Some resources for learning and teaching through experimentation
DC circuits provide a wonderful gateway not only to electrical technology and electronics, but also to energy concepts, the structure of matter, electromagnetism and problem-solving. It should be introduced through manipulation of physical objects, observation and reasoning. It can be introduced as early as the curiosity about such matters is aroused, at ages 10-11, when the manipulative and mental capabilities are certainly up to the task. By age 13-14, electronics can be appreciated, and significant skills developed. An understanding of these matters will greatly facilitate learning in advanced subjects in science and engineering.
The procrustean, one-size-fits-all nature of American education is ill-suited to bringing electricity to its pupils. They will not all develop a curiosity in electricity, and those that do will not develop it all at the same time. Teachers will present it, if and when it is prescribed, as a collection of facts and recipes coming from authority, with no understanding of the value of the subject in education, and probably no understanding of the subject itself. It will be that or Gee Whiz, and either way will be worthless. I have no idea how to overcome this. I recommend to students to learn on their own--then they will not be wasting their time.
In 1950, any hardware store had stuff for electrical experiments. There were #6 dry cells with the screw terminals, electromagnetic bells and buzzers, wire, Fahnestock clips, knife switches, small lamps, and so forth. This has largely disappeared, but in its place we have Radio Shack. Radio Shack is truly excellent in bringing electricity to the people, but it is not quite as good for the beginner. There are electronic lab kits that look all right, but I have not yet investigated them closely. The most important thing to have is some way to connect the wires to make up circuits, and a support for the parts. At one time, a breadboard (a real board, available in hardware stores in the kitchen department) was the support. Fahnestock spring clips could be screwed to the board to make temporary connections. Things could be screwed or glued to the board as necessary. This gave rise to the term "breadboarding" that is still used.
Later came phenolic board with a matrix of holes. Terminals could be pressed through the holes, and even spring-loaded terminals are still available. There are a number of techniques for prototyping circuits on such boards, but they get into soldering and wire-wrapping, and the used of integrated circuits, which is out of the range of what we want here.
The best method is the "solderless breadboard" with its array of connection points. There are short rows of five holes, and any wires stuck into the holes will be connected with each other. This is an excellent method, but it is restricted to electronic components with wire leads no larger than #20 AWG. However, it should be used as soon as possible. It is used with #22 AWG thermoplastic insulated solid wire, and the only tool necessary is a wire cutter and stripper. For making circuits with resistors, it is absolutely excellent. Use only solid wire in the solderless breadboards, best #22 -- larger may strain the springs, and smaller may not make a good connection. Use different color wire for clarity, but this is not as important with simple DC circuits.
Any connection not made with a terminal block, spring clip or solderless breadboard must be soldered. Use a small soldering iron (12 W or so) with a fine conical tip, and rosin-core solder. Connections must not be made only by twisting wire together. This really does not work, and is an extremely bad habit to get into.
For power sources, the only economical choices are batteries or plug-in wall supplies. Either will provide up to 12 V DC at 0.5 A, which is enough for most experiments. As for batteries, the most easily available are zinc chloride or alkaline cells, with an open circuit voltage of about 1.6 V when fresh. Alkaline cells cost about twice what zinc chloride cells cost, but the two are interchangeable. An alkaline cell will say "alkaline" on it, while a chloride cell will be called something like "heavy duty." Alkaline cells have a better shelf life, and may have a longer life in service, but probably do not give twice the service of a chloride cell. The commonly available sizes are D, AA and AAA. D cells weigh 138 g, AA cells 18 g, and AAA cells 11 g. The power available is roughly proportional to the weight, so a D cell has about 8 times the capacity of an AA cell, but costs less than twice as much. AAA cells cost even more than AA cells, so they are definitely uneconomic. Chloride D cells can be obtained in bulk for about $0.50. They must be used in a battery holder so that connections can be made to them. The voltage drops very slowly with service, and falls more rapidly as the cell approaches exhaustion. It is best to use only fresh chloride cells with a terminal voltage above 1.500 volts. If the cells are not already dated, it is well to attach a label with the date of purchase. The D cell has an internal resistance of about 3 Ω. The smaller cells have a smaller internal resistance, down to about 1 Ω for the AAA.
There is a great variety of batteries available, most more expensive than the common D cell. The #6 dry cell would be excellent for our purpose, but is no longer easily available. 9-volt batteries cannot provide much current, and are best for electronics only, and electronics with a small current drain at that. They are relatively expensive, and generally have a short life when used for experiments. There are lantern batteries of 6 V or more that have good capacities, but they are not cheap. The ordinary 6 V lantern battery that is square and has spring terminals is a reasonable value, but leads will have to be soldered to the inconvenient (for us) springs.
