2.1 Text

2.1.1 Resistors

A resistor is an electrical component that resists the flow of electrical current. The amount of current (I) flowing in a circuit is directly proportional to the voltage across it and inversely proportional to the resistance of the circuit. This is Ohm law and can be expressed as a formula: . The resistor is generally a linear device and its characteristics form a straight line when plotted on a graph.

Resistors are used to limit current flowing to a device, thereby preventing it from burning out, as voltage dividers to reduce voltage for other circuits, as transistor biasing circuits, and to serve as circuit loads.

Generally, resistors (Fig 2.1) consist of carbon composition, wire-wound, and metal film. The size of resistors depends on power ratings. Larger sizes are referred to as power resistors. Variable resistors are adjustable: rheostats, potentiometers, and trimmer pots. Precision resistors have a tolerance of 1% or less.

If you are a bit serious about the electronics hobby I recommend learning the "Color Code". It makes a lot easier. The same color code is used for everything else, like coils, capacitors etc. Again, just the color code associated with a number, like: black=0 brown=1 red=2, etc.

Fig 2.2 is an example. It is a 4-band resistor. The first band is the tens values; the second band gives the units; the third band is a multiplying factor, the factor being 10's band value. The fourth band gives the tolerance of the resistor. No band implies a tolerance of ± 20%, a silver band means the resistor has a tolerance of ± 10% and a gold band has the closest tolerance of ± 5%.

For a 5-band resistor, the first band is the hundreds values, the second band gives the tens and the third band gives units, the forth band is a multiplying factor, the factor being 10's band value. The colors brown (1%), red (2%), green (0.5%), blue (0.25%), and violet (0.1%) are used as tolerance codes on 5-band resistors only. All 5-band resistors use a colored tolerance band.

Can you "create" your own resistors? Of cause and not difficult. Here is how to do it. Draw a line on a piece of paper with a soft pencil, HB or 2HB will do fine. Make the line thick and about 2 inches (5 cm) long. With your multimeter, measure the ohm's value of this line by putting a probe on each side of the line; make sure the probes are touching the carbon from the pencil. The value would probably be around the 800kΩ to 1.5 MΩ depending line. The resistance will drop considerably, if you erase some of it (length-wise obviously!). You can also use carbon with silicon glue and when it dries measure the resistance, etc.

2.1.2 Capacitors

A capacitor is an electrical device that can temporarily store electrical energy. Basically, a capacitor consists of two conductors (metal plates) separated by a dielectric insulating material (Fig 2.3(a)), which increases the ability to store a charge. The dielectric can be paper, plastic film, mica, ceramic, air or a vacuum. The plates can be aluminum discs, aluminum foil or a thin film of metal applied to opposite sides of a solid dielectric. The conductor- dielectric-conductor sandwich can be rolled into a cylinder or left flat，the symbols of capacitor are shown in Fig 2.3(b).

A capacitor will block DC current, but appears to pass AC current by charging and discharging. It develops an AC resistance, known as capacitive reactance, which is affected by the capacitance and AC frequency. The formula for capacitive reactance is XC=1/(2πfCC),with units of ohms.

Capacitors are available in various shapes and sizes (Fig 2.4). Usually, the value of capacitance and the working DC voltage are marked on them, but some types use a color code similar to resistors. Small-value capacitors of mica and ceramic dielectrics are indicated in pico farads (10-12 F), but only the significant digits are shown on the package, for example, '105' (Fig 2.4(d))means 10×105 pF=1 μF. Tuning capacitors (such as used in radio) use air as a dielectric, with one set of plates, which can be rotated in and out of a set of stationary plates. Trimmer capacitors are used for fine adjustment with a screw, and have air, mica and ceramic as dielectrics.

2.1.3 Inductors

An inductor is an electrical device, which can temporarily store electromagnetic energy in the field about it as long as current is flowing through it. The inductor is a coil of wire that may have an air core or an iron core to increase its inductance. A powered iron core in the shape of a cylinder may be adjusted in and out of the core.

An inductor tends to oppose a change in electrical current, it has no resistance to DC current but has an AC resistance to AC current, known as inductive reactance, this inductive reactance is affected by inductance and the AC frequency and is given by the formula XL=2πfL L, with units of ohms. Inductors are used for filtering AC current, increasing the output of the RF (radio frequency) amplifier.

Inductors are available in variety of shapes (Fig 2.5): air core, iron core (which may look like a transformer, but has only two leads), toroidal (doughnut shaped), small tubular with epoxy, RF choke with separate coils on a cylinder, and tunable RF coil with a screwdriver adjustment.

Technical Words and Phrases