Tab Electronics Guide to Understanding Electricity ... - Sciences Club

Power Control
equal in both. Therefore, under these circumstances, the power dissipation
of either is
E
P 2 60
2 3600
72 watts
50 50
R
287
Because the purpose of the circuit shown in Fig. 9-3 is to control the
power delivered to the load (R L
), all of the power dissipated by the rheostat
is wasted. In the previous example, the efficiency of the power control
is 50%. At different settings of the rheostat, different efficiency levels
occur; but it is obvious that this level of waste is unacceptable in highpower
electrical applications.
A disadvantage of using a single SCR for power control, as illustrated
in Fig. 9-2, is that it is not possible to obtain full 360-degree control of an
AC waveform (only 180 degrees, that is, the forward-biasing half-cycle,
can be controlled). To overcome this problem, two SCRs might be incorporated
into a circuit for full-wave power control.
The Triac
Another member of the thyristor family, the triac can be used for fullwave
power control. A triac has three leads designated as the gate, M1,
and M2. Triacs are triggered on by either a positive or negative pulse to
the gate lead, in reference to the M1 terminal. Triacs can also conduct
current in either direction between the M1 and M2 terminals. Like an
SCR, once a triac has been triggered, the gate loses all control until the
current flow through the M1 and M2 terminals drops below the manufacturer’s
specified holding current. Triacs are considered current
devices. The electrical symbols used for triacs are illustrated in Fig. 9-4.
The principle of efficient power control is essentially the same for the
triac as it is for the SCR. Because a triac operates in only two modes (ON
Figure 9-4
Additional thyristor
symbols.
M2
Gate
M1
Triac
Emitter
UJT
B2
B1
Neon
tube