Snap Switches – Definitions & Descriptions
Snap switches, also called micro switches, are activated by a spring-operated (or “snap-action”) mechanism. Depressing the actuator triggers the switching operation, with a pre-defined force and travel. The switching speed itself is largely independent of the speed of actuation.
Actuator
Applying force to the actuator of a snap switch releases the snap-action mechanism, which in turn triggers the switching operation.
Auxiliary actuator
It is possible to attach an auxiliary actuator to a snap switch in order to meet the specific requirements of a given application. Doing so usually alters the travel and forces involved in the switching operation, depending on the length of the levers. By attaching an appropriate auxiliary actuator, it is possible to increase travel and/or reduce the actuating force required.
Terminals
- COM (Common = 1): Base terminal
- NC (Normally Closed = 2): The contact is closed in the rest position, that is, the terminal is connected to COM. When the switch is actuated, the contact opens.
- NO (Normally Open = 4): The contact is open in the rest position, that is, the terminal is separated from COM. When the switch is actuated, the contact closes.
Contact gap (contact opening distance)
The contact gap is the distance between a pair of open contacts. For snap switches, it is usually around 0.3 mm. Generally speaking, for switches with contact gaps < 3 mm, additional measures are necessary for sepa ration from the mains. These switches bear the mark µ for European approvals. Switches with a contact gap > 3 mm can generally be used directly for separation from the mains. Please check the device specifi cations applying to your particular product, and if there is any doubt, please clarify with the responsible testing agencies.
Clearance and creepage distance
Clearance is the shortest distance through the air between two electrically conductive parts. The creepage distance is the shortest distance along the surface of an isolating material between two electrically conductive parts.
| Description | Function | Circuit Symbol |
| S.P.D.T. Single Pole Double Throw (Changeover contact) | In rest position the COM terminal is connected to the NC contact.When the actuator is depressed,COM & NC break contact COM and NO make contact. |  |
| S.P.D.T. – N.O. Single Pole Single Throw Normally Open (Make contact) | Normally Open (Make contact)When the switch is actuated, contact is made. |  |
| S.P.D.T. – N.C. Single Pole Single Throw Normally Closed (Break contact) | When the switch is actuated, contact is broken. |  |
Positions, Forces & Travels
| Position | Description |
|---|---|
| Actuator positions | Dimensions for actuator positions are always specified in relation to a given reference line. |
| Rest Position | The position of the actuator when no external force is applied. |
| Operating Point (mech.) | The point along the actuator’s travel path at which the spring-operated mechanism is actuated. |
| End Position | The position of the actuator at the end of its travel. |
| Reset Point (mech.) | The point along the actuators path, as it travels back towards the rest position, at which the spring-operated mechanism snaps back to it’s original position. |
| Travel Distance | Description |
|---|---|
| Pre-Travel | The distance travelled between the actuator’s rest position and the switching point. |
| Overtravel | The distance travelled between the switching point and the end position. To make absolutely sure that the switching operation takes place, the actuator should use up at least 50% of the available overtravel. |
| Reset Travel | The distance travelled between the operating force and the release point. |
| Free Travel | The distance travelled between the reset point and the rest position. |
| Total Travel | The difference between the rest position and the final position. |
| Movement Differential | The distance travelled between the operating point and the reset point. |
Acutator Position
| Force | Description |
|---|---|
| Initial Force | The force required to move the actuator away from it’s rest position. |
| Operating Force | The force required to move the actuator through the operating point. |
| Sustaining Force | The force required to keep the actuator in it’s final position. |
| Reset Force | The level to which the operating force must be reduced in order to allow the spring-operated mechanism to return to it’s original position. |
| Differential Force | The difference between the operating force and the rest force. |
Diagram showing relationship between operating force and travel
Diagram showing the relationship between contact force and travel
Operating Life, Temperature Resistance, Vibration & Electric Resistance
The operating life specifies the minimum number of switch cycles within the specific values. It depends on a large number of parameters that are determined by the intended application case. Among these are, for example:
- switched current and switching voltage
- type of load (e.g. ohmic, inductive or lamp load)
- Combination of materials in actuating element/actuator
- Actuator type
- Actuator speed
- Switching frequency (switching cycles/min)
- Pretravel/Overtravel
- Environmental factors such as climate conditions or harmful gases (e.g. SO2).
