Basic Technical Information

Definition of Relay Terminology

COIL

·         Coil Resistance - This is the DC resistance of the coil in DC relays usually specified at 23°C. 

CONTACTS

·         Contact Forms Form A (normally open contact) - Form A contacts are also called N.O. contacts or make contacts. Form B (normally closed contact) Form B contacts are also called N.C. contacts or break contacts. Form C (both normal open and closed contacts) Form C contacts are also called change over contacts or transfer contacts.

·         Rated Switching Power - The design value in watts (DC) or volt amperes (AC) which can safely be switched by the contacts. This value is the product of switching voltage x switching current, and will be lower than the maximum voltage and maximum current product.

·         Maximum Switching Voltage - The maximum open circuit voltage, which can safely be switched by the contacts. AC and DC voltage maximums will be different .

·         Maximum Switching Current - The maximum current which can safely be switched by the contacts. AC and DC current maximums will be different.

·         Maximum Switching Power - The upper limit of power which can be switched by the contacts. Care should be taken not to exceed this value.

·         Maximum Carrying Current - The maximum current which the contacts can safely pass without being subject to temperature rise in excess of their design limit, or the design limit of other temperature sensitive components in the relay (coil, springs, insulation, etc.). This value is usually in excess of the maximum switching current.

·         Minimum Switching Capability - The minimum value of voltage and current which can be reliably switched by the contacts. These numbers will vary from device type to device type. Factors affecting minimums include contact material, contact pressure, wipe, ambient conditions and type of relay enclosure (sealed vs. non-sealed).

·         Maximum Switching Capacity - This is listed in the data column for each type of relay as the maximum value of the contact rating and is an inter relationship of the maximum switching power, maximum switching voltage, and maximum switching current.

·         Contact Resistance - This is a value of the resistance when the contacts are closed which includes the resistance of the terminals and contact springs. The contact resistance is measured using the voltage-drop method. Usually measured at 1 amp at 5 VDC.

·         Capacitance - This value is measured between the terminals at 1 kHz and 20°C.

PERFORMANCE

·         Insulation Resistance - The resistance value between all mutually isolated conducting sections of the relay, between coil and contacts, across open contacts and between coil or contacts to any core or frame at ground potential.

·         Breakdown Voltage (Hi-Pot or Dielectric Strength) - The maximum voltage, which can be tolerated by the relay without damage for a specified period of time, usually The ability of the device to withstand an abnormal externally produced power surge, as in a lightning strike, or other phenomenon.

·         Operate Time (Pull-In or Pick-Up Time) - Measured from time coil is energized until normally open contact closure. (With multiple pole devices the time until the last contact closes.) This time does not include any bounce time.

·         Operate Bounce Time - Measured immediately following operate time ,point that contacts first closes and until contact stops bouncing..

·         Release Time (Drop-Out Time) - Measured starting immediately when coil power is removed and normally closed contacts first makes contact. (last contact with multi-pole) this time does not include bounce.

·         Release Bounce Time - Measured immediately following release time ,point that contacts first closes and until contact stops bouncing.

·         Shock Resistance, Destructive - The acceleration which can be withstood by the relay during shipping or installation without it suffering damage, and without causing a change in its operating characteristics. Usually expressed in "G"s.

·         Shock Resistance, Functional - The acceleration which can be tolerated by the relay during service without causing the closed contacts to open.

·         Vibration Resistance, Destructive - The vibration which can be withstood by the relay during shipping, installation or use, without it suffering damage, and without causing a change in its operating characteristics. Expressed as an acceleration in G's or displacement, and frequency range.

·         Vibration Resistance, Functional - The vibration which can be tolerated by the relay during service, without causing the closed contacts to open.

·         Mechanical Life - The minimum number of times the relay can be operated under nominal conditions (coil voltage, temperature, humidity, etc.) with no load on the contacts.

·         Electrical Life - The minimum number of times the relay can be operated under nominal conditions with a rated load being switched by the contacts.

CONSTRUCTION DETAILS

Relays come with the following construction to offer different types of protection against dust, flux, contaminating environment and automatic cleaning processes.

·         Open Type - Lowest cost type of construction, relay without dust cover. Protection must be provided by customers' enclosure.

·         Dust Cover Type - Dust cover protects the relay from large particles and protects user from electrical shock. May be soldered to PC board by hand to prevent flux and washing fluids from entering relay.

·         Sealed Type - Sealed relays are designed to prevent penetration of fluxes and gases during wave soldering and PC board washing. Best for use with wave soldering operations. Not for use in flammable environment.

·         Hermetic Seal - True hermetic sealed relays have metal to metal and glass to metal seals. The entire device is purged with dry nitrogen gas prior to sealing, improving reliability.

·         Terminals Types
PC board terminals (designed for use in PC board applications) Solder lug (designed with hole for wire insertion before soldering)
Spade lug terminals (designed for standard spade type lugs)
Screw terminals (design for use with solder lug terminals)
Combination of PC board and spade lug terminals

·         Mounting Means
Solder lug terminals
Flange mounting
Socket mounting

Application Guidelines

Relay applications may encounter a wide range of operating conditions. Life testing under actual operating conditions is recommended if possible to insure relay meets required relay life and performance. Careful review of relay specifications is recommended. Standard coil voltages are recommended to be used where possible to insure availability and the lowest relay cost possible for your application.

