|
|
|
Singapore |
|
|
|
|
Singapore |
|
Maximum contact voltageThe maximum contact voltage that can be switched. Never exceed this value during operation.
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
Single stable coil |
Double-winding latching coil |
Single-winding latching coil |
||
|
Polarized coil |
Non-polarized coil |
4-terminal coil |
3-terminal coil |
|
|
|
|
|
|
|
Standard voltage applied to the operating coil when a relay is used under normal conditions (according to JIS C4530).
The standard current flowing to the coil to enable use of a relay under normal conditions (JIS C4530). The value is given at a coil temperature of 23°C The tolerance, unless otherwise specified in the model specifications, is ＋15% and −20%.
Coil resistance is the resistance between
coil terminals when the coil temperature is 23°C
The tolerance is ±15% unless otherwise specified in the model
specifications. (The coil resistance for AC specifications and the coil
inductance are the reference values.)
The power consumed by the coil when rated voltage is applied to the coil (rated voltage × rated current). The rated power consumption for AC specifications is the value at a 60-Hz frequency.
The minimum voltage required to operate the relay (JIS C5442). The value is given for a coil temperature of 23°C
The maximum voltage that the relay for which
the relay will release when the voltage drops dramatically or gradually
decreases (JIS C5442).
The value is given for a coil temperature of 23°C
Example: MY4 DC Models
The distributions of the must-operate voltage and the must-release voltage
are shown in the following graph.
As shown in the graph, the relay operates at voltages less than 80% of the
rated voltage and releases at voltages greater than 10% of the rated
voltage.
Therefore, in this catalog, the operating and must-release voltages are
taken to be 80% max. and 10% min. respectively of the rated voltage.
The status where power is supplied to
contacts and the power that has been supplied to the coil is turned OFF
then ON. Also the must-operate voltage at that time.
(The coil voltage, contact current, and ambient temperature are set as
conditions.)
For latching relays, the minimum pulse width of the rated voltage applied to coils to set and reset the contacts. The value is the rated voltage applied to the coil at an ambient temperature of 23°C
For DC relays, the value found from the time constant by adding rectangular waves. For AC relays, the value at the rated frequency. The values are different for operation and release.
A magnetic body inserted in the coil to effectively operate the magnetomotive force in an electromagnet.
Note: The term core is used mainly
for fixed magnetic objects. Those that move inside the coil are called
moving cores. Sometimes pole pieces are attached to effectively utilize
the magnetic attraction.
A short-circuited coil for partially delaying change in magnetic flux by using the current generated by mutual inductance between the magnetic pole of a DC electromagnet that has been partially encased and an excitation coil. Shading coils reduce the vibration of the moving parts.
The time between the moment the rated voltage is applied to the coil when the contact operates. For relays with more than one contact, the operating time is the time until the slowest contact operates, unless otherwise defined (JIS C5442).
The operating time is given for a coil temperature of 23 °C and does not
include bounce time.
The time from the moment when the rated voltage is applied to a set coil until the contact operates. For relays with more than one contact, the set time is the time until the slowest contact operates, unless otherwise defined (JIS C5442).
The set time is given for a coil temperature of 23 °C and does not include
bounce time.
The time from the moment the rated voltage is removed from the coil until the contact releases. For relays with more than one contact, the release time is the time until the slowest contact releases, unless otherwise defined (JIS C5442).
For relays with only NO contacts, the release time is the time until the
slowest NO contact opens.
The release time is given for a coil temperature of 23 °C and does not
include bounce time.
The time from when the rated voltage is applied to the reset coil until
the contact releases. For relays with NO contacts only, it is the time
until the slowest NO contact opens.
For relays with more than one contact, the reset time is the time until
the slowest contact releases, unless otherwise specified.
The reset time is given for a coil temperature of 23 °C and does not
include bounce time.
Intermittent switching between contacts due to shock and vibration caused by the impact of the moving parts of relays (armatures) colliding with the core or other contacts (JIS C5442).
The bounce time for NO contacts when the coil rated voltage is applied at a coil temperature of 23°C.
The bounce time for NC contacts when the coil rated voltage is removed at a coil temperature of 23°C.
Number of relay operations per unit time.
The resistance of the isolated sections between contacts and coils, conducting terminals and uncharged metallic parts (e.g., core frame and core), or between contacts.
This value is given for the relay and does not include lands on PCBs.
