RES-SELECT Datasheet, PDF (1/12 Pages) –

Resistor Selection
Application Notes
R e s i s t o r F a c t s an d F a c t o r s
A resistor is a device connected into an electrical circuit to
introduce a specified resistance. The resistance is measured in
ohms. As stated by Ohm’s Law, the current through the resistor
will be directly proportional to the voltage across it and inverse-
ly proportional to the resistance.
The passage of current through the resistance produces
heat. The heat produces a rise in temperature of the resis-
tor above the ambient temperature. The physical ability of the
Selection Requires 3 Steps
resistor to withstand, without deterioration, the temperature
attained, limits the operating temperature which can be permit-
ted. Resistors are rated to dissipate a given wattage without
exceeding a specified standard “hot spot” temperature and the
physical size is made large enough to accomplish this.
Deviations from the standard conditions (“Free Air Watt
Rating”) affect the temperature rise and therefore affect the
wattage at which the resistor may be used in a specific applica-
tion.
Simple short-cut graphs and charts in this catalog permit rapid
determination of electrical parameters. Calculation of each
parameter is also explained. To select a resistor for a specific
application, the following steps are recommended:
1 . (a) Determine the Resistance.
(b) Determine the Watts to be dissipated by the Resistor.
2 . Determine the proper “Watt Size” (physical size) as controlled by
watts, volts, permissible temperatures, mounting conditions and
circuit conditions.
3 . Choose the most suitable kind of unit, including type, terminals and
mounting.
S t e p 1 d e t e r m i n e r e s i s t an c e an d w a t t s
Ohm’s Law
(a)
R
=
V
I
or
I
=
V
R
or
V = IR
Ohm’s Law, shown in formula form
above, enables determination of the
resistance when the required voltage and
current are known. When the current and
voltage are unknown, or the best values
not decided on, at least two of the three
terms in Ohm’s Law must be measured
in a trial circuit.
(b) P = I²R

or
P = VI
or
P
=
V²
R
Power in watts, can be determined from
the formulas above, which stem from
Ohm’s Law. R is measured in ohms, V in
volts,I in amperes and P in watts.
Why Watts Must Be Accurately Known
Stated non-technically, any change in
current or voltage produces a much
larger change in the wattage (heat to be
dissipated by the resistor). Therefore,
the effect of apparently small increases
in current or voltage must be investi-
gated because the increase in wattage
may be large enough to be significant.
Mathematically, the wattage varies as
the square of the current, or voltage, as
stated in the formulas (b). For example,
an increase of 20% in current or voltage
will increase the wattage 44%. Figure 1
below graphically illustrates the square
law relation. Hence, the actual current
must be used in figuring the wattage and
the increase in wattage due to appar-
ently small changes, then determined in
order to select the proper size resistor.
Allowance should be made for maximum
possible line voltage.
400
300
200
100
0
0
100
200
Percent Current
or Voltage
Fig. 1: Rapid increase of wattage with current or voltage.
Step 2 Power Rating or Physical Size of Resistor
A resistor operated at a constant wattage will attain a steady tem-
perature which is determined largely by the ratio between the size
(surface area) and the wattage dissipated, The temperature stabilizes
when the sum of the heat loss rates (by radiation, convection and
conduction) equals the heat input rate (proportional to wattage). The
greater the resistor area per watt to be dissipated, the greater the
heat loss rate and therefore the lower the temperature rise. The rela-
tion between the losses varies for different resistors.
Free Air Watt Rating
The wattage rating of resistors, as established under specified stan-
dard conditions, is defined as the “Free Air Rating” (“Full Rating” or
“Maximum Power Rating”). Several standard methods of rating are
in use based on different service conditions. The method of both
the “National Electrical Manufacturers Association” (NEMA) and the
“Underwriters’ Laboratories, Inc.” (UL) can be described as follows:
The relation of the “Free Air Watt Rating” of tubular type, vitre-
ous enameled resistors to the physical size, is to be set at such a
figure that when operated at their rated watts, the temperature rise
of the hottest spot shall not exceed 300°C (540°F) as measured by
a thermocouple when the temperature of the surrounding air does
not exceed 40°C (104°F). The temperature is to be measured at
the hottest point of a two-terminal resistor suspended in free still air
space with at least one foot of clearance to the nearest object, and
with unrestricted circulation of air.
A slightly different definition of temperature limit used as a basis
for wattage rating, and which results in a slightly higher attained
temperature, was originally established in military specification MIL-
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