Other applications with single polarity triggering from an IC or digital drive circuit operate in quadrants 2 and 3, where MT1 is usually connected to positive voltage (e.g. +5V) and gate is pulled down to 0V (ground).

Part of the gate current (dotted line) is lost through the ohmic path across the p-silicon, flowing directly into MT1 without passing through the NPN transistor base.

In this case, the injection of holes in the p-silicon makes the stacked n, p and n layers beneath MT1 behave like a NPN transistor, which turns on due to the presence of a current in its base.

In particular, TRIAC always has a small current flowing directly from the gate to MT1 through the p-silicon without passing through the p-n junction between the base and the emitter of the equivalent NPN transistor.

This is because it is the only quadrant where gate current is injected directly into the base of one of the main device transistors.

[4] Quadrant 2 operation occurs when the gate is negative and MT2 is positive with respect to MT1.Figure 1 Figure 5 shows the triggering process.

So, in the end, the structure which is crossed by the major portion of the current is the same as quadrant-I operation ("3" in Figure 5).

Therefore, the red arrow labeled with a "3" in Figure 6 shows the final conduction path of the current.

Therefore, the red arrow labeled with a "3" in Figure 6 shows the final conduction path of the current.

A TRIAC starts conducting when a current flowing into or out of its gate is sufficient to turn on the relevant junctions in the quadrant of operation.

By putting a resistor or a small capacitor (or both in parallel) between these two terminals, the capacitive current generated during the transient flows out of the device without activating it.

A careful reading of the application notes provided by the manufacturer and testing of the particular device model to design the correct network is in order.

[5] Normal TRIACs, except for low-power types marketed as sensitive gate,[6] already have such a resistor built in to safeguard against spurious dv/dt triggering.

and, as mentioned before, is in relation to the tendency of a TRIAC to turn on from the off state after a large voltage rate of rise even without applying any current in the gate.

A high rate of rise of the current between MT1 and MT2 (in either direction) when the device is turning on can damage or destroy the TRIAC even if the pulse duration is very short.

The device typically starts to conduct the current imposed by the external circuitry after some nanoseconds or microseconds but the complete switch on of the whole junction takes a much longer time, so too swift a current rise may cause local hot spots that can permanently damage the TRIAC.

[2] The commutating dv/dt rating applies when a TRIAC has been conducting and attempts to turn off with a partially reactive load, such as an inductor.

The reason why commutating dv/dt is less than static dv/dt is that, shortly before the device tries to turn off, there is still some excess minority charge in its internal layers as a result of the previous conduction.

When used to control reactive (inductive or capacitive) loads, care must be taken to ensure that the TRIAC turns off correctly at the end of each half-cycle of the AC in the main circuit.

[3] An electric motor is typically an inductive load and off-line power supplies—as used in most TVs and computers—are capacitive.

Snubber circuits are also used to prevent premature triggering, caused for example by voltage spikes in the mains supply.

This, however, increases the required trigger current or adds latency due to capacitor charging.

On the other hand, a resistor between the gate and MT1 helps draw leakage currents out of the device, thus improving the performance of the TRIAC at high temperature, where the maximum allowed dv/dt is lower.

Values of resistors less than 1kΩ and capacitors of 100nF are generally suitable for this purpose, although the fine-tuning should be done on the particular device model.

Because each SCR will have an entire half-cycle of reverse polarity voltage applied to it, turn-off of the SCRs is assured, no matter what the character of the load.

To overcome the problem DC or a pulse train may be used to repeatedly trigger the TRIAC until it turns on.

When mains voltage TRIACs are triggered by microcontrollers, optoisolators are frequently used; for example optotriacs can be used to control the gate current.

Alternatively, where safety allows and electrical isolation of the controller isn't necessary, one of the microcontroller's power rails may be connected to one of the mains supply.

These devices are made specifically for improved commutation and can often control reactive loads without the use of a snubber circuit.

The first TRIACs of this type were marketed by Thomson Semiconductors (now ST Microelectronics) under the name "Alternistor".