Switch#Contact bounce

When bouncing a mechanically induced interference effect is referred to in electromechanical switches and buttons: Instead of instant electrical contact actuation of the switch calls briefly produced a multiple opening and closing of the contact. Much less common, however, it comes when you turn the switch or the button is released after the first interruption for repeated re- contact. The reason for the bounce are suspensions of components of the switch mechanism, which are caused by the physical effect of the elastic collision.

Effects

This effect of the multiple closing and opening of the contact leads in fast electronic circuits, the time resolution is high enough to detect the bouncing, undesirable multiple events. This affects, for example, digital input devices such as a computer keyboard, input controller to keypads or electronic circuits which detect a relay or other electro- mechanical contact. By bouncing closing operations would be erroneously registered as a multiple stop without debouncing a keystroke.

But even with electromechanical switches, relays and contactors occurs the bouncing of the contacts and thereby leads to increased contact and failure of the contacts. The contact welding, which arises in the simultaneous occurrence of streams is greatly favored by the bounce phase, because there the resulting arcing contact material is repeatedly melted, then what can be welded with a renewed closing during the bounce. If the contact is designed for over- current, which may occur during switching of loads, there is a risk of the welding or bonding of the contact halves. Whether a contact welds therefore strongly depends on the connected load. Especially when switching on, at least initially capacitive loads such as switching power supplies, causing an inflated by a factor of 20-50 inrush current at the first contact closure occurs on melting. This leads to a strong contact wear after only a few thousand circuits, which then leads to the failure of the switch and or welding. The picture shows a switch for 250 V, 16 A served in a power strip to turn on two laptop power supplies with only 50 VA. The rated current of the two power supplies together amounted to only about 1 amp, which are no problem for the 16 A switches. You can see in the picture with the times evaporated from the contacts Contact material that has been deposited in Schaltergeäuse and of course missing on the contacts.

Upon contact, open relays, contactors or circuit breakers on the other hand usually occurs no bouncing up and so no welding a.

The duration and the number of multiple contact transfer during the bounce is determined by the mechanical properties of the switch, the size, operating force, restoring spring force, shape and material and mass of the contacts and their fortifications. Typical bounce time in electromechanical switches and buttons are in the time range from 100 microseconds to 10 milliseconds. The bounce time for a 16 A toggle switch for mains voltage, as shown in the picture, is approximately 5 msec. For large contactors for several 100 A, the bounce time can be greater than 100 msec. be. The bouncing depends in large contactors with operating coil Ac also strongly influenced by the switch- off of the contactor coil, whereby the magnetic attractive force and thus the impact velocity of the moving contact is affected.

Countermeasures

Since the beginning of the electronic signal processing and the associated relevance of this phenomenon in signal switches and relays various hardware and software techniques have been developed to counteract the bounce and its effects. These measures are called debouncing. Debouncing by means of a low-pass filter or a lock logic.

• In the simplest case, an RC element as a low pass filter and a Schmitt trigger is provided for signal shaping. The low-pass filter suppresses high frequency signals according to the contact bounce, the Schmitt trigger provides the appropriate voltage level safe for subsequent digital circuit.
• Means locking logic in the form of a changeover switch. For locking is an asynchronous RS flip-flop or adequate circuitry used the " bounce-free " button. It must be ensured by the mechanical structure of the changeover switch that contacts can not oscillate between the two contact positions. The contact path when switching between the two states must be chosen to be sufficiently large.

Debouncing by an activation of the AC magnetic coil of a relay or contactor that reduces the impact energy and equalized for all closing operations. For this purpose, biased solenoid, among others, and enabled by phase gating with reproducible voltage curves.

• The change in state of the contact will only be registered if it is present a certain time, the so-called debounce. This is a form of low pass filtering, and may as a digital low-pass filter can be realized. Usually as simple, this filter is implemented in the form of a counter. The counter value, which triggers the event, together with the counting speed, represents the cutoff frequency of filter
• Furthermore, a debouncing can also be performed in software by means of locking logic. Because but for each key a toggle switch and two digital inputs, together with the associated higher circuit complexity are needed this kind of debouncing is rarely used.

If the lock is simulated by a time control logic in the form of a monostable multivibrator in software, the pulse will be detected with the first edge signal and ignore any additional change in signal for a certain time following, however, this method is sensitive to high-frequency noise pulses. It is because it is a form of high-pass, no safe debouncing dar. also provides the sub-sampling is not a secure debouncing, since it only the probability for the detection of short disturbance pulses is reduced, but not avoided.

By special mechanical construction and service skills also virtually bounce- free switches can be realized. Liquid contact materials, for example in the form of the mercury switch are virtually bounce-free. Due to the toxicity of mercury and its chemical compounds, these switches find to avoid the bounce but no practical application more.

Sensor keys with integrated electronics such as piezo - and Hall- keys contain internal threshold already and usually deliver bounce-free signals.