Summary： Various specifications and relays accompanying every power electronic component cannot be left behind on this crucial issue. Here, we shall look at various significant specifications of a relay that every engineer should be equipped with when working with relays.
It is essential to take seriously the relay specifications listed on their datasheets. They are not just numbers, and a sound engineer must work with them if they need to achieve quality. The operation of the relay outside the tabulated specifications may drastically reduce its lifespan, leading to the failure of its switching systems and sometimes damaging the electrical unit that is under test. With this in place, let us focus on some of the most common specifications of the relay and their impact on the switching systems.
Figure 1: Relay image by Simon Mugo
A relay as a gadget is a composition of moving parts subjected to wear and tear due to stresses contributing to its failure. The life expectancy of a relay provides us with the necessary data and information on when we expect the relay to fail in terms of mechanical aspects of tear and wear.
Reed and electromechanical relays are the only existing mechanical relays. Instruments with reed relays are proven to have a longer lifespan than EMRs because they have very few moving parts. The reed relay contacting blade operates in a bending rather than a moving mechanism, and its contact is hermetically placed in a glass envelope. This makes reed relays safe from contaminants and physical defects. The EMRs have a lesser mechanical life than the reed relay but can handle high power capacity.
Relays are built to sustain voltages higher than the rated maximum switching voltages across their contacts while the electrical signal is applied. This relay specification is referred to as cold switching voltages. Cold switching voltage is also known as standoff voltage.
Relays of high cold switching voltages are sometimes helpful in insulation testing but never try to switch the relay. In contrast, the voltage is switched on since it is always beyond the relay operating voltage. A switching system with standoff voltage has PCB traces designed to withstand such applied voltages.
This is the maximum amount of current that can pass across the relay’s contact in open or closed mode. Operating a relay under high voltages is accompanied by arcing, and it affects the contacts, which causes eroding, hence reducing its performance. When working on electrical designs that focus on switching modules, we monitor the voltage rating on the design to determine the lowest spacing limit we can use for the circuit board, components, and traces.
For systems where there is the presence of both negative and positive voltages, then the differences between the two voltages should be taken into consideration. For example, you might have a relay-switching power of three-phase; the voltage going across the given relay is higher than the voltage through each electrical power phase. It is vital to note that a switching system’s maximum voltage may be lower than the relay’s maximum switching power since relay specifications are computed using the resistive loads available in them.
Some electrical relays in the market work under minimal switching current, which must be available for the given relay to operate. This is very significant for relays that operate in the hot switching environment, and the contacts are subjected to wear and tear, exposing the materials used to build them. Therefore, the minimum voltage must be available for wetting of the contacts to achieve low resistance on the contact. Reed relays are suitable for such operations.
This is the maximum current a relay is rated to sustain during switching ON and OFF without destroying its parts, such as the contacts. Therefore, engineers should not miss this specification, which is available in the datasheet.
Carry current happens when the contacts of the relay are already closed, and the relay can overcome higher currents than the current of the switching labeled as switch current. This current is limited by the resistance of the contact, which leads to the heating up of the duplicate contacts. Never open a relay that is carrying a current more remarkable than the switch current offers; to be able to do so, you have to ensure that you reduce the current.
Some relay operates under pulsed carry current. This current heats the relay contacts without causing the arcing associated with the hot switching. Pulsed carry current can be categorized into a single event or repetitive. The repetitive event should be cautiously overlooked to protect the device against thermal challenges like overheating.
This is one of the specifications that can appear confusing to relay users, but it is very significant regarding the precise timing of situations. We can capture wrong measurements if an application does not work well with the relay timing.
On the datasheet of the switching system, the specified operation time includes the overall time that the system software will take to process the driver instruction plus the time the electric relay takes during operation and settling. The driver ensures no one accesses the switching system until settling time is completed. But sometimes, this can be overridden if the driver's waiting state is overridden.
Throughout my interaction with users of the relays, some ignore the power rating specification in the relay datasheet. They don’t know that this hurts the relay and might reduce its lifespan. If you use a signal that operates on both maximum switch current and maximum switch voltage, expect the outcome to exceed the relay power rating.
Let us have a simple example of a relay with a power rating of 60W, a maximum switching voltage of 250V, and a 2A maximum switching current. On calculating the power, you will get 500W, which is far beyond the relay rating by almost an order of power magnitude. To get the correct relay rating, a relay with a rating of 250W should not have a current above 250mA passing through the system.
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