Between coils and contacts: Between coil terminals and all contact terminals
Between contacts with different polarity: Between contact terminals of different polarity
Between contacts with the same polarity: Between contact terminals with the same polarity
Between set coils and reset coils: Between set coil terminals and reset coil terminals
The maximum value before insulation damage occurs when voltage is applied for one minute to an isolated metallic part (especially charged metal). The voltage is applied at the same location as the insulation resistance.
The leakage current (the current used to detect insulation damage) is
normally 1 mA.
Sometimes, however, leakage currents of 3 mA and 10 mA are used.
The maximum abnormal voltage that the relay can withstand when the voltage surges momentarily due to lightning, switching an inductive load, etc. The surge waveform, unless otherwise specified, is the standard impulse voltage waveform according to JIS C5442,i.e., 1.2 × 50 μs.
FCC Part 68 specifies 10 × 160 μs ± 1,500 V.
The vibration resistance of a relay is divided into two categories:
Destruction, which quantifies the characteristic changes of, or damage to,
the relay due to considerably large vibrations which may develop during
the transportation or mounting of the relay, and malfunction durability,
which quantifies the malfunction of the relay due to vibrations while it
is in operation.
The shock resistance of a relay is divided into two categories:
Destruction, which quantifies the characteristic change of, or damage to,
the relay due to considerably large shocks which may develop during the
transportation or mounting of the relay, and malfunction durability, which
quantifies the malfunction of the relay while it is in operation.
The durability of contacts when no load is applied and the contact is switched at a specified switching frequency.
The durability of contacts when a rated load is applied and the contact is switched at a specified switching frequency.
If different metals are attached at both ends and the temperatures where the metals are connected are held at different temperatures, current will flow in one direction in the circuit. The electromotive force that causes this current is called thermoelectromotive force.
Thermoelectromotive force occurs between the different metals in
terminals, armatures, and contacts in relays. This Thermoelectromotive
force is the reason the actual temperature and the measured temperature
are different when relays are used to switch thermocouples.
Indicates the degree of high-frequency signal leakage between contact terminals that are in an open status and unconnected terminals.
The loss of high-frequency signal between contact terminals in a closed circuit.
The quantity of high-frequency signal reflection that occurs in a transmission path.
The voltage standing-wave ratio that occurs in transmission paths.
Note: Formula for converting return
loss to V.S.W.R.
Example Method for Measuring High-frequency Characteristics
Contacts not related to the measurement are terminated at 50Ω.
The maximum high-frequency signal power that can pass between contact terminals in a closed state.
The maximum high-frequency signal power that
a contact can switch.
The electrical durability will be shorter than for rated loads.
The degree of high-frequency signal leakage between contact circuits.
The TV rating is one of the common ratings used to evaluate the inrush current resistance characteristics in the UL and CSA standards. It indicates the load switching level for a relay, including the inrush current.
Relays used in television power supplies, for example, need to have a TV
rating.
A tungsten lamp is used as the load in the switching test (durability
test) and a switching durability of 25,000 times total is required.
|
TV rating |
Inrush current |
Steady-state current |
Example models |
|
TV-3 |
51 A |
3 A |
G2R-1A |
|
TV-5 |
78 A |
5 A |
G2R-1A-ASI |
|
TV-8 |
117 A |
8 A |
G2R-1A-TV8-ASI |
The problem where discharge between opposing
conductors causes a short-circuit.
This often occurs with contacts used with medium and large currents.
Welding, locking, or gluing causing contacts to have difficulty opening.
The wear of contacts due to mechanical causes, such as wear during repeated operation.
The expending of contacts due to electrical, thermal, chemical, and other causes during the repetition of contact switching.
The problem where contact surfaces become dirty and discharge occurs more easily.
Note: If precious metal contacts
switching in an environment with certain types of organic gases present,
the organic gas that attaches to the surface of the contacts will break
down as a result of the discharge and create black powder (e.g., carbon),
which makes discharge more likely to occur.
Metal oxides, sulfides, and other film that is generated on or attaches to contact surfaces and cause boundary resistance.
The magnetic characteristics caused by the shape around directly opposing magnetic surfaces.
Noise due to mechanical vibration caused by AC poles or rectifier wave drive with insufficient smoothing.
Removing the difference due to the effect of magnetic history by applying a saturation current to the operating coil during measurement of operation or release voltage (or current) or during testing.
Note: The current used is called the
soak current.
Relays where the contacts switch based on the non-excitation and excitation of the coil and otherwise have no special functions based on operating elements.
Relays with set coils and reset coils and a latching configuration to hold the set status or reset status.