Any electronics supplier offers a large selection of small power supplies that plug into a wall receptacle, or have a line cord (called "desk top" supplies), at prices of $5.00 up, and these are excellent value. Regulated supplies are recommended, since they are protected from overcurrent. The voltage of an unregulated supply will vary greatly with load (a nominal 9V supply will give about 12 V with no load). Variable-voltage power supplies are not as useful as they might seem, and are expensive. The most commonly available voltages are 5, 6, 9 and 12 V. Any voltage less than 24 V is quite safe, and will not give a noticeable shock. A supply giving 5 V, +12 V and -12 V is quite convenient for all kinds of experiments.
Experimenting with AC suggests the household supply. Unfortunately, this can be very dangerous because of the practice of grounding one side of the circuit, and bringing ground into every piece of equipment. The only good thing is that contact with the hot wire usually just produces an unpleasant buzz that is a learning experience, unless one is standing in water or touching something grounded (like the metal parts of anything that is plugged in!). It is difficult to get a fatal shock from 120 V, unless you really try. I personally do all my AC experimenting with power from an isolating transformer. Then, touching any one thing has no consequences at all. Low-voltage AC (under 24 V) coming from a transformer is quite safe to experiment with. A fused transformer in a plastic box with AC terminals is satisfactory. There are also wall transformers available. Whenever using household current, a GFCI (ground fault circuit interrupter) receptacle is a very good idea, since they will protect against dangerous shocks to ground.
Incandescent lamps make a good load, because they give an indication of the current passing through them. On the other hand, their resistance varies greatly from cold to hot (this can be the basis of an experiment). LED's are excellent for the same reason, but they are not even approximately resistances, but constant-voltage-drop elements. Resistors are good resistors, about constant in value, but give no indication of the power dissipated in them except by heat. One could heat water with them, and find the relation between the joule and the calorie. Electrolysis is another good load, and the gases given off can be investigated. The effects of electricity--heating, chemical, magnetic, shock, and so forth are important to observe.
Switches can be simulated by moving wires in and out of a solderless breadboard, or touching two wires together. The usual electronic switches are small and it requires some skill to attach leads with solder. The switches themselves have to be mounted somehow. You can also not see what is going on inside. Knife switches are easy to understand, but have to be mounted on something like a breadboard, and the same goes for bell pushbuttons. If these things are used, a breadboard with a solderless breadboard screwed to it is what you want.
Some components need protection against abuse, and this can be done by permanently attaching resistors. A 10 Ω resistor in series with a D cell will limit the current to 150 mA, and will make paralleling easy, while protecting the cell against a short circuit. A potentiometer should be protected by a resistor in series with the slider that will limit the current to the normal value for the potentiometer. For a 10 kΩ, 2 W potentiometer, a 1000 Ω resistor is about right. LED's need protection against excessive current, because they cannot limit the current themselves. A 220 Ω resistor is satisfactory for the usual LED.
To make quantitative measurements, meters are necessary, and the common DMM (digital multimeter) is the tool of choice. It measures volts, amperes and ohms, and can be used to check the results of calculation. This is not a mere exercise, like so many school "experiments" but part of the actual design process. One designs a circuit on paper, then breadboards it to see if it works as intended. The proper use of a meter is a valuable skill to learn. The engineer usually measures just voltages, and this is always safe. Measuring currents is more troublesome. The multimeter (set as a voltmeter!) can be used from an early age. The DMM is much more rugged than the expensive Weston meters of past days. However, any DMM should be arranged so that the leads must be reconnected to change from voltage to current/ohms. The leads should normally be kept in the voltage position. This is a very important point. The fuse will protect the meter, but changing fuses is a bother.
Magnetism can also be studied at the same time as DC circuits. This includes not only electromagnets, but induction as well. Nails do not make good cores; wrought iron or even mild steel is better. Bend circular rod in a U-shape, and use rectangular bar for end pieces. Try for closed magnetic circuits, and experiment with air gaps. Use coils wound from lacquer-coated magnet wire, say #30 AWG, or even wire-wrap wire. Wind the coils on a bobbin made from plastic tube with end pieces, and use many turns (hundreds). Then the cores can be inserted or withdrawn at will. Transformers can be made. They will probably have lots of leakage, but will be actual transformers, and can be studied at low AC voltages. An inexpensive compass can be used to detect a magnetic field. Edmund Scientific has lots of permanent magnets.
These suppliers will send you catalogs for mail order, or you can order from the websites.
The part numbers shown are suggestions; there are many equally good alternatives. All of these things are also used for electronics.
Composed by J. B. Calvert
Created 7 February 2001