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Understanding Relays

Actual Relay Design

Current flows through the control coil, which is wrapped around an iron core. The iron core intensifies the magnetic field. The magnetic field attracts the upper contact arm and pills it down, closing the contacts and allowing power form the power source to go to the load.


Ambient Temperature: Temperature measured directly near the relay. The maximum allowed value may not be exceeded, otherwise there is a chance for relay failure. (e.g. Coil overheating)

Bounce Time: Time interval between the first and final closing (or opening) of a contact, caused by a mechanical shock process in contact movement. These shock processes are called contact bounce.

Break Contact: A contact that is closed in the rest state of the relay and open in the operating state. (refer Normally Closed Contacts)

Bridging Contact: Special contact assembly in which two stationary contacts are connected by a movable bridge. In open contact condition the bridge is separated on both its sides from the stationary contacts. Due to this double interruption a bigger contact gap can be achieved. This is of advantage especially at very high contact loads or when there are safety requirements. (refer Form Z type contacts)

Coil Current: The current (by design) drawn by the coil for generating the magnetic pull force. At the moment of switching the coil On, the current is high earthen in continuous use.

Coil Resistance: Electrical resistance of the relay coil at reference temperature. Coil resistance varies with temperature. (refer Pick-up voltage change due to coil temperature rise)

Contact Forms: This denotes the contact mechanism and number of contacts in the circuit. Form a contacts are also called NO contacts or make contacts. Form B contacts are also called NC contacts or break contacts. Form c contacts are also called Changeover contacts.

Contact Gap: Distance between the contacts in the open contact circuit condition.

Contact Rating: The current a relay can switch ON and OFF under specified conditions of voltage and environmental parameters.

Contact Resistance: Electrical resistance of a closed contact circuit, measured at the terminals of the relay with indicated measuring current and voltage.

Change Over Contact: Compound contact consisting of a make contact and a break contact with a common contact spring. When one contact circuit is open the other one is closed. (refer Form C type contacts)

Make Contact: A contact that is open in the rest state and closed in the operating state. (refer normally open contacts)
Continuous Current (Contacts): Maximum value of current (RMS value at AC), which a previously closed contact can continuously carry under defined conditions.

Creepage Distance: Closest distance between two conductive parts, measured along the surface of insulated parts.
Dielectric Strength: Voltage (RMS Value in AC Voltage, 50Hz 1 min) the insulation can withstand between relay elements that are insulated from one another.

Driver Protection circuit: When the coil energization is switched off, a very high negative peak voltage is produced by the coil and it may reach more than 10-20 times the nominal coil voltage. Possible destruction of the semiconductor device (Driver) the coil circuit is the result. A solution is provided by a driver protection circuit that is a damping component, which is connected in parallel to the coil. It protects the driver but does slow the release time of the relay. Also known as coil snubber circuit.

Dropout Voltage: The Voltage at or below which all the contacts of an operated relay must revert to unoperated position. Also known as release voltage. Dust Proof Relays / Solder Proof Relay: Relay with case for protection against dust and touch. With specified solder conditions are kept, no harmful amounts of flux or solder vapor penetrate into the relay.

Duty Cycle: Ratio of the duration of energization to the total period of intermittent duty.

Electrical Endurance (Electrical Life): Number of operations until switching failure of a relay under defined Conditions of load and of ambient influences. The reference value specified for the life apply, unless otherwise specified, to a resistive load. At lower contact loads a substantially longer electrical life is achieved. At higher loads the electrical life is reduced substantially.

Inrush Current: This value specifies the instantaneous current that may flow on the defined conditions. (Voltage, Power Factor, Duration) when the contact closes. Depends on type of load.

Insulation Resistance: Electrical resistance, measured between insulated relay parts at a test voltage of 500VDC.

Material Transfer: During the switching procedure the arc heats up the two contacts differently, depending on the load and polarity. This result in a material transfer from the hotter to the cooler electrode. With a higher DC loads on the contact, a 'pip' is build up, the other contact loose material and it creates a chatter.

Maximum Carrying Current: The maximum current which after closing or prior to opening, the contacts can safely pass without being subject to temperature rise in excess of their design limit.

Maximum Continuous Voltage: The maximum voltage that can be applied to relay coil continuously with out causing damage.

Maximum Switching Current: The maximum current that can safely be switched by the contacts.

Maximum Switching frequency: The maximum switching frequency which satisfies the mechanical or electrical life under repeated operations by applying a pulse train at the rated voltage to the operating coil

Maximum Switching Power: The maximum value that can be switched by the contacts without causing damage.

Maximum Switching Voltage: The maximum open circuit voltage that can be safely be switched by the contacts.

Mechanical Endurance (Mechanical Life): Number of switching operations without contact load during which the relay remains within the specified characteristics.

