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RESISTORS

A resistor will limit the current flow through itself to calculable value based upon its resistance and the applied voltage (see Ohms Law). This means a resistor can be used to run a low voltage device from a higher voltage power supply by limiting the required power to a predermined level. Resistors are not polarity sensitive.

Tolerance
The tolerance of a resistor refers to how close its actual resistance has to be to the value marked on it. Common tolerances are 5% and 1%

Wattage
Depending on the power requirements of a circuit, resistor wattage needs to be calculated to ensure that they don't over heat. The more common ratings available for resistors are 1/4 Watt, 1/2 Watt, 1 Watt & 5 Watt. The wattage required for different circuits can be calculated by using the power formula described later.

Values
Because it would be impractical to carry every possible value of resistor, they are available in pre-selected ranges. These ranges are known as preferred values. The E 12 series, which is the most common series, (12 values per 100) is denoted as: 10ohms, 12ohms, 15ohms, 18ohms, 22ohms, 27ohms, 33ohms, 39ohms, 47ohms, 56ohms, 68ohms, 82ohms.

This does not limit the range of resistors to a total of twelve values, but each resistor value must begin with a number from the series and be a multiple of  X0.1, X1, X10, X100, X1000, X10000 etc. i.e. 1.5ohms, 15ohms, 150ohms, 1500ohms, 15000ohms.

The E24 series has 24 values per 100 which includes the above sequence plus these extra values: 11ohms, 13ohms, 16ohms, 20ohms, 24ohms, 30ohms, 36ohms, 43ohms, 51ohms, 62ohms, 75ohms, 91ohms.

CAPACITORS

A capacitor works on the principal of having two conductive plates which are very close and are parrallel to each other. When a charge is applied to one plate of the capacitor, the electrons will generate an approximately equal, but opposite charge on the other plate of the capacitor.
Vapacitors will pass AC current, but will block DC current. A capacitor can also be used to smooth out voltage ripple, as in DC power supplies. Capacitance is measured in Farads (F).

Capacitor Parameters
Capacitors have five parameters. Capacitance (Farads), Tolerance(%), Maximum Working Voltage (Volts), Surge Voltage (Volts) and leakage. Because a Farad is a very large unit, most capacitors are normally measured in the ranges of pico, nano and micro farads.

Working Voltage
This refers to the maximum voltage that sould be placed across the capacitor under normal operating conditions.

Surge Voltages
The maximum instantaneous voltage a capacitor can withstand. If the surge voltage is exceeded over too long a period there is a very good chance that the capacitor with be destroyed by the voltage 'punching' through the insulating material inside the casing of the capacitor. If a circuit has a surging characteristic, choose a capacitor with a high rating surge voltage.

Leakage
Refers to the amount of charge that is lost when the capacitor has a voltage across its terminals. If a capacitor has a low leakage it means that very little power is lost. generally leakage is very small and is not normally a consideration for general purpose circuits.

Tolerance
As with resistors, tolerance indicates how close the capacitor is to its noted value. These are normally written on the larger capacitors and encoded on the small ones

Capacitor Markings
There are two methods for marking capacitor values. One is to write the information numerically dirctly onto the capacitor itself. The second is to use the EIA coding system.

EIA Codeing
The EIA code works on a very similar principle to the resistor color code. The first two digits refer to the value with the fourth character represents the tolerance. When the EIA code is used, the value will always be in Pico-farads (see Decimal Multipliers).

Example 1: 103K
This expands to:
1 = 1
0 = 0
3 = x1,000
k = 10% (see Capacitor Tolerance for listings)
Then we combine these number together:

1 0 x1,00 = 10,000 pico farads = 0.01micro farads,
at + or - 10% tolerance.

Example 2 : 335K
This expands to:
3 = 3
3 = 3
5 = x100,000
K = + or - 10%
Then we combine these numbers together:

3 3 x 100,000 = 3,300,00 pico farads = 3,300 nano farads = 3.3 micro farads
at 10 % tolerance.