For DC switching circuits this “one-way” switching characteristic may
be acceptable as once triggered all the DC power is delivered straight
to the load. But in Sinusoidal AC Switching Circuits
this unidirectional switching may be a problem as it only conducts
during one half of the cycle (like a half-wave rectifier) when the Anode
is positive irrespective of whatever the Gate signal is doing. Then for
AC operation only half the power is delivered to the load by a
thyristor.
In
order to obtain full-wave power control we could connect a single
thyristor inside a full-wave bridge rectifier which triggers on each
positive half-wave, or to connect two thyristors together in inverse
parallel (back-to-back) as shown below but this increases both the
complexity and number of components used in the switching circuit.
Thyristor Configurations
There is however, another type of semiconductor device called a “Triode AC Switch” or Triac for
short which is also a member of the thyristor family that be used as a
solid state power switching device but more importantly it is a
“bidirectional” device. In other words, a Triac can
be triggered into conduction by both positive and negative voltages
applied to its Anode and with both positive and negative trigger pulses
applied to its Gate terminal making it a two-quadrant switching Gate
controlled device.
A Triac behaves
just like two conventional thyristors connected together in inverse
parallel (back-to-back) with respect to each other and because of this
arrangement the two thyristors share a common Gate terminal all within a
single three-terminal package.
Since
a triac conducts in both directions of a sinusoidal waveform, the
concept of an Anode terminal and a Cathode terminal used to identify the
main power terminals of a thyristor are replaced with identifications
of: MT1, for Main Terminal 1 and MT2 for Main Terminal 2 with the Gate terminal G referenced the same.
In most AC switching applications, the triac gate terminal is associated with the MT1 terminal,
similar to the gate-cathode relationship of the thyristor or the
base-emitter relationship of the transistor. The construction, P-N
doping and schematic symbol used to represent a Triac is given below.
Triac Symbol and Construction
We
now know that a “triac” is a 4-layer, PNPN in the positive direction
and a NPNP in the negative direction, three-terminal bidirectional
device that blocks current in its “OFF” state acting like an
open-circuit switch, but unlike a conventional thyristor, the triac can
conduct current in either direction when triggered by a single gate
pulse. Then a triac has four possible triggering modes of operation as
follows.
- Ι + Mode = MT2 current positive (+ve), Gate current positive (+ve)
- Ι – Mode = MT2 current positive (+ve), Gate current negative (-ve)
- ΙΙΙ + Mode = MT2 current negative (-ve), Gate current positive (+ve)
- ΙΙΙ – Mode = MT2 current negative (-ve), Gate current negative (-ve)
And these four modes in which a triac can be operated are shown using the triacs I-V characteristics curves.
Triac I-V Characteristics Curves
In Quadrant Ι, the triac is usually triggered into conduction by a positive gate current, labelled above as mode Ι+. But it can also be triggered by a negative gate current, mode Ι–. Similarly, in Quadrant ΙΙΙ, triggering with a negative gate current, –ΙG is also common, mode ΙΙΙ– along with mode ΙΙΙ+. Modes Ι– and ΙΙΙ+ are,
however, less sensitive configurations requiring a greater gate current
to cause triggering than the more common triac triggering modes of Ι+ and ΙΙΙ–.
Also, just like silicon controlled rectifiers (SCR’s), triac’s also require a minimum holding current IH to
maintain conduction at the waveforms cross over point. Then even though
the two thyristors are combined into one single triac device, they
still exhibit individual electrical characteristics such as different
breakdown voltages, holding currents and trigger voltage levels exactly
the same as we would expect from a single SCR device.
Triac Applications
The Triac is
most commonly used semiconductor device for switching and power control
of AC systems as the triac can be switched “ON” by either a positive or
negative Gate pulse, regardless of the polarity of the AC supply at
that time. This makes the triac ideal to control a lamp or AC motor load
with a very basic triac switching circuit given below.
Triac Switching Circuit
The circuit above shows a simple DC triggered triac power switching circuit. With switch SW1 open, no current flows into the Gate of the triac and the lamp is therefore “OFF”. When SW1 is closed, Gate current is applied to the triac from the battery supply VG via resistor R and
the triac is driven into full conduction acting like a closed switch
and full power is drawn by the lamp from the sinusoidal supply.
