12.4: Procedure

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12.4.1: Low Voltage

1. Consider the circuit of Figure 12.3.1. With a 12.6 volt secondary rating and a 120 volt primary rating, the turns ratio is approximately 10:1. In other words, for any reasonable input signal at the primary, the output at the secondary is expected to be one tenth the voltage and ten times the current.

2. Connect the primary side of the transformer to the function generator as shown in Figure 12.3.1. Set the generator to a 10 volt peak sine at 60 Hz. Place the oscilloscope probe grounds at the bottom of the secondary. Connect probe tip one to the top of the secondary and probe tip two to the center tap. Record the peak amplitudes in Table 12.5.1 and capture an image of the scope display. Compute and record the primary/secondary voltage ratio as well (for the full secondary).

3. Build the circuit of Figure 12.3.2 using Rsense = 22 \(\Omega\), Rload = 10 \(\Omega\) and Vin = 5 volt peak sine at 60 Hz. Place one scope probe across the load and the other across Rsense. Record the peak amplitudes in Table 12.5.2 and capture an image of the scope display.

4. Using the voltage measured across the sense resistor, determine the primary side current. Using Ohm’s law and the measured load voltage, determine the load (i.e., secondary) current. Based on these, compute the primary/secondary current ratio. Record these values in Table 12.5.2.

12.4.2: Line Voltage

5. This section uses the 120 VAC line. Treat it with the caution it deserves. Connect the circuit of Figure 12.3.3 leaving Rload unconnected. Measure the secondary voltage with the DMM (AC Volts). Record the value in Table 12.5.3 under “Unloaded”.

6. Add the load resistor, 20 \(\Omega\), and measure the load voltage with the DMM. Record the value in Table 12.5.3 under “Loaded”. Determine the percent change between the loaded and unloaded voltages. Also, measure the load voltage using the oscilloscope and capture an image of the display.