Despite wide use of “soft” on/off switching, the traditional electromechanical switch is still often required or preferred and is available in countless versions.
As with all components, there are some top-tier parameters as well as many secondary ones, and some of those secondary ones may be very important in a specific application.
As expected, the two obvious switch specifications are voltage and current. The critical value for voltage is the maximum voltage value the switch can “hold off” when it is open, before it arcs, flashes over, or otherwise fails. Even a small, inexpensive basic switch can usually handle tens of volts. For current, it is the maximum amount of current that the switch can handle when the contacts are closed and current flows.
EM switches used for signaling are generally need only modest voltage and current. In contrast, switches for power must have ratings commensurate with the application plus a safety factor.
Another performance metric for some situations is the switch bandwidth. This is not a concern for on/off switches handling AC line or DC power. However, many switches must handle audio signals with little or no attenuation or frequency roll-off, while some handle RF. Some switches have truly innovative and unique designs and can handle frequencies into the GHz range (admittedly, these are relatively costly, but the fact that they can even be done is impressive). Many of these higher-frequency switches are shielded, so they do radiate RF.
Contacts and materials Another factor that affects switch performance is the materials used for the switch contacts. Switch ratings and performance are also determined, to some extent, by the switch use of AC or DC, which have different effects on contact ratings and wear, micro-arcing, tarnish, and other less-obvious characteristics.
Some switches are designed to provide a self-wiping action at the contact surface, to dislodge and breakthrough any tarnish that may have formed, especially a problem if the switch is not used for a long time). Some switches use a special plating of the contacts to minimize contact wear or prevent tarnish buildup. Some of these special finishes are standard catalog items, while others are available only by special order (higher cost and longer lead time).
Switch bounce Another characteristic of interest in applications where the switch position is “read” by software is switch bounce time. When the metal contacts close or open, they “bounce” before settling to their final position rather than going in a step-function from one position to the other. The system software may misread these bounces as multiple on or off triggering. To avoid this problem, designers use a dedicated debounce circuit usually based on a Schmitt trigger or a small software algorithm that takes bounce into account.
Knowing the maximum bounce period to successfully implement the debounce algorithm is important. There is no single “typical” bounce profile, as they are a function of many mechanical variables (Figure 1). The bounces period can range between one and 20 milliseconds, and the bounces can range between ten and 100 cycles. (This can be a tricky area; there are anecdotal cases where the software algorithm was based on one maximum bounce-time value for a switch and it worked, but then the switch was replaced with another functionally identical one but with a longer bounce period – and the debounce algorithm no longer worked.)
Wire terminations Switch vendors offer many types of termination for the electrical and connections wires of the switch. These include terminations compatible with surface-mount PC boards, through-holes PC boards, screw terminals, eyelets(the wire goes through and is soldered), and crimp.
The next part of this article looks at some actual switch examples.
Related EE World Content
Switches: The complete guide
Miniature Toggle Switches
Dual Seal Waterproof All-in-One Toggle Switch
In defense of the toggle switch
Panel Mount Toggle Switches Feature One Through Four Pole Configurations
References
- RS Components, “The Comprehensive Guide To IP Ratings”
- Electronic Design, “11 Myths About Switch Bounce/Debounce”
- Maxim Integrated Products/Analog Devices, Application Note 287, “Switch Bounce and Other Dirty Little Secrets”
