1996 Nov 15 1
Philips Components
Leaded resistors General Introduction
INTRODUCTION
Data in data sheets is presented, whenever possible,
eaccording to a ‘format’, in which the following chapters
are stated:
•TITLE
•FEATURES
•APPLICATIONS
•DESCRIPTION
•QUICK REFERENCE DATA
•ORDERING INFORMATION
•FUNCTIONAL DESCRIPTION
– Product characterization
– Limiting values
•MECHANICAL DATA
– Outlines
– Mass
– Marking
– Mounting
•TESTS AND REQUIREMENTS
The chapters listed above are explained in this section
“General Introduction Leaded resistors”
, with detailed
information (including
“Packaging”
) in the relevant data
sheet.
DESCRIPTION
Most types of conventional resistors have a cylindrical
ceramic body, either rod or tube. For special purposes, a
high-grade aluminium ceramic is used. The resistive
element is either a carbon film, metal film, thick film or a
wound wire element. Film types have been trimmed to the
required ohmic resistance by cutting a helical groove in the
resistive layer. This process is controlled completely by
computer and yields a high reliability. The terminations are
usually iron end caps onto which tinned connecting wires
of electrolytic copper are welded.
All resistor bodies are coated with a coloured lacquer or
enamel for protection. Dependent on types, this lacquer
provides electrical, mechanical and/or climatic protection,
also against soldering flux and cleaning solvents, in
accordance with
“MIL-STD-202E”
, method 215 and
“IEC 68-2-45”
.
ORDERING INFORMATION
Resistors are ordered by their ordering code, a 12-digit
number. The packaging method and resistance code are
integral parts of this number.
FUNCTIONAL DESCRIPTION
The functional description includes: nominal resistance
range and tolerance, limiting voltage, temperature
coefficient, absolute maximum dissipation, climatic
category and stability.
The limiting voltage (DC or RMS) is the maximum
voltage that may be continuously applied, see
“IEC publications 115-1 and 115-2”
. Where applicable,
derating details and performance nomograms are
given, showing the relationship between power
dissipation, ambient temperature, hot-spot temperature
and maximum resistance drift after prolonged operation.
For power resistors, graphs indicate the relationship
between temperature rise and dissipation with lead-length
or heatsinks as parameters.
The temperature rise in a resistor due to power dissipation,
is determined by the laws of heat - conduction, convection
and radiation. The maximum body temperature usually
occurs in the middle of the resistor and is called the
hot-spot temperature.
Heat conducted by the leads - which can be considerable
in power types - must not reach the melting point of the
solder at the joints. This condition may require the use of
heatsinks and/or longer leads.
In the normal operating temperature range of film resistors
the temperature rise at the hot-spot, ∆T, is proportional to
the power dissipated: ∆T = A × P. The proportionally
constant ‘A’ gives the temperature rise per Watt of
dissipated power and can be interpreted as a thermal
resistance in K/W. This thermal resistance is a function of
the dimensions of the resistor, the heat conductivity of the
materials used and to a lesser degree, the way of
mounting. The sum of the temperature rise and the
ambient temperature is:
Tm=T
amb + ∆T
where:
Tm= hot-spot temperature
Tamb = ambient temperature
∆T = temperature rise at hot-spot.
The stability of a film resistor during endurance tests is
mainly determined by the hot-spot temperature and the
resistance. The lower the resistance - other conditions
remaining constant - the higher the stability due to greater
film thickness.
1996 Nov 15 2
Philips Components
Leaded resistors General Introduction
Summarizing
Performance
When specifying the performance of a resistor, the
dissipation is given as a function of the hot-spot
temperature, with the ambient temperature as a
parameter.
From ∆T=A×P and Tm=T
amb +∆T it follows that:
If P is plotted against Tm for a constant value of A, parallel
straight lines are obtained for different values of the
ambient temperature.
DESCRIPTION RELATIONSHIP
Dimensions and conductance
of materials determine heat resistance
Heat resistance × dissipation
gives temperature rise
Temperature rise + ambient
temperature give hot-spot temperature
Hot-spot temperature and
resistance value determine stability
PTmTamb
–
A
---------------------------
=
The slope of these lines,
is the reciprocal of the heat resistance and is the
characteristic for the resistor.
