Thyristor Rectifier

ATTENTION: During this experiment, do not touch any part of the circuit while energized. Do NOT ground the VARIAC.

For this experiment, the variable transformer (VARIAC) at a low frequency of 60 Hz and peak of 35 V is used as the main AC source.

1. Setup

1. Before starting, connect the differential probe to one scope channel.
   1. Set the button on the differential probe to 1/20 (or 20X) attenuation.
2. On the scope channel menu, set the probe to be at 10X unless 20X is available for the differential probe. If 10X is chosen, manually multiply any measurements or results by two to reach the 20X desired.
3. To setup the VARIAC, make sure the VARIAC output (looks like a regular receptacle) is not connected to any cable.
   1. Keep the VARIAC OFF and make sure its knob is set to zero.
   2. Slowly adjust the VARIAC knob to around 15% output.
4. Before connecting the differential probe to the circuit, tie the probe's terminals together and adjust its measured waveform on the screen to show zero offset voltage.
5. Connect the output cable to the VARIAC, and the differential voltage probe across the VARIAC output banana plugs.
   1. Turn the VARIAC ON.
   2. Slightly adjust the VARIAC to achieve 35V peak.
6. Take a copy of V_in to use for reference. Show two to five fundamental cycles.
7. Turn the VARIAC OFF. Do not adjust its knob setting for the rest of the experiment.

2. Half-Wave Rectifier SCR Circuit with Resistive Load and Zero Firing Angle

1. The main rectifier component is the SCR (S), which is a TYN058. The load resistor (R) is 51 Ω. The SCR control circuit is enclosed in the dotted box of Fig. 1.
2. The control circuit uses diodes (1N4004), a 1 kΩ resistor (R₁), a control resistor that is manually changed (R₂), and a ceramic (no polarity) 1 µF capacitor (C).
   1. Make sure the SCR and diode polarities are correct. The dash on the diode is at the cathode while the SCR pin assignment is shown in Fig. 2.
3. On the proto board, build the circuit shown in Fig. 1. Use a short circuit instead of R₂.
4. Connect the differential voltage probe across the load resistor to observe the output voltage, V_out.
5. Turn the VARIAC ON.
6. Adjust the time base on the scope to show V_out for the same number of fundamental cycles that were captured for V_in. Make a copy of the waveforms.
   1. Measure the average or mean V_out.
   2. Zoom in between the SCR turn-off point and the next SCR turn-on point. Measure the time difference using the scope cursors. Make a copy of the waveform.
7. Keep the differential probe connection and other circuit connections the same for the next part.
8. Turn OFF the VARIAC. Do NOT change the VARIAC voltage setting.

Figure 2: Pin assignment of the SCR.

3. Half-Wave Rectifier SCR Circuit with Resistive Load and Non-Zero Firing Angle

Two different resistors will be used as R₂. The values should be between 100 and 1000 Ω. The resistance can read the resistance color code, or measured with a digital multimeter.

1. Angle Setting #1 (small R₂)
   1. Remove the short circuit, which was previously used instead of R₂.
   2. Connect the small resistance value for R₂.
   3. Turn the VARIAC ON.
   4. Adjust the time base on the scope to show V_out for the same number of fundamental cycles captured for V_in. Make a copy of the waveforms.
   5. Measure the average or mean V_out.
   6. Zoom in between the SCR turn-off point and the next SCR turn-on point. Measure the time difference using the scope cursors. Make a copy of the waveform.
   7. Keep the differential probe connection and other circuit connections the same for the next part.
   8. Turn OFF the VARIAC. Do NOT disassemble the circuit or change the VARIAC voltage setting.
2. Angle Setting #2 (small R₂)
   1. Replace R₂ with the larger value resistor.
   2. Turn the VARIAC ON.
   3. Adjust the time base on the scope to show V_out for the same number of fundamental cycles captured for V_in. Make a copy of the waveforms.
   4. Measure the average or mean V_out.
   5. Zoom in between the SCR turn-off point and the next SCR turn-on point. Measure the time difference using the scope cursors. Make a copy of the waveform. The mean value should be what is expected from this equation:
      <V_out>=V₀[1+cos(α)]/(2π) (1)
      which is slightly less than half the peak voltage of the input.
   6. Turn OFF the VARIAC. Disassemble the circuit and return the VARIAC setting to zero.