is
reached. The current is then "ON" for the duration of the cycle, which is
the conduction angle 
. The average or DC current can be
determined by
Objectives
Background
The diode seen in the previous block is a simple switch. It is "ON" whenever the applied voltage provides forward bias. There are many applications where a controlled switch is required. We have already examined one controlled switch, the power MOSFET. Another important controlled switch is the thyristor. Thyristors have the ability to control large amounts of power with a minimum of control energy. They are commonly used for rectification (AC to DC), inversion (DC to AC), and many other applications.
One important type of thyristor, the silicon controlled rectifier (SCR) is different from the power MOSFET in that it only conducts in one direction. It is basically like a diode except it can be turned "ON" at any point in the cycle. The symbol for an SCR is shown in Fig. 1. It has three terminals, the anode, cathode, and gate. A current pulse is applied to the gate to start conduction. Once conduction is started, the pulse is no longer necessary, and the SCR remains in conduction until the current goes to zero or some other technique is used to force it to stop conducting.
An SCR is shown between a sinusoidal source and a resistive load in Fig. 2a.
The applied voltage v and the corresponding current i are shown in
Fig. 2b. Note the two different angles associated with the current
waveform. The current is off until the firing angle is
reached. The current is then "ON" for the duration of the cycle, which is
the conduction angle . The average or DC current can be
determined by
............(1)Another important type of thyristor, the triac, basically consists of two SCRs in reverse parallel (or shunt) so that once turned on, current can flow in both directions. The triac is the device of choice when simple switching of a 60 Hz 120 or 240 V circuit is desired. It is readily available in current ratings up to 40 A or more and in voltage ratings up to 400 V. The triac package often contains electrical isolation like an opto-isolator so it can be turned on by logic level voltages. It turns on at the first voltage zero after the control voltage is applied, and turns off at the first current zero after the control voltage is removed. This prevents transients on both the source and load, and is a highly desirable feature. If higher voltages must be switched, or higher currents, or faster switching speed is needed, then SCRs or power MOSFETs must be used. But where the triacs capabilities of voltage, current, and speed are adequate, it is virtually impossible to beat with any other device.
Discussion and Calculations
= 0o , 45o , 90o, and
135o? Assume R = 12 
and v = 80 sin 
t.
Instructional Activity in Class
With the SCR conducting, increase the variable autotransformer setting until the peak voltage across the load is 80 volts, as read by CH1. Record the AC AMPS and DC AMPS readings and sketch the output voltage. Also record the voltage shown on the DC VOLTS, AC VOLTS, and the digital multimeter set to AC VOLTS. What quantities are these three meters reading? Can you verify the experimental readings by theoretical computations? Do your calculations here, but give the detailed answer in the Conclusion section. Use the measured value of resistance and information from Block E to determine the average power in the load. How do the average and rms currents you measured compare with those you would expect from the information in Block E?
resistor to
give four different firing angles, as close to 0o as possible, (this is
probably around 20o), 45o, 90o, and 135o. Show how you
measured these angles on the oscilloscope face. Adjust the variable
autotransformer so that the peak voltage across the load is 80 volts, as
read by CH1. Then record the average and rms voltages for each firing angle.
Compare results with those predicted from Activity 1. Sketch the voltage
waveform across the load for each firing angle.
Using the Fluke Harmonic Analyzer, measure the harmonic content of the input current for each firing angle. Record the THD and the amplitudes of the first 10 harmonics.
Turn the control knob fully counterclockwise. There should be no voltage or current. Turn the control knob slowly clockwise until the circuit just starts to conduct. Observe this firing angle. Now turn the control knob slowly counterclockwise until the circuit just stops conducting. Observe the firing angle and note its value just before conduction stopped. The difference between these two firing angles is known as the hysteresis of the circuit.
Conclusion
