Introduction
"AC" stands for Alternating Current, which can refer to either voltage or current that alternates in polarity or direction, respectively. These experiments are designed to introduce you to several important concepts specific to AC.A convenient source of AC voltage is household wall-socket power, which presents significant shock hazard. In order to minimize this hazard while taking advantage of the convenience of this source of AC, a small power supply will be the first project, consisting of a transformer that steps the hazardous voltage (110 to 120 volts AC, RMS) down to 12 volts or less. The title of "power supply" is somewhat misleading. This device does not really act as a source or supply of power, but rather as a power converter, to reduce the hazardous voltage of wall-socket power to a much safer level.
Transformer -- power supply
PARTS AND MATERIALS- Power transformer, 120VAC step-down to 12VAC, with center-tapped secondary winding (Radio Shack catalog # 273-1365, 273-1352, or 273-1511).
- Terminal strip with at least three terminals.
- Household wall-socket power plug and cord.
- Line cord switch.
- Box (optional).
- Fuse and fuse holder (optional).
If you decide to equip your power supply with a fuse, be sure to get a slow-acting, or slow-blow fuse. Transformers may draw high "surge" currents when initially connected to an AC source, and these transient currents will blow a fast-acting fuse. Determine the proper current rating of the fuse by dividing the transformer's "VA" rating by 120 volts: in other words, calculate the full allowable primary winding current and size the fuse accordingly.
CROSS-REFERENCES
Lessons In Electric Circuits, Volume 2, chapter 1: "Basic AC Theory"
Lessons In Electric Circuits, Volume 2, chapter 9: "Transformers"
LEARNING OBJECTIVES
- Transformer voltage step-down behavior.
- Purpose of tapped windings.
- Safe wiring techniques for power cords.
SCHEMATIC DIAGRAM
ILLUSTRATION
INSTRUCTIONS
Warning! This project involves the use of dangerous voltages. You must make sure all high-voltage (120 volt household power) conductors are safely insulated from accidental contact. No bare wires should be seen anywhere on the "primary" side of the transformer circuit. Be sure to solder all wire connections so that they're secure, and use real electrical tape (not duct tape, scotch tape, packing tape, or any other kind!) to insulate your soldered connections.
If you wish to enclose the transformer inside of a box, you may use an electrical "junction" box, obtained from a hardware store or electrical supply house. If the enclosure used is metal rather than plastic, a three-prong plug should be used, with the "ground" prong (the longest one on the plug) connected directly to the metal case for maximum safety.
Before plugging the plug into a wall socket, do a safety check with an ohmmeter. With the line switch in the "on" position, measure resistance between either plug prong and the transformer case. There should be infinite (maximum) resistance. If the meter registers continuity (some resistance value less than infinity), then you have a "short" between one of the power conductors and the case, which is dangerous!
Next, check the transformer windings themselves for continuity. With the line switch in the "on" position, there should be a small amount of resistance between the two plug prongs. When the switch is turned "off," the resistance indication should increase to infinity (open circuit -- no continuity). Measure resistance between pairs of wires on the secondary side. These secondary windings should register much lower resistances than the primary. Why is this?
Plug the cord into a wall socket and turn the switch on. You should be able to measure AC voltage at the secondary side of the transformer, between pairs of terminals. Between two of these terminals, you should measure about 12 volts. Between either of these two terminals and the third terminal, you should measure half that. This third wire is the "center-tap" wire of the secondary winding.
