Choosing the most suitable relay for process control systems and industrial automation applications can be perplexing given the enormous variety of relays that are available on the market today.
By far, the most common type of relay in use is the electro-mechanical relay. It consists of an electrical component, the coil, and a mechanical part, the contact. The inherent simplicity of the electro-mechanical relay and its low cost, has lead to its use in nearly every switching process for inputs and outputs of control systems.
In the case of electro-mechanical relays, coil voltages are available for all industrial requirements from 5VDC to 240VAC. Engineers and technicians can also choose from contacts that switch up to 16A. The contacts can be arranged as normally open, normally closed or as a changeover, and in groups of 1 to 6 poles.
Electro-mechanical relays tend to be highly reliable and can operate for years without malfunctioning. They are best suited to applications that require switching up to five times per minute. They are ideal for standard industrial use for PLC/DCS output 0.5 to 10A resistive load and where isolation up to 4kV is required.
Typically, electro mechanical relays tend to have a contact made of silver tin oxide. This is a very hardwearing material but is designed for minimum switching levels of at least 12V@100mA. To meet the needs of low level switching, relay manufacturers developed gold plated contacts. The contact material features a plating of around 5um of pure gold.
As gold does not tarnish and has the lowest resistance, signals can be switched down to 100mV at 100uA to accommodate most low-level signals. If the relays are required to switch at high speeds or for a very long duration then solid state relays are a more appropriate option. This is because high speed switching leads to deformation of the contact and overheating of the coil in electro-mechanical relays.
Purposely designed to meet the needs of high speed switching, solid state relays incorporate Triac and MOSFET technology. A device called an opto-coupler which is a light emitting diode adjacent to a photosensitive transistor has replaced the coil.
When the LED is energised it produces light that falls on the transistor and causes it to switch on. Depending on the output configuration triacs are used to switch AC and MOSFETs to switch DC. The solid state relay incorporates no moving parts eliminating the possibility of wear and tear.
Extremely reliable, solid state relays should last the life of the installation. They are perfect for applications that require switching at more than 5 times per minute up to 100kHz.
They are available specifically as input devices with switching capacities of up to 100mA or as output devices switching up to 10A inductive loads.
For more information about relays contact Weidmuller on free call 1800 739 988 or email.
Selecting the most appropriate relay Electronics News
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