When a switch is actuated and contacts touch one another
under the force of actuation, they are supposed to establish continuity in a
single, crisp moment. Unfortunately, though, switches do not exactly achieve
this goal. Due to the mass of the moving contact and any elasticity inherent
in the mechanism and/or contact materials, contacts will "bounce" upon
closure for a period of milliseconds before coming to a full rest and
providing unbroken contact. In many applications, switch bounce is of no
consequence: it matters little if a switch controlling an incandescent lamp
"bounces" for a few cycles every time it is actuated. Since the lamp's
warm-up time greatly exceeds the bounce period, no irregularity in lamp
operation will result.
However, if the switch is used to send a signal to an electronic
amplifier or some other circuit with a fast response time, contact bounce
may produce very noticeable and undesired effects:
A closer look at the oscilloscope display reveals a rather
ugly set of makes and breaks when the switch is actuated a single time:
If, for example, this switch is used to provide a "clock"
signal to a digital counter circuit, so that each actuation of the
pushbutton switch is supposed to increment the counter by a value of 1, what
will happen instead is the counter will increment by several counts each
time the switch is actuated. Since mechanical switches often interface with
digital electronic circuits in modern systems, switch contact bounce is a
frequent design consideration. Somehow, the "chattering" produced by
bouncing contacts must be eliminated so that the receiving circuit sees a
clean, crisp off/on transition:
Switch contacts may be debounced several different
ways. The most direct means is to address the problem at its source: the
switch itself. Here are some suggestions for designing switch mechanisms for
minimum bounce:
Reduce the kinetic energy of the moving contact. This will reduce the
force of impact as it comes to rest on the stationary contact, thus
minimizing bounce.
Use "buffer springs" on the stationary contact(s) so that they are
free to recoil and gently absorb the force of impact from the moving
contact.
Design the switch for "wiping" or "sliding" contact rather than direct
impact. "Knife" switch designs use sliding contacts.
Dampen the switch mechanism's movement using an air or oil "shock
absorber" mechanism.
Use sets of contacts in parallel with each other, each slightly
different in mass or contact gap, so that when one is rebounding off the
stationary contact, at least one of the others will still be in firm
contact.
"Wet" the contacts with liquid mercury in a sealed environment. After
initial contact is made, the surface tension of the mercury will maintain
circuit continuity even though the moving contact may bounce off the
stationary contact several times.
Each one of these suggestions sacrifices some aspect of switch
performance for limited bounce, and so it is impractical to design all
switches with limited contact bounce in mind. Alterations made to reduce the
kinetic energy of the contact may result in a small open-contact gap or a
slow-moving contact, which limits the amount of voltage the switch may
handle and the amount of current it may interrupt. Sliding contacts, while
non-bouncing, still produce "noise" (irregular current caused by irregular
contact resistance when moving), and suffer from more mechanical wear than
normal contacts.
Multiple, parallel contacts give less bounce, but only at greater switch
complexity and cost. Using mercury to "wet" the contacts is a very effective
means of bounce mitigation, but it is unfortunately limited to switch
contacts of low ampacity. Also, mercury-wetted contacts are usually limited
in mounting position, as gravity may cause the contacts to "bridge"
accidently if oriented the wrong way.
If re-designing the switch mechanism is not an option, mechanical switch
contacts may be debounced externally, using other circuit components to
condition the signal. A low-pass filter circuit attached to the output of
the switch, for example, will reduce the voltage/current fluctuations
generated by contact bounce:
Switch contacts may be debounced electronically, using
hysteretic transistor circuits (circuits that "latch" in either a high or a
low state) with built-in time delays (called "one-shot" circuits), or two
inputs controlled by a double-throw switch. These hysteretic circuits,
called multivibrators, are discussed in detail in a later chapter. |