It would be advisable to keep this project assembled for use in powering other experiments shown in this book. From here on, I will designate this "low-voltage AC power supply" using this illustration:
COMPUTER SIMULATION
Schematic with SPICE node numbers:
Netlist (make a text file containing the following text, verbatim):
transformer with center-tap secondary v1 1 0 ac 120 sin rbogus1 1 2 1e-3 l1 2 0 10 l2 5 4 0.025 l3 4 3 0.025 k1 l1 l2 0.999 k2 l2 l3 0.999 k3 l1 l3 0.999 rbogus2 3 0 1e12 rload1 5 4 1k rload2 4 3 1k * Sets up AC analysis at 60 Hz: .ac lin 1 60 60 * Prints primary voltage between nodes 2 and 0: .print ac v(2,0) * Prints (top) secondary voltage between nodes 5 and 4: .print ac v(5,4) * Prints (bottom) secondary voltage between nodes 4 and 3: .print ac v(4,3) * Prints (total) secondary voltage between nodes 5 and 3: .print ac v(5,3) .end
Build a transformer
PARTS AND MATERIALS- Steel flatbar, 4 pieces
- Miscellaneous bolts, nuts, washers
- 28 gauge "magnet" wire
- Low-voltage AC power supply
CROSS-REFERENCES
Lessons In Electric Circuits, Volume 2, chapter 9: "Transformers"
LEARNING OBJECTIVES
- Effects of electromagnetism.
- Effects of electromagnetic induction.
- Effects of magnetic coupling on voltage regulation.
- Effects of winding turns on "step" ratio.
SCHEMATIC DIAGRAM
ILLUSTRATION
INSTRUCTIONS
Wrap two, equal-length bars of steel with a thin layer of electrically-insulating tape. Wrap several hundred turns of magnet wire around these two bars. You may make these windings with an equal or unequal number of turns, depending on whether or not you want the transformer to be able to "step" voltage up or down. I recommend equal turns to begin with, then experiment later with coils of unequal turn count.
Join those bars together in a rectangle with two other, shorter, bars of steel. Use bolts to secure the bars together (it is recommended that you drill bolt holes through the bars before you wrap wire around them).
Check for shorted windings (ohmmeter reading between wire ends and steel bar) after you're finished wrapping the windings. There should be no continuity (infinite resistance) between the winding and the steel bar. Check for continuity between winding ends to ensure that the wire isn't broken open somewhere within the coil. If either resistance measurements indicate a problem, the winding must be re-made.
Power your transformer with the low-voltage output of the "power supply" described at the beginning of this chapter. Do not power your transformer directly from wall-socket voltage (120 volts), as your home-made windings really aren't rated for any significant voltage!
Measure the output voltage (secondary winding) of your transformer with an AC voltmeter. Connect a load of some kind (light bulbs are good!) to the secondary winding and re-measure voltage. Note the degree of voltage "sag" at the secondary winding as load current is increased.
Loosen or remove the connecting bolts from one of the short bar pieces, thus increasing the reluctance (analogous to resistance) of the magnetic "circuit" coupling the two windings together. Note the effect on output voltage and voltage "sag" under load.
If you've made your transformer with unequal-turn windings. try it in step-up versus step-down mode, powering different AC loads.
Variable inductor
PARTS AND MATERIALS- Paper tube, from a toilet-paper roll
- Bar of iron or steel, large enough to almost fill diameter of paper tube
- 28 gauge "magnet" wire
- Low-voltage AC power supply
- Incandescent lamp, rated for power supply voltage
CROSS-REFERENCES
Lessons In Electric Circuits, Volume 1, chapter 14: "Magnetism and Electromagnetism"
Lessons In Electric Circuits, Volume 1, chapter 15: "Inductors"
Lessons In Electric Circuits, Volume 2, chapter 3: "Reactance and Impedance -- Inductive"
LEARNING OBJECTIVES
- Effects of magnetic permeability on inductance.
- How inductive reactance can control current in an AC circuit.
SCHEMATIC DIAGRAM
ILLUSTRATION
INSTRUCTIONS
Wrap hundreds of turns of magnet wire around the paper tube. Connect this home-made inductor in series with an AC power supply and lamp to form a circuit. When the tube is empty, the lamp should glow brightly. When the steel bar is inserted in the tube, the lamp dims from increased inductance (L) and consequently increased inductive reactance (XL).
Try using bars of different materials, such as copper and stainless steel, if available. Not all metals have the same effect, due to differences in magnetic permeability.
