General Information

General Information
Vishay
Rated Resistance
Resistance value indicated upon the resistor
Critical Resistance
Resistance value at which the rated voltage is equal to the
limiting element voltage
Resistance Tolerance
Permitted variation of the nominal resistance value
expressed as a percentage of that value
Nominal Dissipation
Maximum permitted load at a defined ambient temperature
e.g. T@ = 70°C, which ensures that resistance stability limits
in the relevant specification are not exceeded.
Limiting Element Voltage
Maximum d.c. or a.c. effective voltage which can be applied
continuously to the resistor
Thermal Resistance
Under electrical load a film resistor generates heat which
increases the film temperature. At the same time heat is
dissipated to the environment, so that with constant electric
load and constant convection a thermal balance appears
between the heat, generated by the electrical load and the
heat lost by convection.
These proportions are characterized by the thermal
resistance. The thermal resistance is defined by the
mechanical dimensions of a resistor, the heat dissipation by
the wire leads as well as the convection, radiation and the
mounting of the resistor.
The thermal resistance Rth is defined as follows:
Rth = (Js -JU)) / P = JÜ / P
Js = film temperature in °C
Ju = ambient temperature in °C
Temperature Coefficient
The permissible change of the resistance value depending
on temperature and can be described by the following
equation:
Jü = temperature rise
P = load in Watts
TC (10-6/K) = (RJ - RTREF.) / (RTREF * DJ) * 10-6
The thermal resistance measurement is made under defined
conditions according to DIN.
DJ is the difference between Reference Temperature (TREF)
and the corresponding ambient temperature.
The maximum permissible increase of the resistance value
by the TC, in case of electric load can be determined by way
of the maximum permissible film temperature. The change of
resistance value is calculated by:
The maximum power rating can be calculated using the
following equation:
RJmax = RN[(1+(Jsmax-20°C)*TCmax)]
Consequently the maximum permissible current for the
voltage for P70 can be calculated by RJmax
Insulation Voltage
Maximum peak voltage which may be applied under
continous operating conditions between the resistor
terminations and any conducting mounting surface.
Insulation Resistance
Electrical resistance value of the encapsulent measured
between the terminations of the resistor and applied V-block
according to IEC60115-1
Derating
Boundary curve of maximum allowable dissipation at T@
between upper and lower category temperatures.
Document Number: 20000
Revision: 16-Oct-03
Pmax = (Js - Ju) / Rth
Thus, the maximum permissible power rating Pmax is
dependant on the maximum permissible film temperature,
the ambient temperature Ju and the thermal resistance.
Current Noise
The current noise voltage expressed in µV, is that portion of
noise voltage that arises from d.c. current in a resistor in
addition to the thermal noise voltage. The relative noise
voltage, expressed in µV/V is independent of the applied
dc-voltage U=
Non linearity A3
The harmonic index and the voltage coefficient of resistors
are a criteria for the non-linearity of the current voltage
characteristic. The harmonic index is defined as the
logarithm of the ratio of the fundamental U1 to the 3rd
harmonic E3. It is specified in dB:
A3 = 20 lg ( U1 / E3 ) in dB
Measurements are according to IEC 60440
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General Information
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Stability
The change of resistance values at certain loads and ambient temperatures can be obtained from the Stability Nomogram which
consists of 4 diagrams; these can also be used independently. The stability nomogram for different products can be seen on
the relevant data sheets. Additionally the limiting values stated in the data sheets such as maximum load, surface temperature
etc., have to be observed.The following examples show how to use a nomogram:
Example 1:
Known: size D
R= 1KW, P = 0.5W, ULEV= 350V, t = 5000h, Ju = 70°C
Example 2:
Known: size F; R = 1M, P70 = 1.5W, ULEV= 500V,
t = 2000h at Ju = 50°C
Unknown: DR /R after 5000h
From Diagram A we see a temperature rise of
Jü = 65°C for size D at P = 0.5W
Unknown: DR /R after 2000h
For R = 1M the following equation applies:
P = (U2LEV / R = 0.25W as U = P* R > ULEV (see the dotted
line in the nomogram).
