ECE121 Experiment No. 3 - SCR AC Power Control

SCR AC POWER CONTROL

Laboratory Exercise No. 3

Performance Objectives

A. Familiarize yourself with the operation of a half-wave variable resistor phase-control circuit.

B. Familiarize yourself with the operation of a half-wave RC diode phase control-circuit.

C. Determine the true power delivered to the load in a half-wave phase control circuit.

Basic Concepts

1. The amount of power delivered top a load can be controlled with an SCR conducting over a portion of the input cycle.

2. Phase control means controlling the phase of the trigger with respect to the phase of the anode voltage thus limiting SCR conduction time.

3. The conduction angle of an SCR is the amount of time in electrical degrees that the SCR conducts and delivers power to the load.

4. Phase retard is the amount of time in electrical degrees that the gate trigger is delayed with respect to the anode voltage when both are energized from the same AC source.

Equipment and Materials

ü Power Source 6.3 Vac, 200mA

ü Electronic VOM

ü Bread board

ü Oscilloscope

ü C1 0.1μF

ü CR1, CR2 Silicon Diode

ü DS1 Miniature Lamp

ü Q1 SCR, C106B1

ü R1 10KΩ, 1W

ü R2 470KΩ, 1W

ü R3 100KΩ, 1W

ü S1 SPST component module

Objective A. Familiarize yourself with the operation of a half-wave variable resistor phase control circuit.

1. a) Look at the circuit shown in Fig. 1. Closing S1 applies 6.3 Vac between anode and cathode of Q1 through DS1, and across gate trigger circuit R1, CR1 and R2. DS1 represents the load and remains off until Q1 fires. R1 controls the amplitude of the firing voltage and thus controls the conduction time of Q1. Firing occurs only on the positive half-cycle, the SCR turns off automatically on the negative cycle. Diode CR1 blocks the negative half-cycle from the gate. Connect the circuit as shown and adjust R1 for minimum trigger circuit resistance.

Figure 1

b) Turn on the 6.3 Vac power supply.

c) Close switch S1. Does DS1 light? Yes.

d) Monitor the waveform across DS1 using the oscilloscope. Describe the waveform.

The waveform is a half-wave with an 8 ms half-cycle and 1.3 Vp.

e) Slowly increase the resistance of R1 and describe what happens to the half sine-wave.

As R1 increases, the firing delay angle of the half sine-wave also increases with a 90^o maximum conduction angle.

f) Would you say that Q1 is conducting during the portion of the waveform that is not displayed? No.

g) Refer to Fig. 2. The conduction angle is the amount of time that Q1 conducts in electrical degrees. Record the minimum and maximum conduction angle of Q1 in your circuit.

Conduction angle _(min) = 90 degrees

Conduction angle _(max) = 180 degrees

h) The triggering delay introduced by R1 with respect to the anode voltage is referred to as phase retard, and is stated in electrical degrees. In this circuit it is the difference between the conduction angle and 180^o (Phase retard = 180^o – conduction angle). Record the minimum and maximum values of phase retard in your circuit.

Phase Retard _(min) = 0 degrees

Phase Retard _(min) = 90 degrees

i) How does R1 control phase retard in this circuit?

As R1 increases, SCR turns off because I_G is less than the holding current of SCR. R1 limits the gate current during the positive portion of the input signal. When R1 increases, I_G decreases which increases the phase retard of the circuit. Thus, varying R1 controls phase retard.

j) Why can’t R1 retard the phase more than 90 degrees?

As R1 is decreased from the maximum, I_G will increase from the same input voltage. The control cannot exceed 90 degrees phase retard since the input is at maximum at this point.

Figure 2

k) Would you say that R1 is capable of controlling power delivered on the load? Yes.

l) Open switch S1. DS1 should go out.

Objective B. Familiarize yourself with the operation of a half-wave RC-diode phase control circuit.

2. a) Change your circuit to that shown in Fig. 3. The firing point is controlled by the RC time constant of R1 and C1 in this circuit. C1 charges during the positive half cycle to fire the SCR. It is discharged through CR2 on the negative half cycle, resetting if for the next charging cycle. The phase retard can be varied over the greater angle with this circuit.

Figure 3

b) Adjust R1 for minimum trigger circuit resistance.

c) Close S1. Does DS1 light? Yes.

d) Monitor the load voltage waveform across DS1 using the oscilloscope. Record the maximum conduction angle.

