FEATURE SWITCHES & RELAYS Choosing the right relay
Single Contact: - this is a single point of contact for the current to be conducted through. This may be made from any of the material options mentioned above. Bifurcated Contacts: - This is a single current path split into two contacts allowing the current to be shared equally across two contact areas made from identical contact material types. Premake Contacts: - These are two sets of contacts similar to the bifurcated contacts but the premake is very slightly offset allowing current flow through one set of contacts before the current can flow through the second path. The initial contact will usually be a
Spencer Manly - Applications Engineer at Transonics, discusses what to look out for when choosing the right relay manufacturer for your lighting application
W
hen designing a lighting controller the most stressed component will
be the switching device. This is usually either a Relay or a semiconductor. All other components in the control circuit are using low level signals which will then be used to turn the main power device on and off. For the main power device the key rating considerations to be explored are: Voltage, Current and Drive Power. However as any design moves forward it becomes evident that there are many other parameters that require careful consideration. With some 90 percent of lighting
controller applications, a safety barrier will be required between the lighting system and the control circuitry. In these cases using a semiconductor solution is not an easy option compared to using a relay. Relays are intrinsically isolated and therefore provide the dielectric isolation required by default. Contact load, contact material and contact type are among the most critical elements that require careful consideration when selecting a relay. At the core of a conventional relay is
the Coil which will be controlled by the low voltage control circuitry. The performance of relay coils is fairly standard across the range of all relays. The main options for selecting a relay coil are a standard coil which might
24 SEPTEMBER 2014 | ELECTRONICS
require from 140mW up to 500mW to operate depending on the device or a latching relay which only draws current for the initial switching of the device reducing the power required from the control circuitry.
LATCHING RELAY SPECIFICS The latching relay, once energised, will hold the contacts in until another signal is sent to the coil contacts. This signal can be either an inverted power signal to the same pins used to energise the coil or it could be a separate coil which then would push the contacts apart again. The way a
Figure 1:
HONGFA relays from Transonics
harder material which has a hard surface to lower the risk of initial degradation due to high inrush etc. but typically will not be a very low resistance path. The second contact will be a low resistance path so lower currents can flow. With a lighting application switching currents are not at a fixed level but starts very high and then diminishes to a steady state after certain duration of time (called Inrush). This Inrush period can vary from load to
load and can be 10’s if not 100’s of times higher than the steady state current. This initial high current surge is the most common cause of failure in a switching device and is why great care must be taken when selecting a control relay.
“Contact load, contact material and contact
conventional relay handles the load current is by a pair of contacts which will either be made of a single material or doped with a material designed for the application concerned. Plating’s including Gold, Silver, Nickel, Tin Oxide and Tungsten are available and each has specific load uses. These different materials can be then
type are among the most critical elements that require careful
consideration when selecting the right relay . . .”
HANDLING HIGH INRUSH CURRENT By using a relay with a “premake” set of contacts the initial high inrush current can be dealt with by a tungsten plated contact and then once the current has started to settle, the lower current path can be introduced reducing the heat in the relay. Any relay selected for a lighting application must be compliant with UL508/IEC60669- 1/IEC60669-2-1. Hongfa is a major global relay supplier and focuses on providing complete and reliable relay solutions to customers in the
lighting control field. The company has successfully provided Lighting Control Relay Solutions to well-known
companies in intelligent building, energy saving socket and street lighting applications around the world.
bonded to a variety of contact options to provide a conventional “Single Contact” or options of “bifurcated Contact” or “premake contacts”.
Transonics
www.transonics.com 0845 567 8899
Enter 209 / ELECTRONICS
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