In today’s automotive market, most door latch systems employ snap-action sub-miniature switches to govern such body electronics functions as auto-lock motors, door chimes, and dome lights, as well as provide latch or lock-state signals to a CPU in controlling a variety of functions.
These switches are ever increasingly incorporated into molded electronic component carriers (ECC’s) which contain stamped metal lead frames within an injection molded plastic frame. The advantage of this construction is that virtually all of the wiring connectors, physical alignment features, and structural elements can be built into a single part, or a minimal number of parts, to reduce both the cost and potential tolerance issues for the automaker.
Bridging the gap
While the end result is a simplified system for the OEM, the design and manufacture of these ECC’s is complex process, requiring expertise in switch and latch applications as well as injection molding and mold design. OEMs understand their latching requirements, but are geared toward component assembly rather than component manufacture. Injection molders understand the intricacies of complex tooling and molding, but are not experts in automotive electronics switch design or application. It is switching system suppliers (such as Cherry Electrical Products) who possess the balance of expertise and experience to work with the OEM or Tier 1 latch manufacturer to apply the proper switch in the correct manner for the application and provide the complete ECC assembly.
One of the tools A "wiping" contact system that is self-cleaning and more robust in certain environments, such as those exposed to silicone vapors
Constant contact force over its entire range of travel, which makes them well suited for applications where tolerance issues may not allow the same degree of over-travel required for a snap-action switch.
In the latch designer’s "tool box" is the sliding contact switch (Cherry’s SK series, for example). Slide switches typically offer a cost advantage over conventional snap-action switches, while providing some inherently different features, including:
A slide switch is not a direct replacement for snap switches in all applications. Their operating characteristics are different and require the latch designer to be aware of those differences (see below).
Fig 1. Snap action switch (SPST)
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Fig 2. Slide Switch
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Pre-travel is the amount of button movement required to change states from normally closed (N.C.) to normally open (N.O.) positions. Slide switches require more pre-travel to actuate than snap-action switches because the button travel is directly proportional to contact movement.
A slide switch has an inherent “dead zone” between the N.C. and N.O. positions where the switch is not in either state. This dead zone is typically in the range of 0.3 mm and is required to assure that the switch does not enter a "make-before-break" condition over its internal tolerance range.
When a slide switch is activated, the movable contact moves from the N.C. contact to the N.O. contact. During the transition, if the moving contact completes the N.O. circuit before opening the N.C. circuit, then the make-before-break condition is met. This condition does not exist in SPST switches because by their design, they only have a N.C. or an N.O. contact and not both.
The dead zone has no counterpart in SPST switches (see Fig. 1 above), as the only parameter that matters is the appropriate contact make/break point. The zero-state effect of the dead zone can be mitigated in SPDT switches in the application software by coordinating the microprocessor sampling rate or signal recognition parameters with actuation speed.