Sensitive audio detector
PARTS AND MATERIALS- High-quality "closed-cup" audio headphones
- Headphone jack: female receptacle for headphone plug (Radio Shack catalog # 274-312)
- Small step-down power transformer (Radio Shack catalog # 273-1365 or equivalent, using the 6-volt secondary winding tap)
- Two 1N4001 rectifying diodes (Radio Shack catalog # 276-1101)
- 1 kΩ resistor
- 100 kΩ potentiometer (Radio Shack catalog # 271-092)
- Two "banana" jack style binding posts, or other terminal hardware, for connection to potentiometer circuit (Radio Shack catalog # 274-662 or equivalent)
- Plastic or metal mounting box
Normally, the transformer used in this type of application (audio speaker impedance matching) is called an "audio transformer," with its primary and secondary windings represented by impedance values (1000 Ω : 8 Ω) instead of voltages. An audio transformer will work, but I've found small step-down power transformers of 120/6 volt ratio to be perfectly adequate for the task, cheaper (especially when taken from an old thrift-store alarm clock radio), and far more rugged.
The tolerance (precision) rating for the 1 kΩ resistor is irrelevant. The 100 kΩ potentiometer is a recommended option for incorporation into this project, as it gives the user control over the loudness for any given signal. Even though an audio-taper potentiometer would be appropriate for this application, it is not necessary. A linear-taper potentiometer works quite well.
CROSS-REFERENCES
Lessons In Electric Circuits, Volume 1, chapter 8: "DC Metering Circuits"
Lessons In Electric Circuits, Volume 2, chapter 9: "Transformers"
Lessons In Electric Circuits, Volume 2, chapter 12: "AC Metering Circuits"
LEARNING OBJECTIVES
- Soldering practice
- Use of a transformer for impedance matching
- Detection of extremely small electrical signals
- Using diodes to "clip" voltage at some maximum level
SCHEMATIC DIAGRAM
ILLUSTRATION
INSTRUCTIONS
This experiment is identical in construction to the "Sensitive Voltage Detector" described in the DC experiments chapter. If you've already built this detector, you may skip this experiment.
The headphones, most likely being stereo units (separate left and right speakers) will have a three-contact plug. You will be connecting to only two of those three contact points. If you only have a "mono" headphone set with a two-contact plug, just connect to those two contact points. You may either connect the two stereo speakers in series or in parallel. I've found the series connection to work best, that is, to produce the most sound from a small signal:
Solder all wire connections well. This detector system is extremely sensitive, and any loose wire connections in the circuit will add unwanted noise to the sounds produced by the measured voltage signal. The two diodes connected in parallel with the transformer's primary winding, along with the series-connected 1 kΩ resistor, work together to "clip" the input voltage to a maximum of about 0.7 volts. This does one thing and one thing only: limit the amount of sound the headphones can produce. The system will work without the diodes and resistor in place, but there will be no limit to sound volume in the circuit, and the resulting sound caused by accidentally connecting the test leads across a substantial voltage source (like a battery) can be deafening!
Binding posts provide points of connection for a pair of test probes with banana-style plugs, once the detector components are mounted inside a box. You may use ordinary multimeter probes, or make your own probes with alligator clips at the ends for secure connection to a circuit.
Detectors are intended to be used for balancing bridge measurement circuits, potentiometric (null-balance) voltmeter circuits, and detect extremely low-amplitude AC ("alternating current") signals in the audio frequency range. It is a valuable piece of test equipment, especially for the low-budget experimenter without an oscilloscope. It is also valuable in that it allows you to use a different bodily sense in interpreting the behavior of a circuit.
For connection across any non-trivial source of voltage (1 volt and greater), the detector's extremely high sensitivity should be attenuated. This may be accomplished by connecting a voltage divider to the "front" of the circuit:
SCHEMATIC DIAGRAM
ILLUSTRATION
Adjust the 100 kΩ voltage divider potentiometer to about mid-range when initially sensing a voltage signal of unknown magnitude. If the sound is too loud, turn the potentiometer down and try again. If too soft, turn it up and try again. This detector even senses DC and radio-frequency signals (frequencies below and above the audio range, respectively), a "click" being heard whenever the test leads make or break contact with the source under test. With my cheap headphones, I've been able to detect currents of less than 1/10 of a microamp (< 0.1 µA) DC, and similarly low-magnitude RF signals up to 2 MHz.
