Handbook for Sound Engineers

244 Chapter 10

curves that gives a zero temperature coefficient at some

temperature. Metal film and wirewound types have

temperature coefficient values of less than 100 ppm/°C.

Thermistors are resistance devices designed to have a

large temperature coefficient.

The percent temperature coefficient of resistance is

(10-4)

where,

TCR is the temperature coefficient in percent per °C,

R is the resistance at reference temperature,

r is the resistance at test temperature,

TR is the reference temperature in °C,

TT is the test temperature in °C.

It is better to operate critical resistors with a limited

temperature rise.

Noise. Noise is an unwanted voltage fluctuation gener-

ated within the resistor. The total noise of a resistor

always includes Johnson noise, which depends only on

resistance value and the temperature of the resistance

element. Depending on the type of element and its

construction, total noise may also include noise caused

by current flow and by cracked bodies and loose end

caps or leads. For adjustable resistors, noise is also

caused by the jumping of the contact over turns of wire

and by an imperfect electrical path between the contact

and resistance element.

Hot-Spot Temperature. The hot-spot temperature is

the maximum temperature measured on the resistor due

to both internal heating and the ambient operating

temperature. The maximum allowable hot-spot temper-

ature is predicated on the thermal limits of the materials

and the resistor design. The maximum hot-spot temper-

ature may not be exceeded under normal operating

conditions, so the wattage rating of the resistor must be

lowered if it is operated at an ambient temperature

higher than that at which the wattage rating was estab-

lished. At zero dissipation, the maximum ambient

temperature around the resistor may be its maximum

hot-spot temperature. The ambient temperature for a

resistor is affected by surrounding heat-producing

devices. Resistors stacked together do not experience

the actual ambient temperature surrounding the outside

of the stack except under forced cooling conditions.

Carbon resistors should, at most, be warm to touch,

40°C (140°F), while wirewound or ceramic resistors are

designed to operate at temperatures up to 140°C

(284°F). Wherever power is dissipated, it is imperative

Figure 10-1. Color codes for resistors. that adequate ventilation is provided to eliminate

Multiplier

1 st significant figure

2 nd significant figure

Body Dot System

1 st significant figure

2 nd significant figure

Multiplier

Tolerance Multiplier

1 st significant figure

2 nd significant figure

Multiplier Tolerance

1 st significant figure

2 nd significant figure

Tolerance

Body-End Band System

Multiplier Tolerance

1 st significant figure

2 nd significant figure

Dot Band System

Multiplier ToleranceMultiplier Tolerance

2 nd significant figure 2 nd significant figure

1 st significant figure 1 st significant figure

Color Band System

Resistors with black body are composition, noninsulated.

Resistors with colored body are composition, insulated.

Wirewound resistors have the 1st color band double

width.

Multiplier

1 st significant figure

2 nd significant figure

Miniature Resistor System

Body-End-Dot System

Carbon composition Carbon film

3 rd significant figure

Black 0^1

Brown 1 10 ±1% 1.0

Red 2 100 ±2% 0.1

Orange 3 1000 ±3% 0.01

Yellow 4 10,000 ±4% 0.001

Green 5 100,000

Blue 6 1,000,000^

Violet 7 10,000,000

Gray 8 100,000,000

White 9 

 Solderable*

Gold 0.1 ±5%

Silver 0.01 ±10%

No Color ±20%

Color Digit Multiplier Tolerance Failure Rate

TCR^ Rr–^100

TR–TTR

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