EDI-T® :
Technological Surprises to Sharpen Cutting
Edges of
A reed switch
is an assembly containing ferromagnetic contact blades, hermetically
sealed in a glass envelope and filled with an inert gas. The blades
(flexible reeds) are overlapped but separated by a small gap. The contact
area of each blade is plated with a noble metal, such as Rhodium or
Ruthenium, to provide the switch with stable characteristics and long
life. It can be operated by an externally-generated magnetic field, e.g.
that from an actuating coil. The inert gas atmosphere protects the contact
resistance of the reed switch, which is one of its most important
electrical performance characteristics. The inert atmosphere is maintained
by the integrity of the critical glass-to-metal seal of the switch. This
seal is defined as a "residual stress seal", which means that
the seal is sensitive to handling. When properly made, this seal with
maintain the inert gas atmosphere of the reed switch for a minimum of
thirty (30) years. However, improper handling of the switch and the
application of excessive mechanical force during modification of the
switch leads may result in a loss of the seal hermeticity. If damage is
caused, it might be immediately visible as a glass crack in the seal area
with or without the evidence of chipped glass particles. Or a latent
defect may be created that will deteriorate the seal hermeticity over time
due to the residual stress of the seal being altered through the improper
application of a mechanical force. Once the seal integrity is compromised,
atmospheric oxygen and other contaminants will degrade the contact
resistance and the life expectancy of the switch will be severely
impacted.
The glass seal of the reed switch has a very high
resistance to pressure, but a low resistance to pull forces. The ability
of the seal to properly withstand a switch lead modification is dependent
on several factors: the relation of the wire-to-glass dimensions, the
length of the seal, the pull force or type of modification to be
performed, and the distance and direction of the mechanical force in
relation to the seal itself. Therefore, the proper support and clamping of
the leads is necessary to avoid damage to the seal. Even then, the plastic
deformation strain of the NiFe lead wires can be transmitted through the
clamping area and into the seal. Depending on the combination of the force
and the distance of the clamping area to the seal, damage may still
result.
Reed switches are designed for, and used in, the most
demanding applications; these include computer keyboards, telephone
equipment, automatic test equipment and automobiles. Therefore their
quality must be exceptionally high with regard to conformity and
reliability. This quality is built into our battery-free LED flashlights
during every stage of their design and manufacture. The typical
characteristic parameters of the reed switches are inclusive of operate
and release values, operate and release times, bounce time, contact
resistance, breakdown voltage, insulation resistance, and life expectancy,
etc.
· Operate and Release Values are dependent on the measuring coil, the rate of energization (0.1AT/ms), the detection of the operate (closing) and the release (opening) moment, the position of the measuring coil relative to the earth's magnetic field and on the environmental conditions.
· Operate and Release Times are dependent mostly on the energization and
de-energization rate. They are proportional to the R/L time of he coil.
Operate time is inversely proportional to the ratio of energization to
operate value. Release time is proportional to the ratio of energization
to release value.
· Bounce Time is almost independent of the energization, however, a high
energization gives a somewhat shorter bounce time. The bounce time is
dependent on the current to be switched; above approximately 100mA the
bounce time is almost zero.
· Contact Resistance is dependent on the wire composition, wire diameter,
energization and contact layer. The commercially published contact
resistance is normally measured with an open contact voltage of 20mV and a
current through the closed contacts of 10mA, using Kelvin method. Distance
between measuring points for all switch types is normally 41mm.
· Breakdown Voltage depends on the gap between the contact blades, kind of
gas filling, gas pressure, material of the contact layer and the
availability of free electrons in the gas.
· Insulation resistance is dependent on the condition of the inside of the
glass envelope and on the environment, e.g. relative humidity, conducting
layers on the outside of the glass envelope.
· Life Expectancy of a reed switch is influenced by the contact layer, the
wire diameter, the load, the load circuit parameters and the applied
magnetic field. The contact layer and the wire diameter are determined by
the manufacturer. Lead, load circuit parameters and magnetic field are
determined by the user. The load should be within the maximum published
values. The load circuit parameters (wiring capacitance and inductance),
should be kept as low as possible and the applied magnetic field must be
stronger than necessary for obtaining the maximum operate value.
FEATURES:
1. Compact
and Light: it
can be mounted in a very limited space and ideal for use in miniaturized
equipment.
2. Hermetically
Sealed: its
switching elements are hermetically sealed in an inert gas atmosphere, so
that they are never exposed to the external environment.
3. Long
Life: it
employs no sliding parts, so there is no fatigue related to degradation in
the equality of the materials used, ensuring a virtually unlimited
mechanical life.
4. High
Speed Operation: every
movable element has a very low mass resulting in a high speed of
operation. This enables the reed switch to be used as an interface to a
transient or integrated circuit.
More information on Reed Switches can be found at FAQs of EDI-T® Battery-Free & Consumption-Free LED Flashlights.