A good demonstration of the detector's sensitivity is to touch both test leads to the end of your tongue, with the sensitivity adjustment set to maximum. The voltage produced by metal-to-electrolyte contact (called galvanic voltage) is very small, but enough to produce soft "clicking" sounds every time the leads make and break contact on the wet skin of your tongue.
Try unplugging the headphone plug from the jack (receptacle) and similarly touching it to the end of your tongue. You should still hear soft clicking sounds, but they will be much smaller in amplitude. Headphone speakers are "low impedance" devices: they require low voltage and "high" current to deliver substantial sound power. Impedance is a measure of opposition to any and all forms of electric current, including alternating current (AC). Resistance, by comparison, is a strictly measure of opposition to direct current (DC). Like resistance, impedance is measured in the unit of the Ohm (Ω), but it is symbolized in equations by the capital letter "Z" rather than the capital letter "R". We use the term "impedance" to describe the headphone's opposition to current because it is primarily AC signals that headphones are normally subjected to, not DC.
Most small signal sources have high internal impedances, some much higher than the nominal 8 Ω of the headphone speakers. This is a technical way of saying that they are incapable of supplying substantial amounts of current. As the Maximum Power Transfer Theorem predicts, maximum sound power will be delivered by the headphone speakers when their impedance is "matched" to the impedance of the voltage source. The transformer does this. The transformer also helps aid the detection of small DC signals by producing inductive "kickback" every time the test lead circuit is broken, thus "amplifying" the signal by magnetically storing up electrical energy and suddenly releasing it to the headphone speakers.
As with the low-voltage AC power supply experiment, I recommend building this detector in a permanent fashion (mounting all components inside of a box, and providing nice test lead wires) so it can be easily used in the future. Constructed as such, it might look something like this:
Sensing AC magnetic fields
PARTS AND MATERIALS- Audio detector with headphones
- Electromagnet coil from relay or solenoid
CROSS-REFERENCES
Lessons In Electric Circuits, Volume 2, chapter 7: "Mixed-Frequency AC Signals"
LEARNING OBJECTIVES
- Effects of electromagnetic induction.
- Electromagnetic shielding techniques.
SCHEMATIC DIAGRAM
ILLUSTRATION
INSTRUCTIONS
Using the audio detector circuit explained earlier to detect AC voltage in the audio frequencies, a coil of wire may serve as sensor of AC magnetic fields. The voltages produced by the coil will be quite small, so it is advisable to adjust the detector's sensitivity control to "maximum."
There are many sources of AC magnetic fields to be found in the average home. Try, for instance, holding the coil close to a television screen or circuit-breaker box. The coil's orientation is every bit as important as its proximity to the source, as you will soon discover on your own! If you want to listen to more interesting tones, try holding the coil close to the motherboard of an operating computer (be careful not to "short" any connections together on the computer's circuit board with any exposed metal parts on the sensing coil!), or to its hard drive while a read/write operation is taking place.
One very strong source of AC magnetic fields is the home-made transformer project described earlier. Try experimenting with various degrees of "coupling" between the coils (the steel bars tightly fastened together, versus loosely fastened, versus dismantled). Another source is the variable inductor and lamp circuit described in another section of this chapter.
Note that physical contact with a magnetic field source is unnecessary: magnetic fields extend through space quite easily. You may also want to try "shielding" the coil from a strong source using various materials. Try aluminum foil, paper, sheet steel, plastic, or whatever other materials you can think of. What materials work best? Why? What angles (orientations) of coil position minimize magnetic coupling (result in a minimum of detected signal)? What does this tell us regarding inductor positioning if inter-circuit interference from other inductors is a bad thing?
Whether or not stray magnetic fields like these pose any health hazard to the human body is a hotly debated subject. One thing is clear: in today's modern society, low-level magnetic fields of all frequencies are easy to find. CONTINUE...
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