Relay with one coil and a latching configuration that can switch to and hold a set or reset status according to the polarity of the applied voltage.
Relay that turns multiple contacts ON and OFF in order each time an input pulse is received.
A type of stepping operation, where the contacts alternate between ON and OFF for each input pulse.
Limited to relays characterized by their small size. The maximum dimensions and the average dimensions (indicated in parentheses and marked with an asterisk *) are both indicated as guides for design.
The maximum dimensions are listed as guides
for design.
|
|
PCB mounting dimensions |
Terminal
Arrangement/ |
|
Symbol |
|
|
|
Usage |
|
|
Note: The external dimensions, PCB mounting dimensions, and terminal arrangement/internal connections all have the direction mark on the left. JIS contact symbols are not used, in order to match the case markings.
The markings on the relay itself include the model, the voltage specifications, etc., as well as the internal connections. Some small relays do not have internal connections shown on the relay itself.
The marks used mostly on PCB relays to show the coil direction. This makes it easier to determine the relay coil direction when designing patterns for PCBs and installing PCBs.
1. Top View
Limited to relays with terminal arrangements that can be seen from the
top, as shown in the diagram. The internal connections are drawn showing a
top view of the relay.
2. Bottom View
Limited to relays with terminals that cannot be seen from the top, as
shown in the diagram. The internal connections are drawn showing a bottom
view of the relay.
3. Bottom View Rotation Direction
The terminal arrangements for PCB relays are shown with the relay is
rotated in the direction of the arrow when the coil is on the left
(direction mark on the left).
1. For a DC Relay, the relationship is as follows:
2. For an AC Relay, coil impedance must be taken into account because it
is affected by the coil inductance. The coil impedance varies with the
frequency. Suppose the characteristics at 60 Hz are 100%, using the same
Relay at 50 Hz will produce the characteristics shown in the following
table. These values will depend on the type of Relay. Check the values
before using the Relay.
|
Rated current, Power consumption and temperature rise |
Approx. 117% |
|
Must-operate current |
Approx. 100% |
|
Must-operate voltage, must-release voltage |
Approx. 85% |
3. Be careful of the following points: DC Relays, such as Keep Relays and Relays with built-in operation indicators or surge absorbing diodes, have polarity. If the Relay is connected incorrectly, elements may be destroyed or malfunction. Applying a DC voltage to an AC Relay will cause the coil to heat. This may lead to burning. Applying an AC voltage to a DC Relay will cause the armature to vibrate and the Relay will not operate properly.
Temperature changes affect the resistance of the copper wires used for
coils by approximately 0.4% per °C This directly affects the Relay
characteristics because the coil current, which generates the attractive
force of an electromagnet, changes. The effect of temperature on the
operating characteristics (such as the must operate voltage and
must-release voltage) of an AC Relay is smaller because the ratio of DC
resistance of the coil to the coil impedance is small.
Changes in coil resistance also affect the coil temperature for DC Relays.
This is because a change in coil current causes the amount of power
consumption to change. The value of temperature rise changes according to
the rate of change in the coil current due to changes in the temperature.
A typical example is shown in the following figure.
* The percentage of coil temperature
rise caused by a 20 °C increase in ambient temperature.
Change in coil temperature caused by change in ambient temperature.
The temperature inside the box increases because of heat generated by the
Relays and other devices. The ambient temperature that must be used is the
temperature inside the box near the Relay.
If the Relay coil is not in an operating state, exposure to high
temperatures or high humidity when there is a potential difference between
the coil and other metals such as the core may cause the copper wire coil
winding to corrode. The corrosion is caused by ionized current passing
between the metals when the insulation between them is insufficient.
This can be made analogous to the process of creating metal plating. The
effect is accelerated when acid or base is involved.
Not a lot of attention has been paid to this effect in the past; however,
good quality plastic has been developed for spooling recently, and
insulation materials such as polyurethane, polyester, polyamide, and
fluororesin have also been developed for the winding. These modern
plastics reduce the effect of electrolytic corrosion.
To prevent electrolytic corrosion, avoid storage in locations with high
temperature or high humidity. Thought must be given to the circuit layout,
such as positioning the switch so that the winding is not subject to a
constant positive voltage, or create a positive ground. Good and bad
examples are shown below.
The operating time of the Relay is determined by the coil time constant,
delay time due to the moment of inertia, and the contact switching time.
