Introduction

It is crucial to carefully consider the relay specifications provided in their datasheets. These values are not mere numbers; they contain essential information that a sound engineer must adhere to in order to ensure proper functionality. Operating a relay beyond its specified limits can significantly shorten its lifespan, cause malfunctioning of its switching mechanism, and, in some cases, damage the electrical unit under test. With that in mind, let us explore some of the most common relay specifications and their effects on switching systems.

Figure 1: Relay image by Simon Mugo

Life Expectancy

A relay is made up of moving parts that undergo wear and tear due to operational stresses, which ultimately leads to failure. The life expectancy of a relay provides vital data on when mechanical failure can be expected due to such wear and tear.

Reed and electromechanical relays (EMRs) are the primary types of mechanical relays. Reed relays are known to have a longer lifespan than EMRs because they contain fewer moving parts. In reed relays, the contact blade operates through a bending mechanism rather than a full-moving mechanism, and the contact is hermetically sealed within a glass envelope, protecting it from contaminants and physical damage. EMRs, while having a shorter mechanical life than reed relays, are capable of handling higher power levels.

Relay Cold Switching Voltage

Relays are designed to withstand voltages higher than their rated maximum switching voltage when the electrical signal is applied. This specification is known as the cold switching voltage, or standoff voltage.

Relays with high cold switching voltage ratings can be useful for insulation testing, but they should never be used for actual switching operations at voltages beyond their rated operating voltage. Systems designed to handle standoff voltage include PCB traces specifically rated to withstand such applied voltages.

Maximum Switching Voltage

This refers to the maximum voltage that can safely pass through the relay’s contacts, whether the relay is in the open or closed state. Operating a relay at high voltages can result in arcing, which causes the contacts to erode, diminishing their effectiveness over time. When designing electrical circuits with switching modules, it’s crucial to monitor the voltage rating to ensure proper spacing of components, traces, and the circuit board itself.

For systems where both positive and negative voltages are present, the voltage difference between these should also be considered. For instance, in a three-phase relay-switching system, the voltage across the relay may exceed the voltage present in each electrical phase. It is important to note that the maximum voltage rating of a switching system may be lower than the relay’s maximum voltage rating, as the latter is typically based on resistive loads.

Minimum Switching Voltage

Some relays are designed to operate at minimal switching currents, which are required for the relay to function correctly. This is particularly important for relays used in “hot switching” applications, where the contacts are subjected to frequent wear. In such cases, sufficient voltage must be available to “wet” the contacts, ensuring low resistance at the point of contact. Reed relays are ideal for such applications.

Switch Current

This is the maximum current a relay can safely handle when switching ON and OFF without causing damage to its components, such as the contacts. Engineers should always consider this specification, which can be found in the relay’s datasheet.

Carry Current

Carry current occurs when the relay’s contacts are already closed, and the relay can handle currents higher than those of the switch current. This current is limited by the resistance of the contacts, which causes heating. It is essential never to open a relay that is carrying more current than the switch current rating allows. To do so safely, ensure the current is reduced beforehand.

Carry Pulsed Current

Some relays are designed to operate with pulsed carry current, which can heat the relay contacts without causing arcing. Pulsed carry current can be either a single event or a repetitive pulse. When dealing with repetitive pulses, caution is necessary to avoid thermal issues such as overheating.

Operation Time

This specification can sometimes be confusing, but it is vital for precise operation in time-sensitive applications. Incorrect timing can result in inaccurate measurements if the relay’s operation time is not carefully considered.

The operation time listed in the datasheet refers to the total time required for the system to process the driver’s instruction, plus the time the relay itself takes to operate and settle. The driver ensures the switching system remains inactive until the settling time is complete, though this can sometimes be overridden if the driver’s wait state is bypassed.

Power Rating

Many relay users overlook the power rating specification in the datasheet, often unaware that this can significantly affect the relay’s lifespan. If a relay is exposed to both the maximum switch current and maximum switch voltage simultaneously, it could easily exceed its power rating.

For example, a relay with a power rating of 60W, a maximum switching voltage of 250V, and a maximum switching current of 2A results in a calculated power of 500W, far exceeding the rated power by nearly an order of magnitude. To ensure the relay operates within its specified limits, the correct relay rating would require a maximum current of 250mA for a 250W-rated relay.

Summary

This article has highlighted the following key points:

• Relay datasheet specifications provide essential information for engineers to use the relay properly and avoid damage.
• The life expectancy specification helps engineers predict when the relay will fail due to mechanical wear.
• The maximum switching voltage indicates the highest voltage that can safely pass through the relay’s contacts.
• Cold switching voltage refers to the relay’s ability to withstand higher-than-rated voltages across its contacts when no electrical signal is applied.
• The minimum switching voltage is the minimum voltage required for a relay to operate effectively.
• Switch current specifies the maximum current a relay can safely switch on and off without damaging its contacts.
• Carry current is the current the relay can handle when its contacts are closed and is higher than the switch current.
• Pulsed carry current heats the relay contacts without causing arcing, though repetitive pulses should be avoided to prevent overheating.
• Operation time defines the total time required for a relay to operate, including the processing time and settling time.
• The power rating is the maximum power the relay should handle to ensure accurate performance.
• The relay’s rated power should be roughly equal to the switching system’s rated power for optimal operation.

 

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