From Diagram B a surface temperature of 135°C can be
obtained for JU = 70°C
From Diagram D a DR /R after 5000h of 0.4% can be
obtained for a surface temperature of 135°C of a 1KW
resistor (see solid line in nomogram)
Diagram B
Diagram A
120
ze
Si
D
100
80
Size
60
F
40
20
J = f [P]
Parameter: size
1.2
Load in W
Temperature rise Jü in ∞C
140
0.8
0.6
0.4
0
Jü
∞C
40
∞C
60
80
∞C
80
C
0∞
60
10
C
0∞
40
12
C
0∞
20
14
40
80
60
100
120
140
160
180
Film temperature J s in ∞C
Diagram C
Diagram D
0.01
h
0
t=
10
0
00
0.1
0h
h
00 00h 00h
00
10 50 20
10
1
10
Resistance change DR/R in %
Stability nomogram typical values
0.1
1
0.01
0.01
Resistance change
DR/R in % after 1000h
DR/R (t) = f [DR/R (t = 1000h)]
Parameter: time
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C
0∞
=2
100
0
20
0
1.0
140 J
ü = f [Jü]
120 Parameter:Jü
10
W
10
0W
1K
10
33 K
K
0.1
1M
DR/R [t = 1000h] = f [Js ]
Parameter: resistance value
1
20
40
60
80
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100 120 140 160 180
Film temperature Js in ∞C
Document Number: 20000
Revision: 16-Oct-03
General Information
Vishay
120
Jü [∞C]
Packaging density
The temperature rise in respect of the surface temperature of the hottest SMD component on the board can be obtained from
the nomogram below. It is necessary that the components are distributed uniformly over the whole circuit board.
100
JU
0.4W
80
JU
A
60
0.25W
JU
40
0.125W
JU
0.0625W
=2
=4
=6
JU
1
2
3
5 7 10
2
3
5
7 100
2
3
5 7
Example 1:
Known: 9 resistors each rated at 0.25W
Ju = 60°C
Unknown: temp.rise Jü, surface temp. Js of the
hottest component
Ju = 65°C (A), Js = 125°C (B), see solid line
Pulse Load
When a resistor is subjected to impulses the following points
have to be observed:
1. The maximum pulse load permissible P^max depends on
the pulse duration ti
This also applies to the maximum permissible pulse
voltage U^max
2. The average load P may not exceed the corresponding
nominal load. For resistors with resistance values greater
than the critical value the nominal value is determined by
the critical value and the maximum operating voltage
permissible.
Required
P =
1
tp ∑ R
t2
Ú U2 ( t ) dtP J
t1
Explanations:
R
= nominal value
tp
= period of time
U (t) = pulse voltage
PJ = nominal load of the resistor for the ambient
temperature J t2 – t1 = pulse duration ti
Document Number: 20000
Revision: 16-Oct-03
60
80
0∞C
0∞C
=8
20
0∞C
0∞C
=1
00
∞C
100
JU
=1
20
∞C
B
140
120
Example 2:
Load of each resistor = 0.0625W, Ju = 50°C maximum
admissible surface temperature 90°C
How many components may be mounted?
- 33 pcs, see dotted line
3. Differences arise when resistors are subject to single shot
(switching-on processes) or repetitive pulses.
Approximate values for the load with rectangular pulses
for each model are stated in the appropriate sections of
the catalog.
All other pulses have to be converted to rectangular
pulses which show the same energy content and the
same pulse voltage.
Example: Exponential pulse
t Us2 / 2R = ti Us2 / R e.g. ti = t / 2
Explanations:
t = time constant of the exponential pulse
ti = pulse duration or the rectangular pulse
Ûs = peak voltage
R = nominal value of the resistor
The maximum permissible pulse voltages Ûmax are also
stated. The permissible pulse loads have been fixed in
such way that the changes which appear in resistance
values are comparable to those stated for the electrical
long time load according to IEC 60115-1.
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For Technical questions in Americas/Asia, contact: [email protected]
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