Resistive Components
Pulse Withstanding
Chip Resistors
PWC Series
Today’s electronic devices are becoming
smaller and smaller. As a result designers
are moving more towards surface mount
components not only for new designs
but also to design out large axial and
other through-hole resistors. In most
cases this is a straightforward task as
several resistor manufacturers offer chip
resistors with performances to match
axial parts. However in some cases, due
to power rating or pulse withstanding
requirements, this has been impossible.
The requirement, in particular, for pulse
withstand capability is growing due to
the need to protect sensitive modern
electronic systems.
To meet this demand Welwyn have
designed a Pulse Withstanding Chip
Resistor (PWC Series). The PWC series is
available in four standard sizes from
0805 to 2512 as shown in the following
table. The table also gives details of the
improved LEV (Limiting Element Voltage)
and increased power rating. Pulse
withstanding details are given on the
following pages.
The enhanced performance of the chips
is made possible by the precise use of
the best resistance inks and a closely
controlled production process.
• Standard chip sizes
(0805 to 2512)
• Improved power
rating
• Improved working
voltage
• Excellent Pulse
Withstanding
performance
Subsidiaries of TT electronics plc
Excellence in Technology
2
Size
Power @70°C
Resistance range
Tolerance
LEV
TCR
Operating temperature
Values
Pulse capability
0805
0.125W
1206
0.33W
1R0 to 10M
0.5, 1, 2, 5%
2010
0.75W
2512
1.5W
150V
200V
400V
500V
<10R:200ppm/°C
≥10R:100ppm/°C
-55 to +155°C
E96 preferred other values to special order
See following pages
Applications
Applications vary from line protection for telecommunications
to surge withstanding resistors for use in circuit breakers.
Details of a typical telecomm and power supply application are
given below.
Telephone lines can be subjected to a large range of voltage
disturbances, many of which can damage switching equipment.
This has led to the need for circuit protection against both high
voltage transients, usually of short duration caused by lighting
strikes and overloads of longer duration, due to direct
connection to mains power lines.
These two faults are separated into primary and secondary
protection. Primary protection handles the high voltage
transients, and is usually located within the exchange.
Secondary protection is usually built into the equipment to be
protected, and deals with both current and voltage limiting.
Voltage limiting prevents damage to the equipment and shock
hazards, and current limiting prevents damage to wiring.
A typical application circuit is shown in fig 1 where the resistors
are designed for ring signal sending through a solid state relay.
The resistors are protected from lightning surge by Over-voltage
Protection in the system and the resistors are required to
withstand pulses of 15 Watts for 1 second and 75 Watts for
0.1 second, repeated 60 times. Test requirements for telecomms
applications are laid down by the International
Telecommunications Union (ITU) and Bell Communications
Research (Bellcore).
Fig 2 shows a typical power supply or battery charger circuit. In
this application the resistor is required to withstand a small
inrush surge, and also a lightning strike surge. The lightning
strike is usually simulated by applying either a 1.2/50µs or
10/700µs pulse shape, the number of pulses and pulse intervals
being specified by the customer or the relevant standard. Typical
standards to be met are Cenelec EN50082-1 and EN50082-2
which are part of the European EMC directives.
To determine the suitability of a PWC series chip resistors for
your application refer to the pulse withstanding data as given
below. Graphs have been produced to show the PWC
performance under single and continuous pulse, maximum
pulse voltage for single and continuous pulses and lighting
surge performance using both 1.2/50µs and 10/700µs pulse
shapes.
Welwyn
Resistor
Lightning Surge
Line
Over
Voltage
Protection
Input
Power
Supply
Circuit
Welwyn
Resistor
Output
Fig 1
Fig 2
3
Description of Performance Tests
Single Impulse
The single impulse graph was the result of 50 impulses of
rectangular shape applied at one minute intervals. The limit of
acceptance was a shift in resistance of less than 1% from the
initial value. The power applied was subject to the restrictions
of the maximum permissible impulse voltage graph as shown.
The formula used to calculate the average power for repetitive
pulses is shown below.
2
P = V t
i
R t
p
For a rectangular impulse
< Pnom
Continuous Load Due to
Repetitive Pulses
The continuous load graph was obtained by applying repetitive
rectangular pulses where the pulse period (tp) was adjusted so
that the average power dissipated in the resistor was equal to
its rated power at 70°C. Again the limit of acceptance was a
shift in resistance of less than 1% from the initial value.
2
For an exponential impulse P = V t
e
< Pnom
R 2t
p
Where
R = nominal resistance
t
p
= time of the pulse period
(1/tp = pulse frequency)
V = peak voltage of the impulse
P = average power dissipation
of continuous pulses
t
i
= impulse time of a rectangular pulse
T
e
= time constant of an exponential pulse
V
t
p
V
t
i
e
t
1
t
2
Rectangular Pulses
t
Exponential Pulse
0,37 .
V
t
Resistive Components
Single Pulse
1000
4
Pulse Power P (W)
100
0805
1206
2010
2512
10
1
0.0001
0.001
0.01
Pulse Duration ti (s)
0.1
1
Continuous Pulses
100
Pulse Power P (W)
10
0805
1206
2010
2512
1
0.1
0.0001
0.001
0.01
Pulse Duration ti (s)
0.1
1
Pulse Voltage
10000
Pulse Voltage (Volts)
1000
O805
1206
2010
2512
100
10
0.0001
0.001
0.01
Pulse Duration ti (s)
0.1
1
Resistive Components
5
Lightning Surge
Resistors are tested in accordance with IEC 60 115-1 using both
1.2/50µs and 10/700µs pulse shapes. The limit of acceptance is
a shift in resistance of less than 1% from the initial value.
1.2/50µs Lightning Surge
10000
Peak voltage (volts)
1000
0805
1206
2010
2512
100
10
1
10
100
1000
10000
Value (ohms)
100000
1000000
10000000
10/700µs Lightning Surge
10000
Peak Voltage (Volts)
1000
0805
1206
2010
2512
100
10
1
10
100
1000
10000
Value (ohms)
100000
1000000
10000000
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