These values differ with the shape of the Relay. For example, Relays with
a large gap between the core and the armature or Relays that have
electromagnets made from materials with a large magnetic resistance, have
small inductance values, and the time constant is small. However, this
weakens the attractive force, hence more time is required to attract the
armature. This phenomenon often occurs in DC operation. The magnetic
attraction is weakened because it is inversely proportional to the square
of the distance between the core and the armature. For High-speed Relays,
the gap is made smaller and material with high magnetic permeability is
used to reduce amount of coil winding.
In AC operation, a current larger than the rated current is drawn when
power is supplied. The shape of the Relay is not as relevant as it is for
DC operation.
The moment of inertia has an indirect driving force that prevents large
loads on the armatures when they begin to move.
Movement of the armatures practically dictates the contact switching time.
For this reason, the loads and the attractive force must be balanced so
that the armature movement is as small as possible, and it moves smoothly
through all stages of the movement. Contact bounce is affected by factors
such as the moving speed of the armature, quality of the moving parts, and
springiness of the contact springs.
Generally, the shape of the contact spring, the contact segment, or the
structure of the stopper is designed to dampen the shocks from the
movements.
The Relay operating time is affected by the voltage/current applied to the
coils.
As shown in the following figure, when a voltage slightly higher than the
must-operate voltage is applied, the time taken for the coil current to
reach the must- operate current, the time taken for the moving parts to
overcome inertia and start moving, and the time taken for the attractive
force to overcome the weight of the loads and switch the contact, all
become longer, therefore the operating time is considerably extended.
When a voltage much higher than the must-operate voltage is applied, all
the times become shorter, and therefore the operating time becomes faster.
The relationship between the voltage applied to the coil and operating
time is as explained above, however, the voltage applied to the coil also
affects other characteristics. This is why a rated coil voltage is
specified.
When the Relay temperature changes, the springiness of the Relay contact
spring, the amount of friction, and the coil resistance, change. Of these,
the coil resistance has the largest impact on the operating time. As
explained in the section on the principles of operation, the movement of
the electromagnet is related to the current. The current of DC
electromagnets can be expressed by the following equation.
At this point, if the coil temperature increases, the coil resistance
increases by 0.4% per °C as stated before. This increases R (DC coil
resistance) of the coil time constant (L/R), therefore reducing the
waiting time of the contacts and shortening the operating time. On the
other hand, if the coil resistance increases, the coil current decreases.
This extends the operating time of the DC Relay. The figure shows the
change in operating time according to coil temperature for voltage and
current operation.
Large Relays that have an operating time of a few 10 ms do not change a
lot even if the temperature changes. Small Relays that have an operating
time of less than 10 ms tend to change more when the temperature changes.
Silver migration describes a phenomenon that occurs when certain humidity
and oxidation-reduction conditions exist, and then DC voltage is applied
to silver electrodes for an extended period of time. This phenomenon
causes the insulation to deteriorate and occasionally causes short circuit
problems.
It is recommended to perform a conductivity test when the Relay is
actually used, or to use Relays with gold-plated or gold-clad.
It is not well known what causes silver migration, or what conditions
accelerate silver migration. However, the following points can be made.
|
Conditions that Cause Silver Migration |
Conditions that Accelerate Silver Migration |
|
Usage of silver |
• Applied voltage is high and insulation is thin (high potential frequency). • High water absorption rate of insulation material. • Oxidation-reduction gases such as (SO2, H2S, NH3) |
|
Applying DC voltage for an extended period of time |
|
|
Insulation with high absorbency of humidity |
|
|
Usage in high-temperature or high-humidity environments |
General-purpose Relays made by OMRON do not use silver plated terminals and will not cause silver migration.
When a plated part is stored for an extended period of time, needlelike
crystals form on the surface. These crystals are called cat whiskers
because of their shape. Depending on the length of the crystals, they may
cause short circuit problems.
The reason why cat whiskers form is not completely understood. However, it
is said that they will form easily when brass or zinc is used as the base
material and tin or zinc is used for the plating.
Example of Whisker Formation
General-purpose Relays made by OMRON have solder plating or special zinc
plating to guard against the formation of whiskers. When designing parts,
print boards, or patterns, keep in mind about the use of zinc and brass,
and allocate enough space for the insulation.
When shipping Relays through tropical zones, regardless whether it is just
the Relays or the Relays are built in to other devices, they will be
exposed to high temperatures and high humidity.
To protect the metallic material from this kind of environment, High
humidity Relays with special external specifications have been developed.
