THIS circuit is used to control brightness of mains lights and the speed of AC motors. It
uses a triac, diac and has a radio-frequency interference
(RFI) noise suppression circuit built into it as well.
Caution: this Kit connects directly to mains power supply.
You must know what you are doing as mains voltages can
be lethal. The board must be put in a suitable enclosed box
before using.
The kit is constructed on a single-sided printed circuit
board (PCB). Protel Autotrax and Schematic were used to
design the board.
ASSEMBLY INSTRUCTIONS
It is generally best to add the lowest height components to
the board first; the resistors. Make sure to get the triac in
the correct way - the metal back of the triac goes in
above the bar marked on the overlay. The diac looks like
a glass diode but it has a mark around the centre, not at
one end. The choke (wire wound ferrite coil) can be
soldered in either way around.
CIRCUIT DESCRIPTION
This is a standard text-book circuit. A triac may be
considered as two SCR's (Silicon Controlled Rectifiers
connected in opposite directions. A diac is a gate trigger
device. Triacs, diacs & SCR's are different types of
Thyristors.
A triac is a 3 terminal ac semiconductor switch which is
triggered ON when a low energy signal is applied to its
Gate. Switching is fast. The low energy of switching
means that a wide range of low cost control circuits can
be used, for example, optically coupled switches. Since
the triac is bilateral (2 SCR's connected in opposite
directions) the terms anode and cathode have no meaning.
So the terms Main Terminal 1 and 2 (MT1, MT2) are
used. It is standard to use MT1 as a reference
point.
The circuit here controls the average power to a load
through the triac by phase control. The ac supply is
applied to the load for only a controlled fraction of each
cycle. The triac is held in an OFF condition for a portion
of its cycle then is triggered ON at a time determined by
the circuit. The main problem with this circuit is radio
frequency interference (RFI.)
RFI. Each time the triac is turned on the load current
changes very quickly - a few micro seconds - from zero to
a value determined by the lamp resistance and the value
of the mains voltage at that instant in time. This transition
generates RFI. It is greatest when the triac is triggered at
90o and least when it is triggered at close to zero or 180o
of the mains AC waveform.
Since there may be long lengths of mains wire between
the triac and the lamp load which will radiate this RFI an
L-C RFI suppression network is usually built into these
types of circuits. You may detect this RFI by bringing an
FM radio close to the dimmer circuit. Short out the choke
coil and notice that the RFI increases. The wire-wound
coil and C1 provide the RFI suppression network.
WHAT TO DO IF IT DOES NOT WORK
Poor soldering is the most likely reason that the circuit
does not work. Check all solder joints carefully under a
good light. Next check that all components are in their
correct position on the PCB.
WORKING OF THE CIRCUIT:
In this circuit, the traic uses the property of phase control and thus controls the average
power supplied to a load. The ac power is supplied to
the load for only a controlled time
frame of each AC cycle.
Triac is
actually a three terminal device, which is triggered on when a positive or
negative
current passes through its gate terminal. The triac
remains in an OFF condition for a portion
of AC cycle
then is turned ON (triggered on by diac) for a time period determined by
the circuit. The triac turns on when the diac starts
conduction. The diac is turned on when the
voltage reaches the break over voltage i.e. VBO. Diode
then starts conduction and allows
current toward the gate of triac. When the voltage
changes to below VBO the diac is off, so it
turns off the triac too. Diac mostly operates at low
voltages, its VBO is usually around 30 V.
There is also a problem with this circuit, which is Radio Frequency
Interference or RFI. This RFI generates in a way that
when the triac is turned on, each time
in a cycle, the current to the load changes very
quickly i.e. in a few microseconds it goes
from zero to a certain value determined by the
resistance of load and the mains line voltage at
that time. This sudden change causes RFI to generate.
This interference is highest when the traic triggers at 90o of the AC wave, and is lowest when
it triggers at 0o or 180o of the
AC wave cycle.
