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**Unformatted text preview: **TYPES Of
amro
TO MA RESISTANCE
Since Rzo of Eq. (3.8) is the resistance of the conductor at 20"C and
EXAMPLE 3.8 If the resistance of a copper wire at freezing (0"C)
7 - 20'C is the change in temperature from 20"C, Eq. (3.8) can be
30 0, what is its resistance at - 40"C?
written in the following form:
Solution: Eq. (3.5)
R = P-[1 + 020 AT]
(3.9)
234 5" C+0 234.5"C -4090
30 0
RZ
providing an equation for resistance in terms of all the controlling
R2 =
(30 02 )( 194.5 . C)
234 5" C
= 24.88 0
parameters,
PPM/'C
EXAMPLE 3.9 If the resistance of an aluminum wire at room tempera
For resistors, as for conductors, resistance changes with a change in tem-
have (20'C) is 100 mall (measured by a millichrometer), at what temper.
perabare. The specification is normally provided in parts per million per
atacre will its resistance increase to 120 mo?
Solution: Eq (35)
degree Celsius (PPM/ C), providing an imn
late indication of the sensi
arity level of the resistor to temper
For resistors, a 5000 PPM Level is
256"C + 20"C 236"C+ Iz
considered high, whereas 20 PPM is quite low, A 100O PPMP'C character.
re results in a change is zenis-
100 m()
120 mo
istic reveals that a 19 change in
rance equal to 1000 PPM, or 1009/1,0m,AND = VLOG of its nameplate
and
I2 = 120 mn 2560 C
valve not a significant change for most applications, However, a 10"
(100 m(2- 2365C
change results in a change equal to 1/ICO (19) of is nameplate valve,
which is becoming significant, The concern, therefore, lies not only with
the PPM level, but also with the range of expected temperalive vacation,
In cavallion form, the change in resistance is given by
TABLE 2.A
Temperature coefficient of resistance for panous
//Temperature Coefficient of Resistance
AR = PARINA (PPM)AT)
(3,18)
conduction ar KyC
There is a second popular equation for calculating the resistance of a
conductor at different temperatures, Defining
Material
Temperature
and IT is the change in temperature fron the reference lord of Life.
Copper
(2,7)
Gold
EXAMPLE .10 FOR & I KEL CANON CORPORATION FORGOT WINA A PIMA
as The temperature coefficient of resistance it & temperature of 20C
6,665
and his as the resistance of the sample at 20VC, We determine the resis
tance Re, at a temperature , by
Solution,
6.606
Contrastan
(2.8,
The values of an, for different materials have been evaluated, and a fewr
are listed in Table 24,
Ex (32) can be written in the following for
# 1160 42
AT
from which the site of scifi for a, we defined,
26 TYPES OF RESISTORS
Since ARIAT is the slope of the sucre in Fig, 211, we can conclude that
Fixed Resistors
the higher the temperature coefficient of resistance for a material, the
Revision are made in many forme, the all belong in side of ers
more SERSHIDE IN THE resistance level in changes in temperature,
Referring 16 Table 2.3, we find that copper is more sensitive to rein
perature variations than to silver, gold, of aluminum, although the differ...

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- Fall '19
- SK Abid