The purpose of this lab is to investigate voltage regulation techniques for generating constant voltages and power supplies.

Full-wave Rectifier.

The source of unregulated voltage in a power supply is usually a half-or full-wave rectified sine wave from a transformer. For this lab we will use a full-wave bridge rectifier, as shown below. Construct the rectifier using 1N4007 rectifier diodes and a 100 uF filter capacitor (be sure to observe the polarity of the capacitor). Do not short out the transformer secondary.

What output voltage would be expected if the voltage of the transformer secondary is 10 Vrms? Measure the output voltage with no load resistance and compare. Use both the oscilloscope and the DMM.

Compute the power that would be dissipated in a load resistor of 1 kilohm. Apply a 1 kilohm resistor capable of handling the power, and measure the voltage and peak-to-peak ripple again. (Use your scope in AC coupling mode to do this.) Compare with theory. What should the ripple frequency be? What is the observed ripple frequency?

What is the peak inverse voltage (PIV) across the rectifier diodes? Check that this does not exceed the PIV rating of the diode. What is the current capability of the diode? For the rest of the experiment, put the unregulated voltage on one of your breadboard busses. Remove the load. You will put the load on the regulated voltage later.

Discrete Component Regulators.

An emitter follower circuit can be used to buffer the output of a simple zener voltage regulator, as shown below. We will use a power transistor for the emitter follower, the TIP31, because of the power demands that we will place on the circuit. Choose R such that a minimum of 12 mA of current will flow through the zener diode.

What is the expected output voltage of the regulator? Construct and test the circuit, using the unregulated output of your rectifier as the supply voltage for the circuit, and with R_load = 1 kilohm, as before. Measure and note the various voltages on your schematic. Then do the following:

• Measure the p-p ripple voltages at the different points. Why is the output ripple less than that of the unregulated source? How much of the output ripple is due to the ripple across the zener diode? Compute the `ripple rejection', defined as the ratio of the p-p output ripple to the p-p input ripple.

• Apply a 180-ohm load to the output of the appropriate wattage rating. What happens to the output voltage and to the ripple, and why? What is the load regulation?

• How much power is being dissipated in the load, and how much in the regulator transistor? How does this compare with the power rating of the transistor?

• How much base current is required to drive the 180-ohm load if the transistor has a beta of 20? How much current does this leave to go through the zener diode? The circuit stops regulating when the zener current drops to zero. At what value of R_load would this occur at?

Feedback in Voltage Regulation

The circuit used above effectively `compares' the output voltage with the voltage across the zener diode and maintains about 0.7 V difference between the two. The following op amp circuit does a better job of comparing the two voltages. Within limits, the op amp drives the base of the regulator transistor such that the output voltage is equal to the zener voltage.

Construct the circuit and measure the output voltage and ripple. How is this different from before, and why? Why is this circuit better than before? What limits the output current of this regulator?

In the above circuit the output ripple should be the same as the ripple across the zener diode. The ripple can be practically eliminated if the reference voltage is derived from an already regulated source (+15 V, say). In this case the zener diode is not needed and the reference voltage can be varied using a potentiometer. Implement the variable reference using a 10-kilohm potentiometer and adjust the output to be 5.1 volts, as before. What is the p-p output ripple now, and what is the ripple rejection?

The regulator can be protected against a short circuit on its output by adding another transistor, as shown below. If the output current exceeds a certain value, Q2 is turned on and `robs' Q1 of its base current. Try this circuit. What would you expect the maximum output current to be? Test by decreasing the value of R_load (add other resistors in parallel).

Integrated Circuit Regulators.

The circuit that you have just tested is the basic technique employed in the LM105 integrated circuit adjustable voltage regulator. Another IC regulator which is somewhat easier to use is the LM317. It is a three-terminal package in which the output is regulated to be 1.25 volts greater than the `adjust' input, which is a high impedance input. For this regulator, V_out = 1.25 (1 + R₂ / R₁).

Design a 5 volt regulator and build and test the circuit, using the unregulated output from your rectifier as the input to the regulator. Measure the load regulation and ripple rejection.

Replace R2 with a 10 kilohm pot and test the 317's operation as an adjustable regulator. What is the minimum output voltage of the adjustable regulator?