Conduction angle _(max) = 180 degrees

e) Slowly increase R1 for maximum resistance while observing the oscilloscope waveform. Record the minimum conduction angle.

Conduction angle _(max) = 22 degrees

f) What are the minimum and maximum values of the phase retard?

Phase Retard _(min) = 0 degrees

Phase Retard _(max) = 158 degrees

g) Why does this circuit give a greater range of phase retard control than the previous circuit?

Because the capacitor charges during the positive half-cycle in the circuit, it supplies and triggers the gate of the SCR to switch on during the

h) Open switch S1.

Objective C. Determine the true power delivered to the load in a half-wave phase control circuit.

3. a) When power delivered to a load in the form of pulses, it must supply the load requirements from one pulse to the next. True power is not the power contained in the pulse but ii is the average power over a complete cycle. To find true power it is necessary to find the effective (rms) value of voltage or current in the complete waveform. The indications provided by standard ac and dc voltmeters and ammeters are not directly useable. Most ac meters are average sensing instruments calibrated to read the rms of a sine-wave rather than a complex waveform. Dc meters will indicate the average of the waveform. The rms value can be calculated mathematically but requires a detailed analysis for each different type of waveform. Fig. 4 shows a graph that plots the ratio E_rms / E_peak as a function of phase retard angle for a half-wave phase control circuit of the type used in this exercise. This graph has been determined mathematically and is similar to various graphs of this type used to find the values in a complex waveform. By measuring E_peak with an oscilloscope and finding the value of the ratio from the graph, E_rms can be calculated. If the value load resistance is known, the true power then can be calculated.

b) Remove lamp DS1 and replace it with 100Ω resistor R3 in your circuit.

c) Adjust R1 for minimum trigger circuit resistance.

d) Close S1.

Figure 4

e) Connect the oscilloscope across load resistor R3 and measure the peak value of voltage.

E_R3 = 2.5 Vpk

f) With R1 set to minimum, the phase retard angle should be zero degrees. Refer to Fig. 4 and locate the ratio E_rms / E_peak for a phase retard angle of 0 degrees.

E_rms / E_peak = 0.5

g) Calculate the value of E_rms across R3 using the value of peak voltage measured in (e) and the E_rms / E_peak ratio determined in (f).

E_rms / E_peak = ratio

E_rms = E_peak x ratio

E_rms = (2.5) (0.5)

E_rms = 1.25 V

h) Calculate the true value of power across the load using your calculated value of E_rms and the indicated value of R3.

P_T = E_rms ² / R3

P_T = (1.25) ² / 100

P_T = 1.5625 mW

i) Adjust the oscilloscope sweep speed for exactly one cycle in 10 cm. Each cm represents 36 electrical degrees.

j) Adjust R1 for phase retard angle of 90 degrees. The conduction angle will be 90 degrees 2.5 cm. Record the peak value of voltage from the oscilloscope waveform then refer to Fig. 4 and record the E_rms / E_peak ratio for a 90 degree phase retard angle.

E_R3 = 2.5 Vpk

E_rms / E_peak = 0.35

k) Calculate E_rms and P_T as in the preceding steps for a phase retard angle of 90 degrees.

E_rms = E_peak x ratio

E_rms = (2.5) (0.35)

E_rms = 0.875 V

P_T = E_rms ² / R3

P_T = (0.875) ² / 100

P_T = 7.656 mW

i) Open S1 and turn off the 6.3 Vac power source.

Conclusion

• Based in the experiment, gate current will flow during the positive portion of the input signal, thereby turning the SCR on since anode-to-cathode connection is forward biased. For the negative region of the input signal, SCR turns off because the anode is negative with respect to the cathode. The diode prevents a reversal in the gate current. While R1 (potentiometer) controls the gate triggering current, and the amplitude of the firing voltage, thus also controlling the conduction time of Q1 with a 0^o to 90^o phase retard and 90^o to 180^o conduction angle.
• By controlling the variable resistance in the half-wave phase control circuit, it also varies the current or voltage (rms) values and thus power to the load. True power is not the power contained in the pulse but it is the average power over a complete cycle. By measuring the E_peak with an oscilloscope and finding its ratio to E_rms, with the corresponding phase retard angle, it can therefore be used to calculate for P_T. [P_T = E_rms²/ RL ]