_______________________________________________________________________ Special Devices
3-49. Figure 3-25 shows the basic circuit, which shows the difference in current control
between the SCR and the triac by comparing their operation. In the circuit shown in Figure
3-25, view (A), the SCR is connected in the familiar half-wave arrangement. Current will
flow through the load resistor (RL) for one alternation of each input cycle. Diode CR1 is
necessary to ensure a positive trigger voltage. In the circuit shown in Figure 3-25, view
(B), with the triac inserted in the place of the SCR, current flows through the load resistor
during both alternations of the input cycle. Since either alternation will trigger the gate of
the triac, CR1 is not required in the circuit.
Figure 3-25. Comparison of SCR and Triac Circuits
3-50. Current flowing through the load will reverse direction for half of each input cycle.
To clarify this difference, Figure 3-26 shows a comparison of the waveforms seen at the
input, gate, and output points of the two devices.
Figure 3-26. Comparison of SCR and Triac Waveforms
23 June 2005