Even if Relays are not used and just stored, the degradation of the
contacts may progress. This is due to the influences of sulfur and
chlorine contained in the atmosphere, as shown in the following table. If
the Relay is to be stored for such a long period as years, it is
recommended to perform a conductivity test when the Relay is actually
used, or to use Relays with gold-plated or gold-clad contacts.
|
Area |
Detected elements |
Results of observation of contact surface (Ag contacts left for 12 months) |
|
Chemical plant |
Ag, S |
Almost
uniform and dense corrosive substances were observed on the entire
surface of the contacts. |
|
Steal mill |
Ag, S |
Irregular projections and recesses were observed and pillars of crystal were dispersed. As a result of analysis, Ag2S was detected. |
|
Highway |
Ag, S, CL |
Circular crystals
were sporadically observed. |
The desirable features of contacts, purely from a usage point of view, are
that they have stable characteristics (such as contact resistance) and
that they have a long life. To meet these conditions, contact follow and
contact pressure are important aspects.
Contact pressure is normally 5 to 50 g for general-use silver and
platinum, and 3 to 10 g for precious metals such as gold, silver, and
palladium. The pressure is smaller for precious metals because the
switching capacity is smaller and it is relatively robust against
environmental influences.
Contact follow requires the contacts to be touching even if the contacts
are somewhat worn out. It is closely related to the contact pressure. The
product of the two is the workload of the contacts. For a certain
workload, the contact pressure can be increased or the contact follow can
be increased to change the contactibility.
For example, when the contact pressure is large and the contact follow is
small, initially it will appear stable, but as the contact begins to wear
out, the contact pressure will rapidly drop and eventually the contacts
will not touch at all.
On the other hand, if the contact pressure is small and the contact follow
is large, the contact resistance may increase, or it may have difficulty
breaking down the film. Therefore, good Relays must have a reasonable
balance between contact follow and contact pressure.
Contact resistance can be regarded as a combination of concentrated
resistance and boundary resistance.
At first glance, the contact looks like it is touching the whole surface.
However, depending on the shape and the roughness of the surface of the
contact, it actually touches only on a single or multiple points. Current
flow is concentrated at these contact points and the generated resistance
is the concentrated resistance.
As described by the structure and principle of Relays formula, the contact
stiffness, contact pressure, and the inherent resistance of contact
material are related. A model of the contacting parts is shown below. The
contact is made over an extremely small area. Current is concentrated in
this extremely small area.
A measured example of the relationship between contact pressure and
contact resistance is also shown below.
If contacts are exposed to the air, the formation of oxidation and
sulphurization films is unavoidable. Resistance caused by these films is
called boundary resistance (film resistance).
Generally, the concentrated resistance takes up a large proportion of the
contact resistance before the contact is used. However, as the contact is
used, arcing and mechanical friction begin to wear it out, and the
proportion of the boundary resistance increases. The proportion depends on
the switching frequency. Contact surfaces subject to higher switching
frequencies are relatively clean and the boundary resistance is low.
Contacts with low switching frequencies generate films with fairly high
boundary resistance.
The contact resistance of a Relay is listed in catalogs but it is only a
provision of the initial value determined using a standard testing method.
The actual contact resistance must be suitable for the application device
and is determined by its tolerance to load impedance. Excluding special
cases, such as the transmission of sound currents where distortion and
attenuation becomes a problem, the contact resistance has a tolerance of
1% to 5% of the load impedance.
Most of the problems that occur to a Relay are caused by the
contactibility of the contacts. Load conditions also influence the type of
problem that occurs. Load conditions can be grouped into micro energy
level (dry circuit), mid-energy level, and high-energy level conditions.
The micro-energy level in a strict sense is a load condition of a
mechanical contact circuit, where the status of the contact is not
affected by heat or discharge. In reality, however, the status of the
contact does not change even when a reasonable voltage is applied, so this
load condition is included in the definition.
The voltage level at which the status of the contact remains unaffected is
called the softening voltage. It is 0.09 V for silver, 0.08 V for gold,
0.25 V for platinum, and 0.6 V for tungsten.
The mid-energy level is a load condition where a there is a mild discharge
effect. It is from the softening voltage to where arc discharge begins.
Flashover begins at 12 V for silver, 15 V for gold,17.5 V for platinum, 15
V for tungsten, and 11 V for 10% palladium silver alloy.
The high-energy level describes voltages that exceed the arc discharge
voltage.
Particular problems can occur depending on how the contacts are used. The
following describes some of them.
1. Abnormal Corrosion from Load Switching
This problem occurs when arcing due to load switching bonds nitrogen and
oxygen in the atmosphere together to form HNO3,which corrodes metallic
material (nitric acid corrosion).
