Refer to the temperature controller of Fig. 17.13.

a. Sketch the waveform of the full-wave rectified waveform across the SCR.
b. What is the peak current through the heater when the SCR is “on” and has a short-circuit equivalent between anode and cathode? Assume each diode has a drop of 0.7 V when conducting.
c. When the SCR is on, what is the maximum current through the thermostat?
d. What is the total time for the rise time of the positive pulse of the applied ac voltage from 0 V to the maximum voltage of the rectified signal?
e. What is the time constant of the capacitor that is charging during the same period of part (d)? How do they compare? Why is this a concern?
f. What is the state of the SCR during this charging period? Why?
g. If the gate-firing potential is 40 V, what is the time period between the successive triggering of the SCR?
h. Once the thermostat reaches its set temperature and assumes the short-circuit state, how will the SCR react?
i. What method was used to turn the SCR off anode current interruption or forced commutation?

Fig. 17.13

Transcribed Image Text:

100-W heater load D1 D2 SCR GE C58 120 V ac 60 Hz D4 R1 510 kΩ 0.1 µF Twist leads to minimize pickup Hg in glass thermostat FIG. 17.13