Feature Interconnection The need for reliable high speed Craig Lowe at Transonics reviews some of the key issues worth considering when designing-in a relay S
imply put a relay is a device used to switch a higher Current/ Voltage load with a lower Current/Voltage signal. A relay also provides elec- trical isolation between the control circuit and the power circuitry. When selecting a relay careful consid-
eration must be given to ensure it is ‘fit for purpose’ and will achieve the required MTBF for the end product or system. Key parameters which must be considered when selecting a relay for load or signal switching application are: 1. Contact Power: The Contact power of the relay is determined by the contact area, the contact material and the power of the primary coil. A 100mA contact power relay will need far less coil power for actuation than a 20A relay. The higher current relay requires more durable contact material to resist damage due to welding or heating when contacts are engaged and disengaged. Choosing the correct relay for the maxi- mum current and voltage type is critical. 2. Poles: A relay will have a configura-
tion of either a single or double pole which is relative to the number of isolated contacts. A single pole would be a ‘one in -one out’ switch configura- tion. A double Pole would be 2 isolated switches in a single package simultaneously controlled. Depending on the application the switched con- tacts may be in a Normally Open or Normally Closed configuration (NO/NC); this is the state the relay is in when it is switched off (not energised). 3. Throw: The Throw of a Relay is defined by the number of output con- tacts on for each POLE. For a single pole you may wish to switch between two output loads: Relay OFF, the common POLE feeds power to output 1. Relay ON , the common POLE feeds power to output 2. You may select a double pole double throw (DPDT) relay where you have 2 common isolated inputs and 4 outputs. Manufacturers use the terminology “1 Form A” or “2 Form B” to describe SPST and DPDT respectively. 4. Non-Latching (Monostable)/Latching
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(Bistable): A non-latching relay requires the coil to be energised continuously to maintain the switched state of the contacts. A latching Relay must be energised to change state but power can then be removed from the coil and the contacts will remain in that switched position. A latching relay significantly reduces power consumption as the relay coil is only powered momentarily to change contact position.
The operation of a latching relay can be carried out in 2 ways, with either a single or double coil. A single coil latching relay would require an inversed voltage pulse applied to the coils to enable the magnetic flux to throw the contacts back to the ‘open’ state. A double coil Latching relay has a standard coil to switch ON and an inverted coil to switch OFF when ener- gised with a pulse. The benefits of each are as follows:-
Single coil typically has a lower
power required to energise but would involve more circuitry on the control side to apply an inverted voltage (typi- cally using an H-bridge circuit). Double coils are easier to control as the voltages can be applied positively and there is no need for a negative voltage rail or ‘H’ bridge circuit. 5. Coil power: When using a standard
relay, the current required to hold the contacts ON will be in the region of 400mW. This may be critical when low power operation is required (eg. solar or battery powered equipment). If the application is power critical then a low power coil option should be considered as this would typically offer a coil power of 250mW. 6: Contact type: Relay contacts are pre- dominantly Silver and are doped or coated with other materials which make the surface of the contacts behave in different ways.
Contact selection is a trade-off between power handling capability and contact resistance. The harder materials have a longer life yet may not give the performance required for the lower current applications. 7. Insulation: Due to the inherent
Figure 1: A selection of relays f rom Transonics
nature of the design of a relay, the Primary Coil and contacts are isolated from each other. This enables switch- ing large voltages (sometimes thou- sands of Volts) with a very small control voltage (as low as 3VDC) with- out any interaction between them apart from the function required. 8. Temperature: When a relay is incorporated into a product design it is important to take into consideration the temperatures the relay may be subjected to. The key element of a relay’s construction is a copper coil which creates a magnetic flux pulling in an armature to close the contacts. It is common to specify the coil’s resist- ance at an operating temperature of 20°C on datasheets. With a temperature rise of 1°C it is likely to find an increase in the coil resistance of 0.4 percent. Operation in an ambient temperature of 85°c would produce a 26 percent change in the resistance of the coil. 9. COST: The materials used in the assembly of a relay should be quality and should not be compromised by opting for a low cost solution. The contact materials alone can contain Silver, Gold, Palladium etc. A properly
designed relay has
mechanical parts that would be expected to perform in excess of 50,000 switching operations. A relay might fail rapidly given high tempera- tures and mechanical stress if cheaper materials are used. There
may be several relay
Craig Lowe is Sales Director of Transonics
manufacturers products which meet the technical requirements for a relay requirement. It is important to review all the options to find the most cost effective solution from a price/perfor- mance standpoint. 10 Longevity. COST: An open discus- sion of any relay requirement with a reputable and experienced supplier will reduce the risk of designing in a part that could have availability issues or long lead times. Transonics
www.transonics.com
Enter 212 DECEMBER/JANUARY 2014 Electronics
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