Snap Action Switches – Technical Guide
Switch construction
These basic principles apply to all
our precision switches. The specific
characteristics of each model are given
in more detail in the relevant production
sections.
Single-pole changeover switch (i.e. 83 161)
Push button
Case
Mounting hole
Introduction
Our switches are high-precision, snap-action
switches and these are a few of the key
features which distinguish our switches:
- High ratings with small dimensions
- Very short travels
- Low operating forces
- Highly dependable force and travel values
- Long life
- Large range of actuators for easy
adaptation to the most varied applications
Terminal NC nº2
Return spring
Contact mobile
Contact fixed
Terminal NO nº4
Mounting hole
Common terminal C nº1
Pivot point
Blade
Electrical function
SPDT
(C)
2
1
4
1
Normally closed
(NC)
2
1
Normally open
(NO)
4
Double-pole changeover switch (i.e. 83 132 0)
Push button
Blade
Case
Contact fixed
Terminal NO nº3
Terminal NC nº1
Terminal NO nº4
Contact mobile
Terminal NC nº2
Mounting hole
Mounting hole
Return spring
Electrical function
SPDT
(C)
3
1
4
2
1
Normally closed
(NC)
2
Normally open
(NO)
3
4
The NO and NC circuits must both be of the same polarity.
Products and specifications subject to change without notice.
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Snap Action Switches – Technical Guide
Mechanical characteristics
Terminology - Forces - Positions - Travel
OF
Position of pushbutton
TTF
OT
RF
PT
RP
TP
OL
DT
RLP
Electrical
circuit
OF Operating Force
Maximum force which must be applied to
the operating device to displace the rest
position RP to the tripping point TP.
Forces
TTF Total travel force
Force required to displace the operating
device from its rest position RP to its
overtravel limit position. (We only specify
this value if it is higher than the operating
force. When not quoted, it is equal to or
less than the operating force).
MOF Maximum overtravel force
The maximum force which can be
applied to the operating device, without
damaging it, in the end of travel position
where it is in abutment internally or
against the face of the case.
RF Release force
Force to which the operating force must
be reduced to allow the snap-action
mechanism to return to its release
position RLP.
3
RLP Release position
Position of the operating device at the
moment when the snap-action mechanism
trips on its return to its original position.
RP Rest position
Position of the operating device
when no external mechanical force is
applied. Also described as "height at
rest".
Positions
TP Tripping point
Position of the operating device relative
to the fixing point (hole, face) at the
moment when the microswitch trips.
OL Overtravel limit
Position of the operating device when an
extreme force has moved it to the
effective end of the available travel.
The reference point for the figures given for travel and forces is a point F situated on the button in the case of a plain microswitch, or, generally, 3 mm in from the end of a plain actuator.
The reference point for the positions is one of the fixing holes, unless otherwise indicated.
Graphs of forces vs. travel
Operating force (external) applied
to the operating device
depression
return
Travel
PT Pre-travel
Distance between the rest position RP
and the tripping point TP.
OT Overtravel
Distance between the tripping point TP
and the overtravel limit OL.
DT Differential travel
Distance between the tripping point TP
and the release position RLP.
Contact force
Force after depression:
return
outward
Button movement
Contact
force
Products and specifications subject to change without notice.
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Snap Action Switches – Technical Guide
Mechanical characteristics
Changeover time
This is the time taken by the mobile contact
when moving from one fixed contact to another
until it becomes fully stable (contact bounce
included). This time is a function of the contact
gap, the mechanical characteristics of the snap
action and the mass of the mobile element.
However, thanks to the snap-action
mechanisms employed, the time is largely
independent of the speed of operation.
It is normally less that 20 milliseconds
(including bounce times of less than 5 ms).
Environmental conditions
Resistance to shocks and vibrations
Resistance to impact and vibration depends on
the mass of the moving parts and on the forces
holding the contacts together.
Generally speaking, for a switch without an
actuator :
- Vibration >10 G 10 at 500 Hz
- Impact
> 50 G 11 ms 1/2 sine-wave
Further information on request.
Ambient operating temperature
The maximum and minimum temperatures at
which the mechanical and electrical
characteristics of the switch will remain
substantially unaltered.
Bounce time
about
Changeover time
Mechanical durability
This is an average value indicating the
purely mechanical performance of a switch
when not subject to any electrical load.
It may be useful for evaluation purposes in
cases where the power levels involved are
very low and the electrical life is thus close
to the mechanical life.
Degree of protection
Under the IEC 529 or NFC 20010 classification
scheme, standards employ an IP code to
define the degree or class of protection which
electrical equipment provides against access
to live components, the entry of solid foreign
bodies and ingress of water.
1st numeral
Protection equipment
provides against the entry
of solid foreign bodies
0
4
5
6
(not protected)
diameter ≥ 1 mm
protected against dust
sealed against dust
Maximum speed and rate of operation
Our switches will work at speeds of operation
varying over a very wide range : normally
from 1 mm/min to 1 ms.
The maximum rate of operation with a low
electrical load may be as high as 10
operations/second.
