Triac

Triac is an abbreviation for Triode for Alternating Current. In German, the terms triac or Symistor can be found. Is an electronic device having the semiconductor layer structure, which is an anti-parallel circuit of two thyristors in principle. This makes it possible to switch AC, whereas a single thyristor may switch in one direction only and thus acts as a diode in operation.

Construction

A triac has a control electrode G ( engl. gate) and two main electrodes H1 and H2 ( in engl. Main Terminal, MT1 and MT2 ) where main electrode H2 (MT2 ) usually has a direct connection to the housing. Thus a sufficient control terminal for the two thyristors, two ignition or Hilfsthyristorenstrecken are installed so that it can be tilted with positive and negative control pulses in the low resistance state in the triac.

Although the analogy suggests the anti-parallel thyristors that H1 ( MT1) and H2 ( MT2) are equivalent and can be exchanged arbitrarily; However, this is not the case, because the internal structure is asymmetrical and H1 ( MT1), as indicated in the diagram, a direct connection to the gate has. Therefore, the control voltage is always based on H1 ( MT1), a triac can be triggered with a positive as well as negative gate current. However, it should be noted that the ignition sensitivity depends on the one hand of the gate current and the polarity of the polarity of H1 ( MT1) and H2 ( MT2). It refers to the Zündarten with I , I -, III and III in relation to the associated output characteristic. The largest ignitablity triacs usually with a positive gate voltage, ie in the hazard type I and III.

Triacs can not be made for very large currents, so individual thyristors are in the field of power electronics is still used.

Opto- triac

The opto - triac ( also called photo - triac ) is composed of two anti-parallel arranged photo - thyristors, which are optically activated. Instead, the control electrode of a light emitting diode is used to flood the pn junctions on the photoelectric effect of the charge carriers and thus to render it conductive. This allows electrical isolation between control and load circuit, which loads such as incandescent lamps a light organ (typ. 5-12 V) can be safely driven with the low voltage source of the control electronics.

Applications

The applications are primarily in the area of ​​the phase control than typical variant of the power position in the AC voltage range, such as for brightness control of incandescent lamps (dimmers ), light organs or speed control of universal motors. The latter are used, for example in power tools, electric motor appliances and more frequently where low-loss speed control and high starting torques are required. Opto- triac found in solid-state relay applications where they are used to electrically isolated switching the actual switching element ( power thyristors or triac ). Due to their compact design, low production costs and the fact that, in principle, arbitrarily high isolation voltage can be achieved by the use of fiber optics, opto- triac have the ignition transformer previously used for this purpose displaced. In rare cases, if only a very small amount of power needs to be connected ( to about 100 mW), an opto - triac can also be used for the direct switching of the load.