Basic Aspects of Alternating current.
In this section, you explore some of the basic aspects of AC current.
The electrical power we get from a battery is called direct current (DC). A 9-volt battery produces a voltage of 9 volts, which doesn't change until the battery is used up. The electricity we get delivered from the electrical power company from the wall socket is AC (alternating current). This is different than a battery because the voltage and current coming from our wall sockets is continuously changing. The voltage reverses direction 60 times every second in North America (and 50 times each second in most of Europe and much of Asia).
AC is how electricity is distributed throughout the world's power grid. Sometimes DC needs to be converted to AC, such as solar electric panels used to provide power for an electrical utility. Sometime AC needs to be changed to DC at a different voltage, such as is done in cell phone battery chargers.
What You Need
Displaying an alternating current
- waveform generator and an oscilloscope
- connector for the oscilloscope (consisting of a BNC connector with two wire leads attached)
- alternative: a source of sound, a microphone, and a computer-based, sound-card oscilloscope. CAUTION: Sound card oscilloscopes can handle only low-voltage inputs, such as from microphones. Attempting to use a sound-card oscilloscope for larger electrical signal may damage your sound card. A high-impedance circuit that will enable using a sound-card oscilloscope for higher voltages can be found at www.geocities.com/~uWezi/electronics/projects/soundcard_osci.html.
Building a transformer
- 2-foot length of insulated wire
- 4-foot length of insulated wire
- large iron nail
- AC power supply, waveform generator, or keyboard output
- 2 AC voltmeters (or multimeters configured as an AC voltmeter)
What a 60-cycle AC signal sounds like
- If you have an adjustable AC power supply, attach one of the terminals of the speaker to the positive terminal of the AC power supply and the other speaker terminal to the negative terminal of the power supply.
- Slowly turn up the voltage and you will start to hear the characteristic 60-cycle hum coming from the speaker. This may be a familiar sound to rock musicians working with preowned PA systems, which often leaks into audio systems.
What an 60-cycle AC signal looks like
- Connect the positive and negative terminals of the AC power supply to a 1000 ohm resistor.
- Attach the two wire leads of the oscilloscope input to the two ends of the resistor. (Do not use a PC-based oscilloscope, which we used in other experiments, unless you have a special circuit to adapt the AC signal for this purpose.)
- Turn on the AC power supply with just enough voltage to produce a display on the oscilloscope.
- Adjust the amplitude, time sweep, and, if necessary, trigger setting to display the AC signal on the oscilloscope screen. Figure 115-1 shows how the electrical components are connected to make this measurement. (The diode used in the next set of steps is shown connected.)