PTF

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Vishay Dale

Revision: 15-Nov-12

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Document Number: 31019

THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT

LOAD LIFE SHIFT DUE TO POWER AND DERATING (AT 85 °C)

The power rating for the PTF parts is tied to the derating temperature, the heat rise of the parts, and the

R for the load life performance.

When the tables/graphs below are used together they show that when the parts are run at higher power ratings, the parts will run hotter,

which has the potential of causing the resistance of the parts to shift more over the life of the part.

LOAD LIFE SHIFT VS. POWER RATING

LOAD LIFE

CONDITIONS OF TEST

MAXIMUM

R (TYPICAL TEST LOTS)

MIL-PRF-55182 Paragraph 4.8.18

1000 h rated power at + 85 °C

 ± 0.04 %

 ± 0.15 %

 ± 0.5 %

 ± 1.0 %

MODEL

POWER RATING AT + 85 °C

PTF51

1/20 W

1/10 W

1/8 W

1/4 W

PTF56

1/8 W

-

1/4 W

1/2 W

PTF65

1/4 W

-

1/2 W

3/4 W

Example: When a PTF56 part is run at 1/8 W in a 70 °C ambient environment, the resistor will generate enough heat that the surface

temperature of the part will reach about 17 °C over the ambient temperature, and over the life of the part this could cause the resistance

value to shift up to ± 0.04 %.

If the same resistor was instead run at 1/4 W in a 70 °C environment, the element will heat up to about 30 °C over ambient, and over the life

of the part the resistance value could shift roughly ± 0.5 %.

And if the resistor was run at its maximum power rating of 1/2 W in a 70 °C environment, it will heat up to about 61°C over ambient, and you

could see the resistance value shift roughly ± 1 % over the life of the part.

TEMPERATURE COEFFICIENT OF RESISTANCE

Temperature coefficient (TC) of resistance is normally stated as the maximum amount of resistance change from the original + 25 °C value

as the ambient temperature increases or decreases. This is most commonly expressed in parts per million per degree centigrade (ppm/°C).

The resistance curve over the operating temperature range is usually a non-linear curve within predictable maximum limits. PTF resistors

have a very unifom resistance temperature characteristic when measured over the operating range of - 20 °C to + 85 °C. The standard

temperature coefficients available are

X = ± 15 ppm/°C, Y = ± 10 ppm/°C and Z = ± 5 ppm/°C.

Some applications of the PTF require operation beyond the specifications of - 20 °C to + 85 °C. The change in temperature coefficient of

resistance is very small (less than ± 0.05 ppm/°C) over the expanded temperature range of - 55 °C to + 150 °C. Therefore, when operating

outside the range - 20 °C to + 85 °C, the designer can plan for a worst case addition of ± 0.05 ppm/°C for each degree centigrade beyond

either - 20 °C or + 85 °C as indicated in the graph. This applies to all three temperature coefficient codes.

Example: Assume the operating characteristics demand a temperature range from - 55 °C to + 125 °C. This requires a ± 35 °C

 below

- 20 °C and a ± 40 °C

 above + 85 °C. The extreme  being ± 40 °C means that the worst case addition to the specified TC limit of

± 0.05 ppm/°C times ± 40 °C or ± 2 ppm/°C. Therefore, a Z which is characterized by a base TC limit of ± 5 ppm/°C over the temperature

range of - 20 °C to + 85 °C will exhibit a maximum temperature coefficient of ± 7 ppm/°C over the expanded portion of the temperature range

of - 55 °C to + 125 °C.

AMBIENT TEMPERATURE IN °C

DERATING

0

20

40

60

80

100

120

- 55

- 25

0

25

50

75

100

125

150

175

200

85

0

20

40

60

80

100

120

APPLIED POWER IN W

0

0.125

0.25

0.375

0.5

0.625

0.75 0.875

1

1.125

THERMAL RESISTANCE

PTF65

PTF56

PTF51

4

3

2

1

0

- 50 - 40 - 30 - 20

90 100 110 120 130 140 150

EXPANDED

OPERATING RANGE

- 55 °C TO - 20 °C

EXPANDED

OPERATING RANGE

- 85 °C TO + 150 °C

- 20 °C TO + 85 °C

BASE TC LIMIT

X, Y

or Z