Handbook for Sound Engineers

Resistors, Capacitors, and Inductors 245

thermal destruction of the resistor and surrounding
components.

Power Coefficient. The power coefficient is the
product of the temperature coefficient of resistance and
the temperature rise per watt. It is given in percent per
watt (%/W), and is the change in value resulting from
applied power.

Ac Resistance. The ac resistance value changes with
frequency because of the inherent inductance and
capacitance of the resistor plus the skin effect, eddy
current losses, and dielectric loss.

Ambient Temperature Effect. When operating a
resistor in free air at high ambient temperature, the
power capabilities must be derated, Fig. 10-2. Free air is
operation of a resistor suspended by its terminals in free
space and still air with a minimum clearance of one foot
in all directions to the nearest object.

Grouping. Mounting a number of resistors in close
proximity can cause excessive temperature rise
requiring derating the power capabilities, Fig 10-3. The
curves are for operation at maximum permissible hot
spot temperature with spacing between the closest
points of the resistors. Derating could be less if operated
at less than permissible hot spot temperature.

Enclosure. Enclosures create a rise in temperature due
to the surface area, size, shape, orientation, thickness,

material and ventilation. Fig. 10-4 indicates the effects

on a resistor enclosed in an unpainted steel sheet metal

box, 0.32 inches thick without vents. Determining the

derating is often by trial and error.

f

Figure 10-2. Resistor derating for elevated ambient temper-
ature. Courtesy Ohmite Mfg. Co.

100

80

60

40

20

0

0 40 80 120 160 200 240 280 320 360 400

Ambient temperature rise—oC

% rated wattage

U.L. - NEMA - Industrial Standard

Mil R-26

Figure 10-3. Power derating for grouping resistors. Cour-

tesy Ohmite Mfg. Co.

Figure 10-4. Effect of the size of an enclosure on a 500 W

(^3) / 4 inch × 6 (^1) / 2 inch resistor. Courtesy of Ohmite Mfg. Co.
12
11
10
9 8 7 6 5 4 3 2 1
40 50 60 65 70 75 80 85 90 95 100
% of single unit free air rating
Number of resistors in group
1 inch space
2 inch space
(^3) / 4 inch space
300
200
100
0
0 25 50 75 100
% rated load
Temperature rise—
oC
500
400
300
200
100
0
Tem
perature rise—
oF
A
B
C
D
E
A. Resistor in 3^3 / 8 inch × 3^3 / 8 inch × 8 inch box.
B. Resistor in 5^13 / 16 inch × 5^13 / 16 inch × 12^3 / 4 inch box.
C. Resistor in free air.
D. Box temperature—small.
E. Box temperature—large.
F. Unpainted sheet metal box, 0.32 inch thick steel,
no vents.
F