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TRIACfrom triode for alternating currentis a generic trademark for a three terminal electronic component that conducts current in either direction when triggered. Its formal name is bidirectional triode thyristor or bilateral triode thyristor.
A thyristor is analogous to a relay in that a small voltage and current can control a much larger voltage and bg136.
Once triggered, SCRs and TRIACs continue to conduct, even if the gate b136 ceases, until the main current drops below a certain level called the holding current. In addition, applying a trigger at a controlled phase angle of the AC in the main circuit allows control of the average current flowing into a load phase control. This is commonly used for controlling the speed of induction motorsdimming lamps, and controlling electric heaters.
Quadrants, 1 top right2 top left3 bottom left4 bottom right. The four quadrants are illustrated in Figure 1, and depend on the gate and MT2 voltages with respect to MT1.
The relative sensitivity depends on the physical structure of a particular triac, but as a rule, quadrant I is the most sensitive least gate current requiredand quadrant 4 is the least sensitive most gate current required. The N region attached to MT2 does not participate significantly. The N region vt136 to MT2 is active, but the N region attached to MT1 only participates in the initial triggering, not the bulk current flow.
In most applications, the gate current comes from MT2, so quadrants 1 and 3 are the only operating modes both gate and MT2 positive or negative against MT1. Other applications with single polarity triggering from an IC or digital drive circuit operate in quadrants 2 and 3, than MT1 is usually connected to positive voltage e.
Quadrant 1 operation occurs when the gate and MT2 are positive with respect to MT1. The mechanism is illustrated in Figure 3. The gate current makes an equivalent NPN transistor switch on, which in turn draws current from the base of an equivalent PNP transistor, turning it on also.
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. This, in turn, causes the p, n and p layers over MT2 to behave like a PNP transistor, which turns on because its n-type base becomes forward-biased with respect to its emitter MT2.
Thus, the triggering scheme is the same as an SCR. The equivalent circuit is depicted in Figure 4. However, the structure is different from SCRs. 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. Generally, this quadrant is the most sensitive of the four. This is because it is the only quadrant where gate current is injected directly into the base of one of the main device transistors.
Quadrant 2 operation occurs when the gate is negative and MT2 is positive with respect to MT1. Figure 5 shows the triggering process. The turn-on of the device is three-fold and starts when the current from MT1 flows into the gate through the p-n junction under the gate. This switches on a structure composed by an NPN transistor and a PNP transistor, which has the gate as cathode the turn-on of this structure is indicated by “1” in the figure. As current into the gate increases, the potential of the left side of the p-silicon under the gate rises towards MT1, since the difference in potential between the gate and MT2 tends to lower: 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.
Quadrant 3 operation occurs when the gate and MT2 are negative with respect to MT1. The whole process is outlined in Figure 6. The process happens in different steps here too. In the first phase, the pn junction between the MT1 terminal and the gate becomes forward-biased step 1. As forward-biasing implies the injection of minority carriers in the two layers joining the junction, electrons are injected in the p-layer under the gate.
Some of these electrons do not recombine and escape to the underlying n-region step 2. This in turn lowers the potential of the n-region, acting as the base of a pnp transistor which switches on turning the transistor on without directly lowering the base potential is called remote gate control. The lower p-layer works as the collector of this PNP transistor and has its voltage heightened: Therefore, the red arrow labeled with a “3” in Figure 6 shows the final conduction path of the current.
Quadrant 4 operation occurs when the gate is positive and MT2 is negative with respect to MT1. Triggering in this quadrant is similar to triggering in quadrant III. The process uses a remote gate control and is illustrated in Figure 7. As current flows from the p-layer under the gate into the n-layer under MT1, minority carriers in the form of free electrons are injected into the p-region and some of them are collected by the underlying n-p junction and pass into the adjoining n-region without recombining.
As in the case of a triggering in quadrant III, this lowers the potential of the n-layer and turns on the PNP transistor formed by the n-layer and the two p-layers next to it. Generally, this quadrant is the least sensitive of the four [ clarification needed Why is quadrant 4 the least sensitive?
See discussion ] In addition, some models of TRIACs logic level and snubberless types cannot be triggered in this quadrant but only in the other three.
TRIAC – Wikipedia
In this section, a few triax summarized. 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. The minimum current able to do this is called gate threshold current and is generally indicated by I GT. In a typical TRIAC, the gate threshold current is generally a few milliamperes, but one has to take into account also that:.
When the gate current is discontinued, if the current between troac two main terminals is more than what is called the latching currentthe device continues to conduct.
Latching current is the minimum current that keeps the device internal structure latched in the absence of gate current. The value of this parameter varies with:. In particular, if the pulse width of the gate current is sufficiently large generally some tens of microsecondsthe TRIAC has completed the triggering process when the gate signal is discontinued and the latching current reaches a minimum level called holding current.
What is a TRIAC – Tutorial
Holding current is the minimum required current flowing between the two main terminals that keeps the device on after it has achieved commutation in every part of its internal structure. In datasheets, the latching current is indicated as I Lwhile the holding current is indicated as I H.
They are typically in the order of some milliamperes. The turn-on is due to a parasitic capacitive coupling of the gate terminal with the MT2 terminal, which lets currents into the gate in response to a large rate of voltage change at MT2.
In many cases this is sufficient to lower the impedance of the gate towards MT1. 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.
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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.
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 reason is that during the commutation, the power dissipation is not uniformly distributed across the device.
When switching on, the device starts to conduct current before the conduction finishes to spread across the entire junction. The device typically starts to conduct the current imposed by the external circuitry after some nanoseconds trac microseconds but tb136 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.
The current and voltage are out of phase, so when the current decreases below the holding value, the TRIAC attempts to turn off, but because of the phase shift between btt136 and voltage, a sudden voltage step takes place between the two main terminals, which turns the device on again.
This is similar to the recovery in standard diodes: Because in the TRIAC there are parasitic resistances, a high reverse current in the p-n junctions inside it can provoke a voltage drop between the gate region and the MT1 region which may make the TRIAC stay turned on. Suppose one wants to turn the inductor off: 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.
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. 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.
TRIACs may also fail to turn on reliably with reactive loads if the current phase shift causes the main circuit current to ttriac below the holding current at trigger time.
Low-power TRIACs are used in many applications such as light dimmersspeed controls for electric fans and other electric ttriacand in the modern computerized control circuits of many household small and major appliances. 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 one of the mains supply.
In these situations it is normal to connect the neutral terminal to the positive rail of the microcontroller’s power supply, together with A1 of the triac, with A2 connected to the live. The TRIAC’s gate can be connected through an opto-isolated transistor, and sometimes a resistor to the microcontroller, so that bringing the voltage down to the microcontroller’s logic zero pulls enough current through the TRIAC’s gate to trigger it.
These devices are made specifically for improved commutation and can often control reactive loads without the use of a snubber circuit. Later versions are sold under the trademark “Snubberless”. Littelfuse also uses the name “Alternistor”. Other three-quadrant TRIACs can operate with smaller gate-current to be directly driven by logic level components. From Wikipedia, the free encyclopedia.
For the green vehicle, see Triac car. For triiodothyroacetic acid, see Tiratricol. Retrieved June 28, Potentiometer digital Variable capacitor Varicap. Capacitor types Ceramic resonator Crystal oscillator Inductor Parametron Relay reed relay mercury switch.