Introduction to Triage

In this article we will examine the triage. Tristors with solid state devices can be used to control lamps, motors or heaters, etc. But one of the problems of using a thyristor to control such circuits is that, like diode, the "thyristor" is also a one-way device. It allows only one direction of current to pass through the anode in the Cathode.

For DC switching circuits, this "one-way" switching characteristic is acceptable because once triggered, all DC power is transmitted directly to the load. But in sinusoidal AC switching circuits, this one-way switching can be a problem, as the Door signal transmits only half the cycle (such as a half-wave rectifier) when the anode is positive, no matter what it does. Then only half of the power for AC operation is transmitted to the load by a thyristor.

To achieve full wave power control, we can connect a single thyristor into a full wave bridge rectifier triggered in each positive half wave, or we can connect the two tricircers in reverse parallel (back to back), as shown below. However, this increases both the complexity and the number of components used in the switching circuit.

Tristor Configurations

Triac
Tristor Configurations

However, there is another type of semiconductor device called "Triode AC Switch", or Triac for short, which is also a member of the tristor family and is used as a solid-state power switching device. But more importantly, it is a "duplex" device. In other words, a Triage can be triggered by both positive and negative voltages applied to anthotine and by both positive and negative trigger pulses applied to the Door terminal. Makes it a door-controlled device with two dial switchings.

A Triage acts like two traditional tristors connected in reverse parallel (back-to-back) relative to each other, and due to this arrangement, the two thyristors share a common Gate terminal in a single, three-terminal package.

Since triage transmits in both directions of the sinusoidal waveform, the concept of an Anod terminal and a Cathode terminal, which is used to identify the main power terminals of a thyristor, is replaced by MT1 for Main Terminal 1 and MT2 for Main Terminal 2. The same reference is taken as gate terminal G.

In most AC switching applications, the triage door terminal is associated with the MT1 terminal, similar to the gate-cathode relationship of the thyristor or the base-emitter relationship of the transistor.

Triage Symbol and Structure

Triac
Triage Symbol and Structure

Now we know that a triage is four layers. A positive PNPN and a negative NPNP are three-terminal duplex devices that block the current in the case of "OFF", acting as an open circuit switch. But unlike a traditional thyristor, the triage can transmit the current in both directions when triggered by a single door impact. Then a triage has four possible trigger operating modes as follows.

  • Ι + Mode = MT2 current positive (+and), gate current positive (+and)
  • Ι – Mode = MT2 current positive (+and), gate current negative (-and)
  • ΙΙΙ + Mode = MT2 current negative (-and), gate current positive (+and)
  • ΙΙΙ – Mode = MT2 current negative (-and), gate current negative (-and)

And these four modes, in which a triage can be operated, are shown using the characteristic curves of the triage I-V.

Triyak I-V Characteristic Curves

Triac
Triyak I-V Characteristic Curves

Triage is usually triggered by a positive gate current labeled mode Ι+ above. However, a negative gate current can also be triggered by mode Ι. Similarly, <ΙΙΙ Çeyreğinde, bir negatif gate akımı ile tetikleme, –ΙG de yaygındır, mod ΙΙΙ– ile birlikte mod ΙΙΙ+. However, Ι– and ΙΙI+ modes are less precise configurations that require a larger gate current than Ι+ and ΙΙ, which are more common triage trigger modes to cause triggering.

In addition, like silicone-controlled rectifiers (SCRs), triage requires minimal holding current IH to maintain transmission in waveforms at transition points. Then, even if the two thyristors are combined into a single triage device, they continue to exhibit individual electrical properties, as we expect from a single SCR device, such as different fault voltages, holding currents and trigger voltage levels.

Triage Applications

Triyak is the most widely used semiconductor device for switching ac systems and power control, as it can be made "ON" with a triage, positive or negative Door impact, regardless of the polarity of the AC source at the time. This makes triage ideal for controlling a lamp or AC engine load with a very simple triage switching circuit given below.

Triyak Switching Circuit

Triac
Triyak Switching Circuit

The above circuit shows a simple DC-triggered triage power switching circuit. When the switch SW1 is on, the current does not flow into the triage gate and therefore the lamp becomes "OFF". When SW1 is switched off, the door current is applied from the VG battery source to the triage, through resistance R, and the triage is driven to full transmission, acting as a closed switch, and full power is drawn by the lamp from the sinusoidal source.

