Page 550 - The Mechatronics Handbook

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FIGURE 20.32 Triac characteristic.

FIGURE 20.33 Biploar junction transistors (BJTs).

Triac
The thyristor can only be turned on in the forward biased direction. A triac is a controlled switch that
is equivalent to a pair of thyristors that are connected in an anti-parallel configuration, see Fig. 20.32.
As depicted in Fig. 20.32, a triac can be turned on in both the reverse and forward directions.
Transistors
A transistor is a semiconductor device that has three or more terminals and can provide power amplifi-
cation and switching. As we have seen in the previous discussions, electronic switching can be accom-
plished through either diodes or thyristors. Diode switching does not provide any control freedom. A
thyristor is a three-terminal device and the third (gate) terminal can be used to control and switching
instant. However, one drawback of thyristor switching is that the switching control is only in one direction,
i.e., the gate terminal can only be used to turn on the device. The thyristor switch can only be turned
off by dropping the anode current to zero.
A transistor is a special semiconductor device that can be used for power amplification by modulating
a relatively large current between or voltage across two terminals using a small control current or voltage,
and switching by effectively opening and closing the connection between two terminals using a controlled
signal on the third terminal.
Transistors form the basis of modern electronics and are the fundamental building blocks for digital
electronics, operational amplifiers, and power electronics. There are three common types of transistors,
bipolar junction transistors (BJTs), metal-oxide field effect transistors (MOSFETs), and insulated gate
bipolar transistors (IGBTs).
Bipolar Junction Transistors (BJTs)
A bipolar junction transistor is a three-layer device that is made of the p-n-p or the n-p-n combinations
of semiconductors, see Fig. 20.33. BJTs have three terminals connected to each of the three layers called

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