Basic Aspects of Alternating current. (page 2)

What a diode does to an alternating current

1. Remove one of the connections to the power supply.
2. Attach a diode in the circuit going from the power supply through the resistor.
3. Turn on the AC power supply.
4. Reattach the leads from the oscilloscope to the ends of the resistor.
5. Display the AC signal on the oscilloscope screen.



6. Turn down the AC power supply.
7. Remove the diode. Reverse the direction of the lead and reattach the diode in the circuit.
8. With the AC power supply turned on, observe how the signal changes.

Building a transformer

1. Wind the 2-foot section of wire around the nail. Leave approximately 6-inch lengths of wire at each end, with about ¾ of the insulation removed from the ends of the wire. Keep track of how many turns you apply.
2. Do the same with the 4-foot section of wire. There should be twice as many turns on this section.
3. Attach the positive and negative of an AC power supply to the two leads of the 2-foot section of wire. (We can call this the primary coil.)
4. Attach the two ends of an AC voltmeter to the points of contact between the power supply and the 2-foot section of the transformer wire.
5. Attach the other AC voltmeter to the two leads of the 4-foot section of wire. What do you read?
6. If you have a DC power supply available, apply a similar voltage to the primary windings. How is the voltage of the secondary affected?

Expected Results

A 60-cycle AC signal is displayed on an oscilloscope with a full wavelength repeating every 0.017 seconds. An AC signal has the form shown in Figure 115-2.

Inserting the diode in the circuit results in only one-half of the waveform flowing in the circuit. This means only the positive (or negative) half of the cycle is displayed, as shown in Figure 115-3 for a diode placed in one direction, or as in Figure 115-4 for a diode placed in the other direction.

Why It Works

Alternating current is constantly changing direction.

A diode is a device that passes the current in only one direction.

A transformer changes the AC voltage of an incoming signal based on the ratio of turns between the input and output sides of a transformer. A transformer only lets AC current through, but it will not pass DC current.

The ratio of the primary (in) to the secondary (out) voltage of a transformer is the ratio of the turns of the secondary to the primary. This is given by the equation V_p/V_s = N_p/N_s where V represents the voltage, N the number of windings, p the primary, and s the secondary windings.

Other Things to Try

If you don't have a stand-alone oscilloscope, here are some other options:

1. Build an adapter for the sound card oscilloscope.
2. Use an audible tone, such as from an electronic synthesizer keyboard, to produce a signal that is compatible with a sound card oscilloscope.
3. You can also generate an AC signal using a magnet suspended by a spring over a coil. The signal can be monitored by a sound card oscilloscope or PASCO voltage sensor, and the effects of the diodes can be studied.

The Point

Alternating current consists of a flow of electrons continuously reversing direction. The voltage of a common form of AC follows the rising and falling pattern of a sine wave.

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