Example of nitric acid corrosion
The following countermeasures may be effective.
(1) Reduce the amount of arcing that occurs during load switching by
creating an arc reduction circuit.
(2) Reduce the switching frequency to eliminate continual arcing.
(3) Reduce the humidity in the atmosphere.
2. Coherer Effect
If there is a film on the surface of a contact and the contacts are
touching, the film breaks down and the contact resistance drops rapidly
when the contact voltage exceeds a certain value.
3. Thermoelectromotive Force
Relay contacts are made from a combination of metals (such as silver and
copper alloy) depending on their function. The temperature varies between
the junctions depending on the distance from the heat emitting body (such
as the coil) and depending on the path of heat conduction. As a result,
thermoelectromotive force from a few to a few tens of μV is created
between the contact terminals. Care must be taken especially when handling
micro signals.
A Latching Relay (Keep Relay) can be used to shorten the time required for
current to pass through the coil, thus limiting the amount of heat
generated by the coil and reducing the thermoelectromotive force. A Relay
with small thermoelectromotive force can also be used. (Relay with
especially designed contact conduction section.)
The effect on the contact is completely different between the micro energy
level and high-energy level load conditions. The micro-energy level has
little contact wear but the existence of contact faults becomes a problem.
contact wear, welding, and transfer become problems in the high-energy
level load condition.
At the micro-energy level, the cleanliness of the contacts is the most
important aspect. If any non-conductive material or film is stuck to the
contacts, it will cause contact failure.
Non-conductive material may be dust, such as sand and fiber, but
micro-load Relays have relatively small contact wipes and pressure so any
particles stuck to the contact surface will cause contact faults. This is
a problem for all Relays regardless of the contact material, which makes
it difficult to select and use the right Relay. Formation of a
non-conductive film may be caused by the type of material the Relay is
made from, or the surrounding environment. The film depends on the
moisture in the air, oily or oxidized substances, organic gases emitted
from other Relays or structures, exhaust gas from vehicles, smoke from
factories, flux of the soldering, and fingerprints of the assembly worker.
Therefore, strategies are required for the structure of the Relay, the
material of the contact, and ambient conditions.
Generic silver contacts oxidize and sulphurize easily. Oxidation films do
not affect the contacts a lot, but sulphurization films have a large
effect. For this reason, precious metals that do not sulphurize easily are
used. Normally, palladium, gold, or platinum gold is combined with silver
to form silver alloys used for the contacts. Contacts made from platinum
alloys generate insulating powder (brown powder) due to the unsaturated
organic gas that is emitted from benzene and gasoline. Gold does not form
any films and the contactibility remains stable, but it is soft, so it
bends under low pressure. It cannot be used by itself, so palladium may be
used to create a double-layer contact where the top layer is a gold film.
In conditions where discharge occurs even with only micro-loads, the
contact may oxidize or combustible gases in the atmosphere may burn and
create a carbonized film. Carbonized films are not perfect insulators, so
the resistance may reach a few ten to a few hundred ohms.
At the high-energy level, flashover continuously generates large energy.
This may cause the contact to melt when it is switching, or the contact to
wear out from scattering of metallic vapor. It may also suffer from
problems such as transfer of metallic powder from one contact to the
other, or welding where the contacts melt and bond together when power is
supplied.
DC voltages and DC currents do not have points that cross at zero like
there is for an AC voltage and current. So even for fairly small loads,
arcing may continue for a long time.
Under these load conditions, adherence of metallic powder and carbonizing
of the insulator may deteriorate the insulation. For this reason, certain
types of insulation material and shapes are chosen.
The type of damage that occurs to the contacts depends on the type of
load. Loads such as transformers, motors, and lamps cause large inrush
currents and can cause welding of the contacts. Lamps, motors,
transformers, and solenoids cause currents of a few to multiples of ten
times the current.
Inductive loads such as motors, transformers, and solenoids cause large
reverse currents when power is shut off. The voltages reach 4 to 20 times
the normal voltage. This may wear out the contacts or damage the loads.
<< What is General-purpose Relays?
 |
General-purpose Relays
General-purpose Relays featuring mechanical indicator and lockable test button. |
 |
Slim Relay
The world's first Industrial Slim Relay with large plug-in terminals. |
 |
General-purpose Relay
Versatile and function-filled miniature Power Relay for sequence control and power switching applications. |
|
|
|
Sitemap
failures can be expected at a
reliability level of 60%.