The stability can be determined experimentally, for
instance after 1000 h, as a function of the hot-spot
temperature with the resistance value as a parameter.
It has been found that the resistance changes
exponentially with temperature, giving a straight line
when log is plotted against Tm.
A combination of the graphs of P and against Tm gives
a nomogram from which the values of several variables
can be determined for a resistor of a given size under
different working conditions. An example of such a
nomogram with fictitious values is given in Fig.1. The
intersection of the broken line with the horizontal axis gives
the hot-spot temperature under chosen conditions.
dP
dTm
----------- I
A
----
=
∆R
R
--------
∆R
R
--------
∆R
R
--------
1996 Nov 15 3
Philips Components
Leaded resistors General Introduction
Fig.1 Performance nomogram (for a fictitious resistor) illustrating the way of specifying
the performance of film resistors.
handbook, full pagewidth
MBC679
0.8
0.4
0.2
0.3
0.1
P
(W)
50 100 150 Tm( C)
o
10 1 0.5 0.1
FE a
A
e
∆ R
R
(%)
amb
T =
20 C
o
60 C
o
70 C
o
100 C
o
140 C
o
150 C
o
B
d
cC
D
b
R =
100 Ω
10 kΩ
1 MΩ
after 10 000 h
after 1000 h
Example 1
Assume that a 10 kΩ resistor, whose characteristics are
described by the nomogram, is to be operated at a power
dissipation of 0.4 W and an ambient temperature of 60 °C.
To establish whether this dissipation is allowable at this
ambient temperature and, if so, what the expected stability
of the resistor will be, draw a horizontal line in the upper
half of the nomogram through point A (power dissipation of
0.4 W). This line intersects the 60 °C ambient temperature
line at point B, corresponding to a hot-spot temperature of
128 °C (point C). This is safely below the maximum
indicated by the broken line at 155 °C; therefore a
dissipation of 0.4 W at an ambient temperature of 60 °C is
well within the allowable limit.
Extend line BC into the lower half of the nomogram until it
intersects the 10 kΩ line at point D. Draw a horizontal line
to the left from point D until it intersects the line ‘after
1000 h’ and extend vertically to point E. This means that at
a hot-spot temperature of 128 °C a resistance change of
about 2.5% (point E) can be expected after 1000 hours of
operation. After 10000 hours, the change will be about 9%
(point F).
1996 Nov 15 4
Philips Components
Leaded resistors General Introduction
Example 2
Assume that a 100 Ω resistor, whose characteristics are
described by the nomogram, is to be operated at an
ambient temperature of 70 °C with a required stability after
1000 h of 0.5% (point a). It is desired to find the maximum
permissible power dissipation. In the lower half of the
nomogram, a line that corresponds to a stability after
1000 h of 0.5% intersects the 100 Ω resistance line at
point b, corresponding to a hot-spot temperature of 112 °C
(point c).
Extending the line (b-c) into the upper half of the
nomogram, it intersects the line indicating an ambient
temperature of 70 °C at point d, corresponding to a
maximum permissible power dissipation of 0.25 W
(point e).
If the power to be dissipated exceeds the value found,
a resistor of higher value should be used.
The temperature coefficient
The temperature coefficient of resistance is a ratio which
indicates the rate of increase (decrease) of resistance per
Kelvin (K) increase (decrease) of temperature within a
specified range, and is expressed in parts per million
perK(×10−6/K).
Example: If the temperature coefficient of a resistor of
Rnom =1MΩ between −55 °C and +155 °C is
±100 ×10−6/K its resistance will be,
at 25 °C:
1000000 Ω (nominal = rated value)
at +155 °C:
1000000 Ω±(130 ×100 ×10−6)×1000000 Ω
= 1013000 Ω or 987000 Ω
at −55 °C:
1000000 Ω±(80 ×100 ×10−6)×1000000 Ω
= 1008000 Ω or 992000 Ω
If the temperature coefficient is specified as ≤100 ×10−6/K
the resistance will be within the shaded area as shown in
Fig.2.