Nominal Coil Power: Power consumption of the coil at nominal voltage and nominal coil resistance. Also known as Rated Power.

Nominal Coil Voltage: The voltage by design intended to be applied to the relay coil. Also known as Rated Voltage.

Open Relay: Relay without case or cover.

Operating Power: Coil Power at which the relay operates

Operate time: The time from the initial application of power to the coil until the closure of the normally open contacts. It is excluding bounce time.

PCB Relays: Relays designed for soldering into printed circuit boards.

Pick-up Voltage: The value of the voltage that should be applied to an un-operated relay coil at or below which all the contacts of the relay should operate. Also known as Pull-in voltage / Must operate voltage.

Plug-in Relay: Relays that are held in the socket by flat Plug-in terminals.

Release Time: The time from the initial removal of power from the coil until the re-closure of the normally closed contacts. It is excluding bounce time.

Sealed Relay: Plastic base and cover are sealed with epoxy resin, after soldering into the PC board the relay. may be cleaned in liquid or coated with varnish. Provides a large measure of protection against aggressive ambient atmosphere. Also known as immersion cleanable relays.

Shock Resistance: It specifies at which mechanical shock (multiple of gravitational acceleration 'g' at half since wave and duration 11 ms) the closed contact has still not opened (failure criteria: contact interrupted for>10μ s).

Vibration Resistance: It specifies the amplitude or the acceleration in a defined frequency range at which the closed contacts should still not open (failure criteria: contact interrupted for>10μ s).

The contacts are the most important element of relay construction. Contact performance is influenced by contact material, voltage and current values applied to the contacts, the type of load, frequency of switching, ambient atmosphere, form of contact, contact switching speed and of bounce.
The contacts are practically not clean because the surfaces are covered by thin layers of low conductivity, semiconductor properties or even isolating characteristics. These layers of oxides, sulphides and other compounds will be formed on the surface of metals by absorption of gas molecules from the ambient atmosphere within a very short time. The growth of these layers will be slowed down and eventually stopped as the layer itself prevents further chemical reaction. The resistance of these layers increases with their thickness. To get a reliable electrical contact these layers have to be destroyed. This can be done by mechanical or electro-thermal destruction.

Mechanical Cleaning
When the contacts are closing, the metal surfaces will collide and hit against each other several times (bouncing), causing elastic deformation of the contact area and mechanical destruction of the thin layers. With high contact pressure also this could be obtained.
The design of most of the relays allows the contact surfaces to wipe across each other destroying the non-conductive films on the contact surfaces. This contact wipe is often enough to clean the surface and reduce resistance to an acceptable level, as well as keeping the resistance stable through out the electrical life of the relay.

Electrical Cleaning
The low and non-conductive layers can also be destroyed by the effects of
a) electrical voltage (fretting)
b) current (heating of contact points)
c) thermal effects (high temperature due to electrical arc)

a) Fritting
The term fritting describes the electrical breakdown of oxide / foreign layer when a sufficiently high voltage is applied across a closed contact. Due to the applied voltage and very short distance between the low potentials a high electric field is generated. This electric field destroys the thin non-conductive layer and establishes a metal bridge electrically linking the two surfaces. The value of fritting voltage depends on the contact material, composition and thickness of layers, conductivity and composition of the contact surface.

b) High Currents
High continuous currents and increased contact resistance due to the layers causes heating of the contact. The layers will eventually be destroyed thermally and a larger effective contact area is created, reducing the constriction resistance.

c) Arc, sparks
Under certain circumstances an electric spark or arc will be generated during contact making or contact breaking under load. The extremely high temperature of these arcs may destroy the contact layers and burn or disintegrate other contaminants or particles in the vicinity of the point of contact.

Contact Material Characteristics
Silver - Nickel AgNi 0.15 Good Electrical conductivity and thermal conductivity. Exhibits low contact resistance, and widely used. Easily develops a sulfide film in a sulfide atmosphere.
Silver - Nickel Used for switching loads in the rage of > 100mA upto power switching. Good resistance to contact wear and contact welding. Slightly high contact resistance.
AgNi 80:20 Mainly used in DC Switching particularly in automotive applications where high inrush current occur.
Silver - Cadmium oxide AgCdO Very Good resistance to contact wear and welding. Good thermal and mechanicalstability. Used for switching inductive or high current loads like Motors, Solenoids etc. High contact resistance and Sulphide films form easily.
Silver Tin Oxide AgSn02 High melting point and high thermal stability and therefore high resistance to welding. Also contact erosion rate is lower because any arc spreads to the outside of the contact preventing creation of a local hot spot and potential weld. Highcontact life minimum material migration. AgSn02 is mainly used for application involving high inrush current like lamp loads or inductive DC loads.
Palladium - Copper PdCu Greater Hardness, low contact wear and stable contact resistance. Good corrosion and sulphidation resistance. Very low material migration compared to other contact materials. Expensive. Mainly used for Flasher applications in Automobiles.
Tungsten W Highest melting point, high wear resistance with heavy loads, little transfer of material, best suited for breaking heavy inductive loads.
Au clad(gold clad) Au with its excellent corrosion resistance is pressure welded onto a base metal. Special characteristics are uniform thickness and the nonexistence of pinholes. Greatly effective especially for low level loads under relatively adverse atmospheres. Often difficult to implement clad contacts in existing relays due to design and installation.
Surface Material
Au plating(gold plating) Similar effect to Au cladding. Depending on the plating process used, supervisionis important as there is possibility of pinholes and cracks. Relatively easy to implement gold plating in existing relays.
Au flash plating(gold thin-film plating) Purpose is to protect the contact base metal during storage of the switch or devicewith built-in switch. However, a certain degree of contact stability can be obtained even when switching loads.

