Section Index Owatonna Steele County Amateur Radio 20-Jul-2007

Resistors

Resistors are electrical components used to control current flow in electrical circuits. They come in all shapes and sizes and could be as simple as a length of wire. Sample illustrations of different types of resistors are shown at right, along with the schematic symbol.

Generally, the larger the physical size of the resistor, the more power it can absorb without damaging itself. Resistors used in electronic circuits will often be small in size, handling under 2 Watts. Due to their small size, they typically have a color code to identify them. Physically larger resistors can absorb a greater amount of power. Because of their physical size, the identification information is printed with eye-readable characters.

As current flows through a resistor, it transforms electrical energy into heat. This causes a voltage drop across the resistor. The amount of energy being transformed dictates the power rating required. Fortunately, the use of Ohm's Law makes the determination easy. If the resistance (R) and voltage drop (E) are known, the current (I) is calculated by I = E / R. The power can then be calcualated by P = E / I.

If a 100 Ω resistor has a 10 V drop across it, I = 10 / 100 = 0.1 A. The power being transformed into heat is P = 10 * 0.1 = 1 W.

Multiple resistors can be connected together to obtain a new resistance value or power rating.

When resistors are added end-to-end (series), the value of each resistor is added together.

Req = R1 + R2 + R3 + ...

When resistors are connected across each other (parallel), the resulting equivalent value is always smaller than the value of the smallest resistor. The equivalent resistance value is calculated by the formula:

Req = ( 1 / R1 ) + ( 1 / R2 ) + ( 1 / R3 ) + ...

For two resistors, the formula can be simplified to:

Req = ( R1 * R2 ) / ( R1 + R2 )

The color bands on small resistors are very simple to decode. Most resistors have 4 color bands along its length (some have 5 color bands). Three bands will be toward one end of the resistor, and the fourth band is located at the opposite end. The first three bands indicate the resistance value. The fourth band indicates the tolerance, or how much different the resistor may actually be from the indicated value.

Once the band colors are identified, the numeric values are determined from the chart below. Decoding the color bands for the resistor at right becomes:
• 1st Band = BROWN = 1st Digit = 1
• 2nd Band = GREEN = 2nd Digit = 5
• 3rd Band = YELLOW -- multiply by 10,000
• 4th Band = RED -- Tolerance is +/- 2%
The value of the resistor is approximately 15 * 10,000 or 150,000 ohms (Ω). Since the tolerance is +/- 2%, the actual value may be:
• 150,000 +/- (0.02 * 150,000) -- or
• 150,000 +/- 3,000 ohms -- or
• Range from 147,000 Ω to 153,000 Ω.
Standard resistor values are not in even increments, such as 100 Ω, 200 Ω, 300 Ω, etc. There is a standard of commonly available values. The range of values is based on the resistance and the tolerance of the resistor. When a wider tolerance resistor is being used, there will be fewer values to choose from.

 Color Band COLOR 1st 2nd 3rd 4th BLACK 0 0 1 BROWN 1 1 10 +/- 1% RED 2 2 100 +/- 2% ORANGE 3 3 1,000 YELLOW 4 4 10,000 GREEN 5 5 100,000 +/- 0.50% BLUE 6 6 1,000,000 +/- 0.25% VIOLET 7 7 10,000,000 +/- 0.10% GRAY 8 8 +/- 0.05% WHITE 9 9 GOLD +/- 5% SILVER +/- 10%