Use sliding switch to improve the flexibility of car design

In the modern automotive market, most door spring lock systems use fast-moving micro-switches to manage body functions such as auto-lock, door bell, ceiling light, and provide lock or status lock signals for various functions of CPU control.

These switches are increasingly being incorporated into moulded electronic component carriers, where the metal lead frame of the die has a moulded plastic frame (ECC). The advantage of this architecture is that virtually all lead connectors, physical adjustment functions, and structural units can be built into a single part or a minimum number of parts to reduce costs and potential prescribed tolerance problems for automotive manufacturers.

Bridging the gap

Although the end result is a simplified system for OEMs, the design and manufacture of these ECCs is a complex process requiring knowledge of switches, spring lock applications, injection moulding and die design. OEMs understand their spring lock requirements, but they specialize in component installation rather than component manufacturing. The injection manufacturer understands the sophistication of complex processing and moulding, but is not good at designing or applying automotive electronic switches. It is the knowledge and experience of switch system vendors, such as Cherry Electrical Products, who, together with OEM or layer lock manufacturers, apply the appropriate switches in the right way and provide complete ECC assembly.

One of the tools

There is a sliding contact switch in the "toolbox" of the spring lock design engineer (e.g. Cherry SK series). Sliding switches are usually cheaper than traditional quick-start switches, while providing some unique features:

1. A "wipe" contact system with self-cleaning function that is more robust in certain environments, such as exposure to silicone;

2. It has a constant contact force throughout the trip, which makes it well suited for applications where the tolerances specified for the quick switch may not allow the same degree of overshoot;

3. Sliding switches do not directly replace fast-acting switches in all applications. They work differently and require the attention of the spring lock design engineer (see figure below).

Trip

The trip is the distance a button needs to move from normal closure (N.C.) to normal opening (N.O.). Sliding switches require a larger trip than fast-acting switches, because the button's travel is directly proportional to the distance the contact moves.

Dead Zone of Switch

Slide switch has an inherent "dead zone" between the N.C. and N.O positions, where the switch is not in any state. The dead zone usually has a range of 0.3mm and it is important to ensure that the switch does not enter a make-before-break state within its inherent tolerance.

When the slide switch is sliding, the movable contact moves from the N.C. contact to the N.O. contact. In the conversion process, if the moving contact completes the action of the N.O. circuit before opening the N.C. circuit, then the condition of connection before cut is satisfied. This condition does not exist in the SPST switch because they are designed with only one N.C. or one N.O. contact, not both.

There is no dead zone in the SPST switch (see Figure 1 above) because the key parameter is the appropriate contact on/off point. By adjusting the microprocessor's sampling rate or signal recognition parameters with sliding speed, the application software can mitigate the dead-zone zero-state effect in SPDT switches.

Overrun is the distance difference between the N.O connection point and the full trip of the button. The contact pressure in the quick switch is proportional to the extension of the spring moving the contact across the center position. Quick switch usually requires a push button displacement of 50% to 100% over run to achieve the desired contact pressure. This may be problematic in some applications because the prescribed tolerance of the spring lock erodes the percentage of available overrun.

The slide does not need as much overrun as the quick switch because contact pressure is independent of button travel. Therefore, in the case of wide prescribed tolerances, they may be suitable as substitutes for fast-acting switches, assuming that the electrical load is suitable for sliding switches.

Displacement difference

The displacement difference is the distance between the button return between N.O. closure and N.C. open. The displacement difference of the slide switch is almost equal to the zero-state dead zone (see Figure 2). All switches have a certain displacement difference in order to prevent the problem of connection before tangent. The internal displacement difference of the slider is greater than that of the quick switch because there is a proportional relationship between the contact movement and the button travel.

Electrical performance

Sliding switches are best suited for use as pure resistance, low current loads, such as I/O signals from microprocessors. However, switches can also be used in some harsh environments, such as capacitive loads, where the scratch function provides benefits.

In the design of automotive spring locks, the switches used for load switching of light bulbs or PTC (Positive Temperature Factor) have been replaced by electronic controls. If the circuit design contains a filter capacitance across the pins of the switch, then there is a capacitive load. Because the impedance on the switch side of the capacitor is the impedance of the switch pin line, there will be voltage and current spikes on the switch pin, which will affect the life of the switch.

The inherent travel time of the slider is slower than the fast switch, so it is important to test the application requirements before selecting a special switch.

Physical and environmental performance

In addition to the function button travel difference mentioned earlier, the physical configuration of the slide switch and the available options at the wiring end can be the same as the quick switch. Both switch types also have the same degree of water seal integrity, life and comparable high and low temperature performance.

Comparison of functions and capabilities

In summary, as a complement to standard fast-acting switches, micro-sliding contact switches provide a cost-effective means for automotive spring lock design engineers to provide constant contact pressure, wipe sliding contacts for longer life and environmental corrosion resistance. By combining the slide switch with the fast switch, the function, capacity and performance of the spring lock can be optimized.