1.) With the SCR board unplugged and no power applied, measure the value of RLOAD (approximately 75 Ohms).

2.) Set up the circuit of Fig. 1 excluding the UJT triggering circuit and verify the following:

a) With the SCR shorted, i.e. point A connected to point B, the input voltage V_i should read 20-30 Vrms and the load voltage V_l should read 20-30 Vrms. CAUTION!: The load resistor should get very with the circuit connected in this way. Verify that it is heating up and then turn off the power and disconnect point A from point B.

b) Reapply the power, and with point A not connected to point B, the load voltage V_l should read nearly 0 V and the input voltage V_i should still read 20-30V.

3.) Build the UJT triggering circuit as shown in Fig. 1. Do not connect the circuit to the SCR board. Instead, apply a 10-20 VDC voltage to it and verify that it will operate as a stand-alone relaxation oscillator. Connect point A to the positive of the power supply and point B to the common and obtain a dual trace HP BenchLink Scope screen dump of the voltage across C₁ as well as the voltage across the 100 Ohm resistor with the DC voltage applied. This step insures that the UJT relaxation oscillator will operate as a trigger pulse generator when it is connected in the circuit. Adjust the 50 K pot as necessary to force the circuit to oscillate.

4.) Remove the DC power supply from the UJT triggering circuit and connect it as shown in Fig. 1 .

5.) Remove one of the two probes from the scope so that only one probe is connected to the scope.

6.) Apply AC power to the circuit and set the value of V_i to 30 V by adjustment of the variable transformer.

7.) Place the scope across the SCR and observe that the conduction angle changes as you vary the pot.

8.) Record the following in 10 degree increments of the conduction angle:

a) The RMS value of the load voltage, V_l, using a DMM.

b) The RMS value of the voltage across the SCR, using the scope RMS measurement.

c) The RMS value of the input voltage, V_i, using a DMM. Use the variable transformer to keep this voltage constant by readjusting it at each new conduction angle.

WARNING: RLOAD gets very

9.) Set the conduction angle to about 135 degrees. Obtain a scope screen dump of the voltage across the SCR and the voltage across the load resistor on the same scope screen.

CAUTION! You cannot connect scope probes across the SCR and RLOAD at the same time without causing a short circuit and possible scope damage. That is why only one scope probe is allowed in this part. Instead use the scope's trace memory and trace storage capability. Obtain one of the waveforms, store it, move the probe and obtain the second waveform. Trigger from the line so that the two traces will be aligned properly.

10.) Calculate and plot the following:

a) Measured and theoretical load power vs. conduction angle.

( Measured = V_l ²/RLOAD)

b) V_l and V_scr vs. conduction angle on the same set of axes.