Experiment 5

Alternating to Direct

Rectifying Alternating Current

Objective: The objective of this experiment is to illustrate how a diode can be used to rectify alternating current. The student will use a galvanometer to determine the direction of current flow, when an AC or DC voltage is applied to a circuit containing a diode in series with a resistor and a galvanometer.

Review of Scientific Principles:

For current to flow through a diode, the electrons must move up an energy hill and across the p-n junction. As a voltage is applied in the forward bias, the size of the hill is decreased, so more electrons have the energy needed to move up the hill and across the junction (making current flow). However, if the voltage is applied in reverse bias, the hill is made bigger, so very few electrons have the energy needed to move up the hill. Thus, a diode generally conducts current in only one direction.

Applications:

When you plug an electrical device or appliance into an ordinary wall receptacle at your house, you are using 110 volt AC (alternating current). The electricity was probably produced at a power plant by using a fuel to produce steam, to turn a turbine, to turn an electric generator. The generator spins at 3600 RPM, which is 60 revolutions per second (60 Hz). Many household items are designed to operate on AC, however, some items such as battery chargers, electric trains, and other toys are designed to operate on DC. Diodes are used as rectifiers, to convert AC to DC.

Time: 20-30 minutes

Materials and Supplies:

AC-DC Power Supply

Lead wires

Galvanometer

1 - 1K ohm resistor

Diode (germanium, zener, or LED)

General Safety Guidelines:

*Make sure the power supply dials are set at zero when building or adjusting a circuit.

*Keep your hands and the work area dry to prevent shock.

Experimental Setup:

Procedure:

1. Build the circuit shown in the Experimental setup, and be sure to connect the positive

terminal of the diode to the positive terminal of the power supply.

2. Use the DC terminals of the power supply.

3. Make sure the voltage dial on the power supply is set at zero.

4. Turn on the power supply.

5. Slowly rotate the voltage dial clockwise, and watch the galvanometer needle. Do not

bury the needle.

6. Record the direction of needle movement.

7. Rotate the voltage back to zero.

8. Reverse the direction of the diode and repeat steps 5-7. Do not increase voltage past 2V.

9. Disconnect the lead wires from the DC terminals, and connect them to the AC terminal

on the power supply.

10. Repeat steps 5-8.

Data and Analysis:

1. Will the current flow through a diode both directions?

2. How should a diode be connected in a circuit so current will flow through it?

3. How does a diode effect AC current?

4. Draw a graph of current (vertical axis) vs. time (horizontal axis) for AC current.

5. Considering how a diode affects AC current, draw what you think a graph of current

vs. time should look like for the circuit you built using AC current and a diode.

6. Draw a graph of current vs. time for a DC current, like that produced by a battery.

7. How does the current produced by the AC - diode circuit differ from DC current

produced by a battery?

8. How does increasing voltage effect the ability of a diode to allow current to flow?

9. Why does a diode with a voltage applied in reverse bias restrict current flow?

10.Will a diode change alternating current to direct current (like the current produced by a

battery)?

Extension:

1. Use a hand generator, a resistor, and a galvanometer to show needle movement with

alternating current. Use a 1K resistor, to protect the galvanometer.

2. Use a frequency generator diode and an oscilloscope to show the wave form of

alternating current and rectified alternating current.

3. Obtain the schematic of a full wave rectifier, which uses diodes and capacitors to

produce approximately steady direct current. Consult an electronics handbook for the

details.

Teacher Notes:

*Teacher preparation time is approximately 30 minutes.

*If the diode is improperly connected, the results will be reversed.

*The teacher should demonstrate proper operation of a power supply.

*If a digital multimeter is used, use the milli-amp or micro-amp scale. The student should record the sign (+,-) of the current value.

Answers to Questions:

1. No

2. Positive terminal of the diode to the positive terminal of the power supply.

3. A diode will rectify AC current, which means the produced current will be a pulsating

direct current. It will pulsate at the same frequency as the frequency of the alternating

current.

4. The graph will be a sine wave.

5.

6.

7. The current produced by a battery is steady, while that produced by the action of a

diode on AC current is pulsating. A 5 amp DC current is more powerful than a rectified

AC current varying from 0-5-0 amps.

8. Increasing the voltage decreases the size of the energy hill that the electrons have to

move up, so more electrons can move up the hill and across the p-n junction, allowing

more current to flow.

9. A reverse voltage increases the size of the hill, so few electrons have the energy needed

to move up the hill. Most meters will show no current flowing in the reverse direction.

10. No, only with the addition of a capacitor will the current begin to level off.

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