Protection for persons
against access to
dangerous parts
(not protected)
1 mm Ø wire
1 mm Ø wire
1 mm Ø wire
Mounting - Operation
• To conform to the leakage paths and air gaps
in the standard EEC24 - EN/IEC 61058 -
EN/IEC 60947:
- An insulation pad must be inserted between
the switch and the fixing surface if the latter is
metal.
- Manual operation of a metal actuator must
only be carried out with the help of a secondary
actuator made of insulating materials.
• The installer must ensure adequate protection
against direct contact with the output terminals.
2nd numeral
Protection equipment
provides against ingress of
water
0
4
5
6
(not protected)
splashed water
hosed water
high-pressure
hosed water
7
temporary immersion
8
prolonged immersion
Fixing - Screw torque
• Unless otherwise indicated in the mechanical
characteristics table, the torque required
for the fixing screws must conform to the
following values :
Ø of fixing screw
2
Screw torque maximum 25
in cm.N
minimum 15
2.5
35
25
3
60
40
3.5
100
60
4
150
100
Under this classification, our switches come
within the following categories :
- Plain switches = IP 00
- Protected switches = IP 40 with isolated
connection
- Sealed switches = IP 66 or IP 67
Products and specifications subject to change without notice.
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Snap Action Switches – Technical Guide
Dielectric characteristics
Current rating
This is the current the switch is capable of
making and breaking which forms the basis for
the life tests.
Circuit types
Resistive circuit
For a circuit with alternating voltage, this is in
phase with the current : Cos
ϕ
= 1.
Thermal rating
This is the current the switch will withstand
when not being operated, for a temperature
rise of not more than 60 °C.
τ
=L
Inductive circuit
A circuit of this type with direct current is
characterised by a time constant.
An inductive circuit, with alternating voltage, for
example, incorporating a motor (cos
ϕ
< 1) can
cause current surges up to 6 times the normal
current. For certain switches, we give electrical
endurance curves with L = 5 ms in DC
R
and cos
ϕ
= 0.8 in AC.
R
Switch rating
AC voltages: see the current rating.
With DC voltages the switch rating is very much
dependent on the voltage, the contact gap
(CG)
and the nature of the load being switched.
There is a risk of prolonged or indeed
permanent arcing if the following limits are
exceeded:
Limit of constant current D.C.
Resistive load
Inductive load L/R = 5 ms
M
220
Lamp and capacitance circuit
The currents at the time when the circuit is
closed are very high in this case, being up to
10 times the nominal figure.
48
24
12
➛
EC=0.4 mm
CG=0.4 mm
➛
0.3 0.5
1
2
For special applications, please enquire.
Operating curves
These indicate the electrical life of the
switches, under standard conditions (20 °C,
1 cycle/2 seconds), by showing the number of
switching operations which can be performed
with given types of load.
Number
of
operations
Resistive circuit
Inductive circuit
Products and specifications subject to change without notice.
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➛
5
110
EC=3 mm
CG=3 mm
3
Internal resistance
10 16
This consists of the intrinsic resistance (fixed)
of the parts carrying current and the contact
resistance (variable).
Close to the tripping point and release position,
the force holding the contacts together drops
considerably and this may cause a rise in
internal resistance.
Insulation resistance
The insulation resistance of the switches is
generally greater than 50,000 MΩ measured at
500 V DC.
Dielectric strength
The dielectric strength of our switches is
generally better than:
- 1500 volts between live parts and earth
- 1000 volts between contacts
- 600 volts between contacts for switches
whose contact gap is less than 0.3 mm.
Rating in
Amps
Snap Action Switches – Technical Guide
Contact materials
Choice of contact material
To choose the best material for the contacts
there are various factors to be considered:
- the current and voltages levels
- the type of load
- the number of operations
- the switching frequency
- the environmental conditions.
Electrical recommendations
Inductive circuits
To increase the life of contacts and their DC
rating, arcing on opening can be cut down by
using the following circuits:
- for DC
Fast diode V R > 5 x V nominal
I nominal > 10 x I winding
Contacts for general-purpose use
Our switches are normally fitted with silver
contacts. These are suitable for the majority of
applications and provide the best compromise
between electrical performance, thermal
performance and life.
A
- for DC or AC
A - RC circuit across inductor
B - RC circuit across switch
B
C
C (nF) ~100 x I nominal (A)
V insulation > V peak
R(Ω) ~ load resistance (Ω)
Contacts for low-power circuits
E < 20 V and/or I < 100 mA
The contacts used in this case are plated with
gold (or a gold alloy) for good reliability even in
corrosive atmospheres.
D
C - Varistor circuit across load
D - Varistor circuit across switch
V > V peak supply
P (V.A.)
E (J) ≥ –––––––
100
Contacts for special applications
We can supply special contacts suitable for
particular applications, such as:
- Ag CdO contacts for very high drawn
currents,
- Cross Bar gold-plated Ag Ni contacts which
allow a very wide range of applications to be
covered by a single type of switch.
Very low power circuits
In very low power circuits (I > 1 mA, V ≤ 5 V),
switching is highly sensitive to environmental
conditions (the atmosphere, pollution).
If the supply is powerful enough, adding a
passive resistor to increase the current broken
by the switch to a few milliamps will
substantially improve reliability of operation.
R - Load resistance
C - Very low current load
Products and specifications subject to change without notice.
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