As the battery supplies a positive Gate current to the triac whenever switch SW1 is closed, the triac is therefore continually gated in modes Ι+ and ΙΙΙ+ regardless of the polarity of terminal MT2.
Of
course, the problem with this simple triac switching circuit is that we
would require an additional positive or negative Gate supply to trigger
the triac into conduction. But we can also trigger the triac using the
actual AC supply voltage itself as the gate triggering voltage. Consider
the circuit below.
Triac Switching Circuit
The
circuit shows a triac used as a simple static AC power switch providing
an “ON”-“OFF” function similar in operation to the previous DC circuit.
When switch SW1 is open, the triac acts as an open switch and the lamp passes zero current. When SW1 is closed the triac is gated “ON” via current limiting resistor R and self-latches shortly after the start of each half-cycle, thus switching full power to the lamp load.
As
the supply is sinusoidal AC, the triac automatically unlatches at the
end of each AC half-cycle as the instantaneous supply voltage and thus
the load current briefly falls to zero but re-latches again using the
opposite thyristor half on the next half cycle as long as the switch
remains closed. This type of switching control is generally called
full-wave control due to the fact that both halves of the sine wave are
being controlled.
As
the triac is effectively two back-to-back connected SCR’s, we can take
this triac switching circuit further by modifying how the gate is
triggered as shown below.
Modified Triac Switching Circuit
As above, if switch SW1 is open at position A, there is no gate current and the lamp is “OFF”. If the switch is moved to position B gate current flows at every half cycle the same as before and full power is drawn by the lamp as the triac operates in modes Ι+ and ΙΙΙ–.
However this time when the switch is connected to position C, the diode will prevent the triggering of the gate when MT2 is
negative as the diode is reverse biased. Thus the triac only conducts
on the positive half-cycles operating in mode I+ only and the lamp will
light at half power. Then depending upon the position of the switch the
load is Off, at Half Power or Fully ON.
Triac Phase Control
Another
common type of triac switching circuit uses phase control to vary the
amount of voltage, and therefore power applied to a load, in this case a
motor, for both the positive and negative halves of the input waveform.
This type of AC motor speed control gives a fully variable and linear
control because the voltage can be adjusted from zero to the full
applied voltage as shown.
Triac Phase Control
This
basic phase triggering circuit uses the triac in series with the motor
across an AC sinusoidal supply. The variable resistor, VR1 is
used to control the amount of phase shift on the gate of the triac
which in turn controls the amount of voltage applied to the motor by
turning it ON at different times during the AC cycle.
The triac’s triggering voltage is derived from the VR1 – C1 combination via the Diac (The diac is a bidirectional semiconductor device that helps provide a sharp trigger current pulse to fully turn-ON the triac).
At the start of each cycle, C1 charges up via the variable resistor, VR1. This continues until the voltage across C1 is sufficient to trigger the diac into conduction which in turn allows capacitor, C1to discharge into the gate of the triac turning it “ON”.
Once
the triac is triggered into conduction and saturates, it effectively
shorts out the gate triggering phase control circuit connected in
parallel across it and the triac takes control for the remainder of the
half-cycle.
As we have seen above, the triac turns-OFF automatically at the end of the half-cycle and the VR1 – C1 triggering process starts again on the next half cycle.
However, because the triac requires differing amounts of gate current in each switching mode of operation, for example Ι+ and ΙΙΙ–,
a triac is therefore asymmetrical meaning that it may not trigger at
the exact same point for each positive and negative half cycle.
This
simple triac speed control circuit is suitable for not only AC motor
speed control but for lamp dimmers and electrical heater control and in
fact is very similar to a triac light dimmer used in many homes.
However, a commercial triac dimmer should not be used as a motor speed
controller as generally triac light dimmers are intended to be used with
resistive loads only such as incandescent lamps.
Then we can end this Triac Tutorial by summarising its main points as follows:
- A “Triac” is another 4-layer, 3-terminal thyristor device similar to the SCR.
- The Triac can be triggered into conduction in either direction.
- There are four possible triggering modes for a Triac, of which 2 are preferred.
Electrical AC power control using a Triac is
extremely effective when used properly to control resistive type loads
such as incandescent lamps, heaters or small universal motors commonly
found in portable power tools and small appliances.
But
please remember that these devices can be used and attached directly to
the mains AC power source so circuit testing should be done when the
power control device is disconnected from the mains power supply. Please
remember safety first!.