SPECIFICATIONS

Review the following specifications to determine the exact relay characteristic for your application

·         Coil
1. Voltage rating
2. Pick-up voltage
3. Drop-out voltage
4. Maximum continue voltage
5. Coil resistance
6. Input frequency (AC type)

·         Contacts
1. Contact arrangement
2. Contact rating
3. Life
4. Contact resistance

·         Operate time
1. Operate time
2. Release time
3. Bounce time
4. Switching frequency

·         Mechanical requirements
1. Vibration requirements
2. Shock requirements
3. Ambient temperature
4. Life requirements

·         Physical requirements
1. Mounting methods
2. Cover
3. Size

·         Agency approvals
UL, CSA ,VDE, and etc

HANDLING GUIDELINES

1. Avoid dropping or hitting relay

2. Do not remove relay cover. This may change relay characteristic.

3. Select relay for proper atmospheric conditions.

4. Do not exceed rated specifications.

5. Don't exceed usable ambient conditions.

6. Used sealed types for automatic soldering and pc board cleaning processes.

7. Avoid ultra sonic cleaning, may damage relay.

APPLICATION INFORMATION

Coil
To guarantee accurate and stable relay operation, the first condition to be satisfied is the application of the rated voltage to the relay. Additionally, details concerning the type of the power source, voltage fluctuation and changes in coil resistance due to temperature rise and the rated voltage must also be considered. If a voltage higher than the rated maximum voltage is applied to the coil for a long time, layer short-circuiting and damage to the coil by burning may take place.

·         Coil Operating Voltage Source
If the supply voltage fluctuates, the relay will malfunction regardless of whether the fluctuation lasts for a long time or only for a moment. If the capacity of the power source is insufficient to operate all the devices using a power supply at the same time, the relay may not operate, because the supply voltage has dropped.

·         Coil Temperature Rise
When a current flows through the coil, the coil's temperature rises to a measurable level. If an alternating current flows, the temperature rises even more. When a current is applied to the contacts, heat is generated on the contacts, raising the coil temperature even higher

·         Pickup Voltage effected by Coil Temperature-Rise.
The coil resistance of a DC-switching relay increases (as the coil temperature rises) when the coil has been continuously energized. This increase in the coil resistance raises the voltage value at which the relay operates. The coil resistance also rises due to ambient temperature the relay is exposed too.

·         Upper limit voltage
The maximum voltage applicable to a relay is determined by the coil temperature rise and the coil insulation materials' heat resistivity, electrical as well as mechanical service life, general characteristics, and other factors.
If a voltage exceeding the maximum voltage is applied to the relay, it may cause the insulation materials to degrade, the coil to be burnt, and the relay to fail. The coil temperature must not exceed the temperature that the coil can withstand. The temperature a relay may be subjected to depends on the class (temperature rating) of the materials used in the manufacture of the relay.

·         How to calculate Coil temperature
t = R2 - Rl/R1 (234.5 + Tl) + T1 (°C)
where,
R1 (): coil resistance before energization
R2 (): coil resistance after energization T1 (°C): coil temperature (ambient) before energization
t (°C): coil temperature after energization
Relay must stay energized until maximum coil temperature is reached.

·         DC Input Power Source
Power sources for DC-operated relays are usually a battery or a DC power supply, either with a maximum ripple of 5%. If power is supplied to the relay via a rectifier, the pickup and dropout voltages vary with the ripple percentage. Therefore, check the voltages before actually using the relay. If the ripple component is extremely large, chatter may occur. If this happens, it is recommended that a capacitor be applied across the coil.

·         AC Input Power Source
Generally, the coil temperature of the AC-switching relay rises higher than that of the DC-switching version. This is because of resistance losses in the shading coil, eddy-current losses in the magnetic circuit, and hysteresis losses. A phenomenon known as "chatter" may take place when the AC switching relay operates on a voltage lower than that rated. For example, chatter may occur if the relay's supply voltage drops. This often happens when a motor (which is to be controlled by the relay) is activated. This results in damage to the relay contacts by burning, contact weld, or disconnection of the self-holding circuit. Therefore, countermeasures must be taken to prevent fluctuation in the supply voltage.
One other point that requires attention is the "inrush current" When the relay operates a current much higher than that rated flows through the coil. This current is known as the inrush current. (When the armature is attracted to the magnet, however, the impedance rises, decreasing the inrush current to the rated level.) Adequate consideration must be given to the inrush current, along with the power consumption, especially when connecting several relays in parallel.

·         Operate time
In the case of AC operation, there is extensive variation in operate time depending upon the point in the phase at which the switch is turned ON and it is expressed as a certain range, but for miniature types it is for the most part 1/2 cycle (about 10msec.).For the somewhat large type relay where bounce is large, the operate time is 7 to 20msec., with release time in the order of 9 to 20msec. Also, in the case of DC operation, the operating time remains the same.

·         Relay operation due to inductive interference
In situations where both control and load wiring are in close proximity, thought should be given to separating or shielding the conductors in order to prevent false relay operation. .

·         Influence of external magnetic fields.
Care should be exercised in the placement of relays where strong, external magnetic fields are present, such as in proximity to power transformers or permanent magnets (speakers, etc.). this can cause operational characteristics to change. This influence is more common in the smaller relays.

CONTACTS

·         Types of Load and Inrush Currents
The type of load and its associated inrush current at turnon effects contact life depending on frequency of operation.

·         Resistive load
Steady state current

·         Solenoid load
10 to 20 times steady state current

·         Motor load
5 to 10 times steady state current

·         Incandescent lamp load
10 to 15 times steady state current

·         Mercury lamp load
3 times steady state current

·         Capacitive load
20 to 40 times steady state current

·         Transformer load
5 to 15 times steady state current

·         Contact Protection
Contact life may be extended with the proper contact protection.

1. R C circuit across contacts or across load
2. Diode across load
3. Diode-zener across load
4. Varistor across load


Further information may be obtained by studying the Engineers Relay Handbook published by the National Relay Association or contacting your vendor.

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Copyright © 2001 [CIMI]. All rights reserved. Terms & Conditions. 
Revised: May 11, 2005