Fig.2 Temperature coefficient.
handbook, full pagewidth
MBC796
Rnom
1.3%
0.8%
0.8%
1.3%
8 kΩ
15525055 T ( C)
o
13 kΩ
1996 Nov 15 5
Philips Components
Leaded resistors General Introduction
THERMAL RESISTANCE (Rth)
Thermal resistance that prohibits the release of heat
generated within the resistor to the surrounding
environment. It is expressed in K/W and defines the
surface temperature (THS) of the resistor in relation to the
ambient temperature (Tamb) and the load (P = dissipation)
of the resistor, as follows:
THS =T
amb +P×R
th
The thermal resistance given in the specification is
determined in accordance with DIN 44050
(Tamb between 20 and 25 °C).
The resistor is mounted on a PCB (see Fig.3) which is set
up vertically, with the resistor horizontal. Using an infrared
camera, a thermal image is made of the resistor, thus
defining the hot-spot and solder-spot temperatures.
It should be noted that different ways of mounting give
differing results, i.e. mounting with a higher heat
conductance gives a lower thermal resistance figure;
mounting with a lower heat conductance gives a higher
thermal resistance figure.
PULSE-LOAD BEHAVIOUR
Knowing the thermal characteristics of a resistor, it is
possible to calculate the dissipation due to a single pulse,
which will cause a resistor to fail by going open circuit. This
theoretical maximum can be expressed in terms of
maximum peak pulse power ( max) and pulse duration (ti);
the straight line in Fig.4 is a typical example for a film
resistor. In practice, owing to variations in the resistance
film, substrate, or spiralling, resistors fail at loads less than
this theoretical maximum; the dashed line in Fig.4 shows
the observed maximum for a resistor under
single-pulse-load.
The magnitude of a single pulse at which failure occurs is
of little practical value. More usually, the resistor must
withstand a continuous train of pulses of repetition time tp
during which only a small resistance change is acceptable.
This resistance change ∆R/R is equal to the change
permissible under continuous load conditions. The
continuous pulse train and small permissible resistance
change both reduce the maximum handling capability.
Using a computer program which takes account of all
factors affecting behaviour under pulse loads, curves
similar to those of Fig.4 are being produced for all resistor
ranges.
Measurements have shown that the calculated value is
accurate to within 10% of the true value. However,
maximum peak pulses as indicated in Fig.5 should not be
exceeded.
P
ˆ
handbook, full pagewidth
MLB380
60
90
ehole O 1.2
1
1.5
0.5 2
connecting
point copper 35 µm
not tinned
Fig.3 Mounting dimensions.
Dimensions in mm.
1996 Nov 15 6
Philips Components
Leaded resistors General Introduction
Fig.4 Maximum permissible peak pulse power as a function of pulse duration (ti) for a typical resistor.P
ˆmax
()
handbook, full pagewidth
10 10 10 10 10 10
654321
1
103
10 1
1
10
102
Pmax
(V)
t (s)
i
P for single pulse
(experimental)
P for single pulse
(theoretical)
P for
max
repeated
pulses
∆ R/R equal to
continuous load
P for continuous load
1000
t /t =
pi
500
200
100
50
20
10
5
2
MBC677
Fig.5 Maximum permissible peak pulse voltage ( max) as a function of pulse duration (ti) for a typical resistor.V
ˆ
handbook, full pagewidth
0
600
10 2
MBC678
10 3
10 4
10 5
10 6
200
400
t (s)
i
Vmax
(V)
300
100
500
1996 Nov 15 7
Philips Components
Leaded resistors General Introduction
Fig.6 Rectangular pulses.
handbook, halfpage
MGA206
ti
V
Vi
t
tp
Fig.7 Exponential pulses.
handbook, halfpage
V
t
τ
MGA207
tp
Definition of symbols (see Figs 4, 5, 6 and 7)
Definitions of pulse-load behaviour; metal film
resistors
SINGLE PULSE
The resistor is considered to be operating under single
pulse conditions if, during its life, it is loaded with a limited
number (approximately 1500) of pulses over long time
intervals (greater than one hour).
