125 Physics Projects for the Evil Genius (67 page)

• waveform generator and an oscilloscope
  
• connector for the oscilloscope (consisting of a BNC connector with two wire leads attached)
  
• diode
  
• 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
  .
  

• 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)
  

1. 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.
   
2. 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.
   

1. Connect the positive and negative terminals of the AC power supply to a 1000 ohm resistor.
   
2. 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.)
   
3. Turn on the AC power supply with just enough voltage to produce a display on the oscilloscope.
   
4. 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.)
   

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.
   

Figure 115-1

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

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?
   

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.

Figure 115-2
Alternating current waveform
.

Figure 115-3
Alternating current with a diode
.

Figure 115-4
Alternating current with a diode facing the other way
.

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
where
V
represents the voltage, N the number of windings,
p
the primary, and
s
the secondary windings.

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.
   

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.

A
diode
is an electronic device that lets current flow in only one direction. Diodes are found in electronic circuits and form the basis for more complicated devices, such as transistors and integrated circuits. LEDs (light-emitting diodes) and solar cells are diodes.

Unlike the resistors we studied in previous experiments, diodes do not follow Ohm’s law. They are called nonlinear devices, which gives them properties that are useful in a wide variety of electronic applications.

• diode
  
• DC power supply
  
• voltmeter
  
• ammeter
  
• jumper wires
  

1. Set up the circuit, as shown in
   Figure 116-1
   . This consists of the positive terminal of the power supply connected to the positive end of the diode (identified by the longer of the two leads). The ammeter is connected in series with the diode and, together, they are attached to the power supply. The voltmeter is connected to the two terminals of the diode.
   
2. Start with the power supply at the lowest level and make sure the voltage and current meters read zero.
   
3. Very slowly, walk the voltage up, taking current and voltage readings at each step. Continue until the current suddenly goes up significantly higher than previous levels. Do not allow too much current to flow or the diode can be damaged.
   

The relationship between voltage and current is not linear.

As the voltage increases, a threshold is reached where a small increase in voltage results in a huge increase in current.