Electrical life
The selection of the optimal contact material has great influence on the operating life. The electrical life test is conducted at rated voltage, rated current and resistive load. The lower the electrical current, the longer the elec- trical life – under some circumstances it may even equal the switch’s mechanical life.
Please note:
Media such as greases, oils and materials which contain silicone must not be used on the switch. There is a distinc- tion between mechanical and electrical operating life.
Mechanical life
Indicates how often a switch can be actuated without an electrical load. Mechanical endurance is calculated by actuating the snap switches axially in relation to the actuator in a sinusoidal pattern using about 80% overtravel at a switching frequency of 4 Hz at room temperature.
Please note:
For switching loads which deviate from the values specified in the catalogue, we recommend that you discuss the issues involved with ZF. This is especially important if you have other than linear resistances. These can be electrical circuits with inductive resistances (motors), capacitive resistances (condensers) or lamp loads. To ensure that a switch reaches the end of its electrical operating life, the switch should not be subjected to pressure in its rest position (pre-stressed) and at least 50% of the available overtravel must be used. Operating life specifications for direct current loads are available on request. Where higher switching capacities are involved, we recommend the use of fuses to provide protection against arcing.
Since the operating life of a snap switch depends on a number of factors, we recommend that field trials be performed in order to establish the likely electrical life of a switch in a given application. This is especially recommended when the application deviates considerably from the test conditions described above. Our specialists are always ready to provide you with more advice regarding possible solutions for your particular application.
Behaviour at different temperatures
Depending on the model, the operating temperatures of our switches range from –25 to +70°C and –40 to +150°C. If you attempt to use a switch at operating temperatures either above or below those recommended for your particular model, the switch’s material properties will change and its reliability will be affected. Where switch model codes start with “T” (e.g. 40T125 in compliance with EN 61058), the switches involved have been approved for use at the corresponding temperatures.
Vibration and shock resistance
Snap switches are naturally fairly resistant to shocks and vibrations thanks to their minimal mass of moving parts. They are at their most resistant when the actuator is in the rest position or end position, when vibration resistance is as high as 5 g at 20 – 200 Hz while shock resistance attains 20 g (6 ms).
Snap switches are more susceptible to vibrations at the switching point and at the release point. In certain conditions, this could result in transient make or break contacts (bouncing) to the detriment of the switch’s operating life. This is why snap switches which are regularly exposed to vibration should, wherever possible, not be actuated slowly.
Electric strength
The electric strength of our snap switches is – in the case of models suited for mains voltages – exceeds 1500 V AC between conducting parts and the earth and 750 V AC between the terminals (open contacts) measured over a period of one minute at an ambient temperature of 23°C ± 5°C, relative humidity of < 70% and normal atmo- spheric pressure.
Operation, Contact Types & Materials
Operating speed
Snap switches are suitable for a broad spectrum if operating speeds. However, extremely slow or fast actuations can affect the switch performance and operating life. For product-specific values, please see the technical specifications. The maximum switching frequency (switchings/s) is limited by the electric load. With low switch loads, up to 10 actuations per second are possible. Sudden actuation must be avoided since it decreases the mechanical operating life.
Contact bounce
Bounce time is the time between the moment closing contacts first touch and final (definitive) contact closure. The typical bounce time for our snap switches is between
1.5 and 3 ms, depending on the series.
Transit time
In two-way (double-throw) switches, transit time is the time between the moment the break contact element (NC contact) first opens and the make contact element (NO contact) first closes. Transit time is generally deter- mined by design features such as e.g. contact travel and elastic characteristics. It generally varies between 3 and 10 ms, depending on the model.
If transit time is critically important to the functioning of your application, don’t hesitate to contact us.
Snap switches are suitable for a broad spectrum if operating speeds. However, extremely slow or fast actuations can affect the switch performance and operating life. For product-specific values, please see the technical specifications. The maximum switching frequency (switchings/s) is limited by the electric load. With low switch loads, up to 10 actuations per second are possible. Sudden actuation must be avoided since it decreases the mechanical operating life.
