Triode Thyristor

If a terminal lead is made to one of the base regions, the result will be a triode thyristor. By applying a forward voltage over this lead to the forward-biased junction, or the gate, we can vary the value of U_fbo at will. The greater the gate current I_G , the lower the value of U_fbo .

Fig. 6.3. Current-voltage characteristic of a triode thyristor for several values of control current

A connection diagram for a triode thyristor is shown in Fig. 6.4. Here, the gra­phical symbol is given for a thyristor with a third terminal lead made to its p -region. This is a gate-triggered thyristor because when a forward voltage pulse is fed over the gate lead to the emitter junction of the device, it is driven into the ON state, of course provided that the supply voltage E_A is sufficiently high.

Fig. 6.4. Circuit using a thyristor with a lead from the p -region

Ordinary triode thyristors cannot be turned off by control over the gate circuit. For a turn-off, the current through such a thyristor must be brought down below I_H. The bidirectional thyristors (also known as symmetrical thyristors or symistors, diac, triac) have also been developed. Their name refers to the passage of current through the device and can be turned on by a voltage of either polarity (Fig. 6.5). They may have an n-p-n-p-n or a p-n-p-n-p structure.

Fig. 6.5. Diac: (a) current-voltage characteristic, (b) structure and (c) circuit symbols

Fig. 6.6 Sawtooth voltage generator using a thyristor

Дата добавления: 2015-11-14