Electric Arc Switching
An electric arc is a current intensive gas discharge which occurs when opening a switch or as a result of a flashover. Under certain
circumstances the air path between two contacts are ionized due to very high electric field. Ionization causes the normally non-conducting air conductive and its conductivity is maintained if sufficient energy is supplied. The arc represents an additional resistive path in the load circuit. The minimum voltage and current required for generation and maintenance of a stable arc depends on the contact material and the length of the air gap. (Ionization of air happens if a potential of 32V or more is applied between two electrodes)

Due to extremely high temperature of the arc the surface of the contact will melt. Evaporation or sputtering of the contact material leads to wear and material migration reducing the service life of the contacts.

Arc in DC Circuits
In DC circuits it is generally during contact breaking that arc occurs. When breaking contacts move further apart and as the gap between the contacts increases the minimum voltage to maintain the arc normally rises above source voltage and the arc is extinguished. If however the supply voltage / current is sufficiently high enough to maintain a stable arc across open contacts, the relay will be destroyed.

In DC Inductive circuits, the counter emf generated whose magnitude Is equal to L "'1/2 (energy stored In the Inductance) act as a secondary energy source which causes the arc to be maintained until the energy in the circuit has been converted to heat. This leads to considerably longer arc duration. To prevent destruction of the contacts and to keep the arc duration within limits, the switching voltage/current has to be within the maximum DC breaking capacity. (Higher the UR ratio or lower the power factor of the load, the arc extinguishing time increases

Type of load and inrush current
The type of the load and its inrush current characteristics together with the switching frequency are important factors that cause contact welding. Particularly for loads with inrush currents, measure the steady state current and inrush current and select a relay that provides ample margin of safety. Table shown below illustrates the typical loads and corresponding inrush currents.

Load Inrush Current and Time

Typical contact protection circuits are given in the table below.
Use of contact protective devices or protection circuits can suppress the Counter emf to a low level. However, note that incorrect use will result
in an adverse effect.

  Circuit Features I Others Device Selection
CR circuit If the toad is a timer, leakage current flows through the CR circuit causing faulty operation
* If used with AC voltage, be sure the impedance of the load is sufficiently smaller than that of the CR circuit.
As a guide in selecting rand c, r: 0.5 to 10per 1μV contact voltage c: 0.5 to 1μF per 1μA contact current values vary depending on the properties of the load and variations in relay characteristics.
Capacitor c acts to suppress the discharge the moment the contacts open. Resistor r acts to limit the current when the power is turned on the next time. Test to confirm. Use a capacitor with a breakdown voltage of 200 to 300V. Use AC type capacitors (nonpolarized) for AC circuits.
If the load is a relay or solenoid, the release time lengthens. Effective when connected to both contacts if the power supply voltage is 24 or 48V and the voltage across the load is 1 00 to 200V.
Diode circuit The diode connected in parallel causes the energy stored in the coil to flow to the coil in the form of current and dissipates it as joule heat at the esistance
component of the inductive load. This circuit further delays the release time compared to the CR circuit. (2 to 5 times
the release time listed in the catalog)
Use a diode with a reverse breakdown voltage at least 10 times the circuit voltage and a forward current at least as large as the load current. In electronic circuits where the circuit voltages are not so high, a diode can be used with a reverse breakdown voltage of about 2 to 3 times the
power supply voltage.
Diode and zener diode circuit Effective when the release time in the diode circuit is too long. Use a zener diode with a zener voltage about the same as the power supply voltage.
Varistor circuit Using the stable voltage characteristics of the varistor, this circuit prevents excessively high voltages from being applied across the contacts. This circuit also slightly delays the release time. Effective when connected to both contacts if the power supply voltage is 24 or 48V and the voltage across the load is 100 to 200V.  

Specifications Subject to change without prior notice.

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