Since the battery provides a positive Door current to the triage every time the SW1 switch is switched off, the triage is constantly switched on in Ι+ and ΙΙ+ modes, regardless of the polarity of the MT2 terminal.

Of course, the problem with this simple triage switching circuit is that we need an additional source of positive or negative Gate to trigger triage transmission. But we can also trigger triage using the actual AC supply voltage itself as the door trigger voltage. Consider the circuit below.

Triyak Switching Circuit

Triac
Triyak Switching Circuit

The circuit shows a triage used as a simple static AC power switch that provides an "ON"-"OFF" function similar to the previous DC circuit. When the SW1 switch is on, the triage acts as an open switch and the lamp passes zero current. When SW1 is turned off, the triage is turned on via current limiting resistance R and locks on itself shortly after the start of each half cycle. Thus, it passes full power to the lamp load.

Since the feed is sinusoidal AC, the triage is automatically turned on as an instantaneous supply voltage at the end of each AC semi-cycle, and therefore the load current briefly drops to zero, but in the next half cycle it locks again using the reverse thyristor half. as long as the key remains closed. This type of switching control is often referred to as full wave control due to the fact that both halves of the sinus wave are controlled.

Since the triage is effectively two SCRs connected in a row, we can take this triage switching circuit further by changing how the gate is triggered as shown below.

Modified Triage Switching Circuit

Triac
Modified Triage Switching Circuit

If the SW1 switch is open in position A, there is no door current and the lamp is "OFF". If the switch is switched to position B, the door current flows in the same way as the previous one with each half-loop, and the triathlete is drawn at full power by the lamp as it operates in Ι+ and ΙΙΙ– modes.

Only this time, when the switch is connected to position C, the diode will prevent the door from being triggered when MT2 is negative, as the diode is inverted. Thus, the triage operates only in positive half loops running in I+ mode, and the lamp lights up at half power. Then, depending on the position of the switch, the load is Off, Half Power, or Fully On.

Triage Phase Control

Another common type of triage switching circuit uses phase control to change the amount of voltage for both the positive and negative half of the input waveform, and therefore the power applied to a load, in this case an engine. This type of AC engine speed control provides completely variable and linear control, as the voltage can be adjusted from zero to fully applied voltage.

Triage Phase Control

Triac
Triage Phase Control

This basic phase trigger circuit uses triage in series with the motor through an AC sinusoidal feed. Variable resistance is used to control the amount of phase shift on the VR1 triage gate, which controls the amount of voltage applied to the engine by turning the motor ON at different times during the AC cycle.

The trigger voltage of the triage is derived from the VR1 – C1 combination via Diac (The Diak is a two-way semiconductor device that helps provide a sharp trigger current pulse to fully TURN on the triage).

At the beginning of each cycle, the C1 variable resistance charges through VR1. This continues until the voltage in C1 is sufficient to trigger the transmission of the dialysis, which allows the capacitor to ejaculate into the triage gate of C1 and make it "ON".

When the triage is triggered and saturated for transmission, the parallelly connected gate trigger phase effectively shorts the control circuit and the triage takes control for the rest of the half-cycle.

As we see above, at the end of the half cycle the triage automatically closes, and in the next half loop the VR1 – C1 triggering process starts again.

However, since triage requires different amounts of door current in each switching mode, such as Ι+ and ΙΙΙ– a triage is asymmetrical, meaning it may not be triggered at exactly the same point for every positive and negative half cycle.

This simple triage speed control circuit is suitable not only for AC engine speed control, but also for lamp dimmers and electric heater control, and is actually very similar to the triage light dimmer used in many homes. However, it should not be used as a commercial triage dimmer motor speed controller, as it is generally intended to use triage light dimmers only with resistant loads such as incandescent lamps.

  • A "Triage" is another 4-layer, 3-terminal thyristor device similar to SCR.
  • Triage can be triggered in both directions.
  • There are four possible trigger modes for triage, 2 of which are preferred.

Electric AC power control using triage is extremely effective when used appropriately to control resistant types of loads, such as incandescent lamps, heaters or small universal motors commonly found in portable power tools and small devices.