Diodes and Power Supplies Lab

PH337 Electronics
Experiment #4

DIODES and POWER SUPPLIES LAB

OBJECTIVES

To study the behavior of transformers, diodes and RC filters and,
To understand how to combine these elements in a circuit to make a DC power supply.

DISCUSSION

You have studied how RC low-pass filters and transformers work. You have also looked at how diodes work. This experiment will give you an opportunity to put these elements together into a simple, unregulated DC power supply. You will be able to see how each element functions to provide an end result--the DC voltage.

PROCEDURE

The Transformer
The transformer you will use is a center-tap transformer that is mounted inside a Heathkit power supply. (We've left it inside to make it safer; you won't be able to come in contact with 115 VAC on the primary of the transformer!) See the schematic that shows you how the three voltage taps are brought out to the front panel connectors.
- First, identify which terminal is the center tap of the transformer. The center terminal is not the center tap on the transformer.
- Measure the zero-to-peak voltages of the other two terminals with respect to the center tap. Also make these same measurements with a DMM. Explain why these DMM and oscilloscope measurements are different.
- Sketch the relative phase of the voltage on the two outer taps.

Diode Action and Half-Wave Rectification
- Before assembling the diodes in your circuit, use the DMM to measure the resistance in the forward and reverse directions of the diodes. NOTE: A single DMM ohmmeter measurement does not always accurately describe the I vs V characteristics of the diode. See the sketch of the I vs V curve and explain when the resistance indicated by the DMM might equal the reciprocal of the slope of the I vs V curve.
- Assemble a single diode into the circuit shown in figure 2. Use 100 < R < 500. Also keep 1000 < R_L < 10,000. But do not put the capacitor, C, in the circuit at this point.
- Use the oscilloscope to measure the voltage between points 2 and 3. Explain how the diode causes the "half-wave rectification" that you observe at this point.

Full Wave Rectification
- Add a second diode to the circuit as shown in figure 3.
- Measure the output voltage peak. Sketch the wave form. You will compare it to the "filtered signal" you get in the next section.

RC Filter Action
- Next add an "electrolytic filter" to the circuit. Start with the lower value, e.g., 1 uF. Be sure to use the correct polarity of the electrolytic!
- Measure the "average" voltage with the oscilloscope seen between points 2 and 3 and sketch the waveform.
- Compare the full wave rectified voltage to the RC filtered voltages (done above).
- Next, use the larger capacitor (47 uF) and repeat your measurements.
- Calculate the corner frequencies for the two capacitors. Explain the action of the RC filter(s) in terms of the corner frequencies and transfer function.

Loading and Output Impedance
Use the larger capacitor for this section.
- Measure the voltage at points 2-3 as you vary the load resistor, R_L. Don't decrease R_L below 1000 ohms. (Why?) Calculate the output impedance of your DC power supply from this data.
- Explain why the voltage changed as the load changed.