Contacts
We supply switches with standard and crosspoint contact technology. For low-voltage and low-current applications, we strongly recommend the use of gold crosspoint contacts. The reduced surface area of the cross-shaped contacts means that the surface pressure is greater, which in turn enhances reliability. Standard contacts are more suitable for higher switched loads.
Contact materials
Gold and gold alloys: primarily AuAg; AuAgPt Silver and silver alloys: primarily AgNi, AgSnO2
Gold alloys are especially suitable for low currents and voltages.
Typically they are used in the range from 5 V, 1 mA DC to 12 V 100 mA DC.
But it may also make sense to use them in switches which are only occasionally operated or in atmospheres with a high sulphur content. For switching heavier loads, it usually makes sense to use silver or silver alloys.
In this case, the range typically extends from 12 V, 100 mA DC to 250 V 21 A AC.
Because choosing the right contact materials depends on a large number of factors, such as switching voltage and current, operating environment, atmospheric conditions, etc., we are always pleased to advise you on the best choice of material for your application. Before making any firm decisions, we do advise you to carry out field trials of our switches in real-life conditions.
Materials & Contact Resistance
Materials
For our standard switches, we use high-quality, cadmium- free plastics which are optimized for the intended application. As a rule, we seek to avoid the use of toxic or hazardous materials. You can find out more about our materials policy by consulting our hazardous substances exclusion list.
Behaviour of materials in fire
Insulating materials which are directly connected to electrically conductive parts are classified according to their degree of flammability. Most of the materials we use to manufacture housing are self-extinguishing and categorised under the UL 94 VO standard.
Proof tracking index
Most of the insulating materials we use in our snap switches have a proof tracking index of PTI 250 (PTI 300, e.g. D4) or PTI 175 (PTI 250, e.g. DB, DC). This means that they are capable of 50 drops of test fluid at a test voltage of 250 V without producing any leakage current (IEC 60112).
RoHS
Switches without leads already conform to RoHS. Switches with leads are available in RoHS-conforming models on request. In case of further processing with lead-free soldering, the product-specific solder recom- mendations must be heeded.
For our standard switches, we use high-quality, cadmium- free plastics which are optimized for the intended application. As a rule, we seek to avoid the use of toxic or hazardous materials. You can find out more about our materials policy by consulting our hazardous substances exclusion list.
Glow wire test
The insulation materials used for snap switches with ENEC approval fulfil the required filament tests GWFI according to the household appliance standard IEC 60335-1 at 850°C and GWIT at 775°C or alter- natively the filament test GWT at 750°C.
Contact resistance
The contact resistance of snap switches is composed of the contact resistance and the resistance of the conductive parts. It depends primarily on the construction and the contact material. The contact resistance of silver contacts is max. 100 mΩ, of gold contacts max. 50 mΩ when they are new.
Insulation resistance
The insulation resistance between the conductive parts of our snap switches and a conductive underlay or between the open contacts exceeds 10 MΩ when they are new, measured over a period of one minute at room temperature with 500 V DC.
Caution:
Humidity and soiling can decrease the insulation resistance.
| Designations | Meaning |
|---|---|
| ASA | Acrylnitrat-Styrol-Acrylester |
| LCP | Liquid Crystal Polymer |
| PA | Polyamide |
| PBT | Polybutylenterephthalat |
| PET | Polyethylenterephthalat |
| POM | Polyoxymethylen (Polyacetal) |
| PPS | Polyphenylensulfid |
| PES | Polyethersulfon |
| SI | Silicone |
| TPE | Thermoplastic Elastomer |
| VMQ | Vinyl-Methyl-Polysiloxan (Silicone rubber) |
| UL | IEC/VDE | In vertical flammability test. goes out after more than> | Drops molten material capable of igniting | Max. duration of afterglow |
|---|---|---|---|---|
| V-0 | FV-0 | 5 secs | No | 30 secs |
| V-1 | FV-1 | 25 secs | No | 30 secs |
| V-2 | FV-2 | 25 secs | Possible | 60 secs |
| HB | FH | Burning rate in horizontal flammability test. upto 3mm thick < 7.5mm/min: over 3mm thick > 3.8mm/min |