REPETITIVE PULSE
The resistor is operating under repetitive pulse conditions
if it is loaded by a continuous train of pulses of similar
power.
SYMBOL DESCRIPTION
applied peak pulse power
maximum permissible peak pulse power
(Fig.4)
iapplied peak pulse voltage (Figs 6 and 7)
maximum permissible peak pulse voltage
(Fig.5)
Rnom nominal resistance value
tipulse duration (rectangular pulses)
tppulse repetition time
τtime constant (exponential pulses)
Tamb ambient temperature
Tm(max) maximum hot-spot temperature of the
resistor
P
ˆ
P
ˆmax
V
ˆ
V
ˆmax
Determination of pulse-load
The graphs in Figs 4 and 5 may be used to determine the
maximum pulse-load for a resistor. The calculations
assume:
Tamb =70°C
T
m
is the maximum permissible hot-spot temperature
for the relevant resistor family
∆R/R equal to the permitted value for 1000 hours at
continuous level.
•For repetitive rectangular pulses:
– must be lower than the value of max given by
the solid lines of Fig.4 for the applicable value of ti
and duty cycle tp/ti.
–i must be lower than the value of max given in
Fig.5 for the applicable value of ti.
•For repetitive exponential pulses:
– As for rectangular pulses, except that ti = 0.5 τ.
•For single rectangular pulses:
– must be lower than the max given by the
dashed line of Fig.4 for the applicable value of ti.
–i must be lower than the value of max given in
Fig.5 for the applicable value of ti.
V
ˆi2
R
------- P
ˆ
V
ˆV
ˆ
V
ˆi2
R
------- P
ˆ
V
ˆV
ˆ
1996 Nov 15 8
Philips Components
Leaded resistors General Introduction
Examples
Determine the stability of a typical resistor for operation
under the following pulse-load conditions.
CONTINUOUS PULSE TRAIN
A 100 Ω resistor is required to operate under the following
conditions: i= 40 V; ti=10
−5
s; tp=10
−3
s.
Therefore:
= = 16 W and
For ti=10
−5s and , Fig.4 gives max =19W
and Fig.5 gives max = 500 V. As the operating conditions
=16W and i= 40 V are lower than these limiting
values, this resistor can be safely used.
SINGLE PULSE
A 1000 Ω resistor is required to operate under the
following conditions:
i= 200 V; ti=10
−4s
Therefore:
max = = 40 W
The dashed curve of Fig.4 shows that at ti=10
−4s, the
permissible max = 70 W and Fig.5 shows a permissible
max of 480 V, so this resistor may be used.
V
ˆ
P
ˆ402
100
---------- tp
ti
---- 10 3–
10 5–
----------- 100==
t
p
t
i
---- 100=P
ˆ
V
ˆ
P
ˆV
ˆ
V
ˆ
P
ˆ2002
1000
-------------
P
ˆ
V
ˆ
MECHANICAL DATA
A dimensional sketch and if applicable, a table of
dimensions is given. The lead length of axial types is not
usually stated if the resistors are only available on tape.
The sketch (see Fig.8) does include however, length (L),
diameter of the body (∅D) and the lead diameter (∅d). For
certain types, the length is stated as L1 and L2; L1 is the
body length, L2 is the body length plus lacquer on the
leads. By specifying L1/L2, the dimensional ‘clean lead to
clean lead’ properties can be determined.
The length of the cylindrical body (L1) is measured by
inserting the leads into the holes of two identical gauge
plates (Fig.9) and moving these plates parallel to each
other, until the resistor body is clamped without
deformation
(“IEC publication 194”)
.
This method does not apply to rectangular resistors,
‘stand-up’ types and wirewound resistors with side
terminations.
1/1 page = 296 mm (Datasheet) 27 mm
DO
MBC684
L1
L2
dO
Fig.8 Component outline.
1996 Nov 15 9
Philips Components
Leaded resistors General Introduction
Fig.9 Measurement of dimension L1.
handbook, 4 columns
DO
MBC673
L1
dO
28 2 28 2
gauge plate
Dimensions in mm.
