it will burn out the
from Lod LW1DSE...

It is impotant to add to your intresting bul, that this devices generates lots
of harmonics, that can be irradisted from the load and line wires, acting as
antennas (aerials), and causing lots of QRM's to some sensitive devices.
(TV's, AMradios, ham's, etc) So, I made one of them incorporating a double LC
filter, one in common mode, an other in differential mode to prevent the
harmonics to exit via the wires. This is my circuit, actually in use:

                   .47F                .22F  
                  250VAC   330R 1W      250VAC 
        2 * 39mH  .01F               100H   
                  1kV        <               
L Oo\ooĴ oO
           ======           >\      ======   
               //    TRIAC                  
220VAC      //     BTA24/800              
           //                <  DB3         LAMP
            //    ///            Ĵ ĿDIAC    
           //                      >         
    2Amp   ======                              
N Oo~o(((O
     1F 250V                              
      AC X2          ĴĴ        
T OĿ           .01uF         250VAC    10K     50K
               1kV                           <Ŀ
    ///                        .1uF               

Good luck, and best regars from Barrio Garay, Buenos Aires Argentina!!!


See my buls on "Transistors, SCRs & TRIACs" & "SCR Drill Speed Controller",

Why don't U send an interesting bul? 

73 De John, G8MNY @ GB7CIP

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