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Term
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Definition
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Alternate
Action
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Switch action
defined as single push to engage switch contacts, another push to
disengage switch contacts. EMC
offers two types of Alternate Action switching:
Push-On/Push-Off: switch plunger is in same position in
ON and OFF positions.
Push-On/Remain-In/Push-Off: switch plunger is physically depressed
in the ON position (defined as Differential).
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Break-before-Make
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A property
of switch operation that opens the preceding circuit before closing the
next.
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Common
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The common
is one of the switches' terminals, and is the terminal that is always
part of the switch circuit regardless of position. The common
connects to the different circuits either the normally open (NO) side or
normally closed (NC) side.
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Contact
Bounce
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Contact discontinuity
(open circuit) experienced when a switch is subjected to physical
vibration or rotated to a new position.
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Contact
Resistance
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The Ohmic resistance of the switch contact points, usually
measured in milliohms. This is
particularly significant in dry circuit or low level applications, since
the resistance varies significantly at the micro-amp level with current
variations.
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Detent
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A mechanical click in
the travel of the switch. In
momentary switches, this may be added as a snap dome switch in the bottom
of the plunger travel.
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D-Flat
Double-D
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 Terms describing the switch
mounting bushing shaft physical configuration. "D-Flat"
describes one flat surface, while "Double-D" describes two parallel
flat surfaces on the periphery of a shaft or bushing.
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Dielectric Withstanding Voltage
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This is
the maximum voltage that can be applied between open switch terminals and
the switch housing or mounting bushing (ground) without arcing or
flashover. Arcing or flashover indicates a breakdown of the
insulating material. A typical maximum voltage is 250 to 1500 Vrms,
depending upon the size of the switch.
See also Insulation
Resistance.
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Differential
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The
difference between the resting points of the switch plunger when in the
ON position and the OFF position in a Push-On/Remain-In/Push-Off switch.
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Dry Circuit
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A term
indicating switch contacts designed for operation at currents less than 10
milliamps and at voltages less than 30 millivolts DC.
Also known as Low Level switching.
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EMI/RFI
Shield
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A special
grounding technique that guards the switch from being affected by ambient
Electro-Magnetic Interference (EMI), or Radio Frequency Interference
(RFI). EMI or RFI may negatively affect circuit functions.
These undesirable energy transients usually come through the wiring
(conducted), or the shaft (radiated).
Tests specified by MIL-S-3786 measure how these transients are shunted to
ground.
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Epoxy Seal
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A sealant
used on a switch assembly to safeguard against solder flux, cleaning
solvents, or other contaminants from entering the inside working portion
of the switch.
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Explosion
Proof
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A design
feature that allows switch actuation of electrical energy, without
causing ignition, while immersed in an enclosed area having a combustible
atmosphere.
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Gang
Switch
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Multi-switch
assembly, often interlocking, from EMC Series 400/500.
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Insulation
Resistance
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This is the
resistance measured between open switch terminals, and between a switch
terminal and ground. The unit of measurement is usually Megohms.
See also Dielectric Withstanding
Voltage.
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Make-before-Break
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A property
of switch operation that closes the preceding circuit before opening the
next.
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MIL-I-45208
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A military
specification that states the requirements for an inspection system
(superceded in 1997 by ISO standards).
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MIL-Q-9858
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A military
specification that states the requirements for a quality system,
including downstream supplier control.
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MIL-PRF-22885
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A military
specification for switches, pushbutton, and pushbutton lens displays.
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MIL-STD-202F
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A military
standard defining test methods for electronic and electrical component
parts.
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Momentary
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A switch position that incorporates a spring return mechanism that
restores the shaft to a return position upon release. Switch contacts are only closed when switch
is physically depressed.
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Panel Seal
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A seal
placed between the panel and switch mounting surfaces to exclude leakage
past the mounting bushing.
Also see Shaft Seal.
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PC
Terminals
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Terminals
on a switch specifically designed for mounting on a printed circuit
board. Usually specified with
modification code “PC” (e.g. 2150PC).
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QPL
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An acronym for "Qualified Product List", it lists the
slash numbers in the MIL-PRF-22885 specifications (e.g. MIL-PRF-22885 /04 ;
/13 ; /20 ; /35 etc.)
that successfully meet the criteria of the qualification tests designated
therein.
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Shaft Flat
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The flat
section of a round shaft provided for the purpose of preventing the knob
from slipping during switch actuation and providing orientation of the
knob during assembly.
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Shaft Seal
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O-ring or
other protective cover to prevent leakage past the shaft.
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Shock
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The impact
a switch must withstand without failure.
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Terminal
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The means
by which a rotary switch connects to external circuitry. Common styles
include:
- printed circuit board terminal (PC
terminal)
- solder lug
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Terminal
Strength
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A measure
of a switch terminal's ability to withstand a pull test, usually measured
in pounds.
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Thermal
Shock
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A test that subjects the switch to rapidly changing temperature
extremes.
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Torque
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A
rotational force around an axis.
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