For dimensions see Table 1.
handbook, halfpage
0.6O
O 2.5 max
0.5
max
23 1
MBC685
11 1
8.7
0.5
0.8O
Fig.10 SFR25 and VR25A are available as
‘stand-up’ types and shown in the
‘mounted’ position.
Dimensions in mm.
The relationship between the diameter of the leads and the
diameter of the holes in the gauge plate is shown in
Table 1.
Table 1 Lead diameter and hole dimensions
Mass
The mass is given per 100 resistors.
Marking
The resistors are either colour coded or provided with an
identification stamp. The colour code consists of a number
of coloured bands in accordance with IEC publication 62:
“Colour code for fixed resistors”
. See also
“IEC 115-1”
,
clause 4.5. The coloured bands indicate the nominal
resistance, the tolerance on the resistance and, if
applicable, the temperature coefficient. A maximum of
bands may be used, but in some instances there are
fewer, e.g. if the products are too small.
∅d
(mm) HOLE DIAMETER
(mm)
0.5 0.8
0.6 1.0
0.7 1.0
0.8 1.2
1996 Nov 15 10
Philips Components
Leaded resistors General Introduction
Fig.11 Marking.
handbook, full pagewidth
MLB747
figures multiplier
(Ω)tolerance temp. coeff.
(.10 /K)
6
0.01
0.1
1
10
100
1K
10K
100K
1M
10M
10 %
5 %
1 %
2 %
0.5 %
0.25 %
0.1 %
0
1
2
3
4
5
6
7
8
9
200
100
50
15
25
10
5
1
silver
gold
black
brown
red
orange
yellow
green
blue
violet
grey
white
Theresistance code consists of either three or four bands
and is followed by a band representing the tolerance. The
temperature coefficient is to the right of the tolerance
band and is usually positioned on the cap (MRS types), as
a wide band. When five or six bands in total are used, the
last band will always be the wider one.
The resistance code includes the first two or three
significant figures of the resistance value (in ohms),
followed by an indicator. This is a factor by which the
significant-figure value must be multiplied to find the
relevant resistance value. Whether two or three significant
figures are represented depends on the tolerance: ±2%
and higher requires two bands; ±1% and lower requires
three bands.
The ‘figures’ refer to the first two or three digits of the
resistance value of the standard series of values in a
decade, in accordance with
“IEC publication 63”
as
indicated in the relevant data sheet and shown on the
inside back cover of this handbook.
Certain resistors are not coded by colour bands but by a
stamp giving pertinent data (alphanumeric marking). This
is adopted with MIL types MR24E/C/D, MR34E/C/D,
MR54E/C/D and MR74E/C/D, as well as PR37 and PR52.
Resistors outside the standard
“IEC 63”
series of types
MPR24 and MPR34, are stamped. All wirewound resistors
are stamped.
Body colours
Table 2 The resistor bodies are lacquered in different
colours to simplify identification
Mounting
Most types with straight axial leads and most in the
‘stand-up’ version (radial leads; see Fig.10) are suitable for
processing on automatic insertion equipment, cutting and
bending machines.
COLOUR TYPE
Tan CR25
Light green SFR25/SFR16
Grey NFR25, NFR25H
Green MR25, MR30, MR52, MR24E/C/D,
MR34E/C/D, MR54E/C/D, MR74E/C/D,
MPR24, MPR34, MRS16T, MRS25, AC04,
AC05, AC07, AC10, AC15, AC20
Light blue VR25, VR37, VR68, SFR16S
Red PR37, PR52, PR01, PR02, PR03
Brown WR0167E, WR0842E, WR0825E,
WR0865E
Red-brown SFR25H
1996 Nov 15 11
Philips Components
Leaded resistors General Introduction
TESTS AND REQUIREMENTS
Essentially all tests on resistors are carried out in accordance with the schedule of
“IEC publication 115-1”
in the specified climatic category and in accordance with IEC publication 68,
“Recommended basic climatic and
mechanical robustness testing procedure for electronic components”
. In some instances deviations from the
IEC recommendations are made.
