Relay Contact Life

The useful life of a relay depends upon its contacts. Once contacts burn out, the contacts or the entire relay has to be replaced. Contact Life can be estimated. Mechanical Life is the number of operations (openings and closings) a contact can perform without electrical current. The mechanical life of a relay is relatively long -up to 1,000,000 operations. Electrical life is the number of operations (openings and closings) the contacts can perform with electrical current at a given current rating. Contact electrical life ratings range from 100,000 to 500,000 cycles.

Figure 59. Relay Contacts

Arcing is one enemy of contact life. Arcing occurs when an electric switch is opened and current discharges across the contact gap. Arcing can be minimized by using the correct contact material for the application, or by using an arc suppressor, a device that dissipates energy across the open contacts.

Contacts are available in fine silver, silver-cadmium, gold-flashed silver and tungsten. While fine silver has th

Solid State Relays

A solid-state relay consists of an input circuit, a control circuit and an output circuit.

1-Input Circuit

The Input Circuit is the portion of the relay to which the control component is connected. The input circuit performs the same function as the coil of an electromechanical relay. The circuit is activated when a voltage higher than the relay's specified Pickup Voltage is applied to the relay input.

The input circuit is deactivated when the voltage applied is less than the specified minimum Dropout Voltage of the relay. The voltage range of 3 VDC to 32 VDC, commonly used with most solid-state relays, makes it useful for most electronic circuits.

2-Control Circuit

The Control Circuit is the part of the relay that determines when the output component is energized or de-energized. The control circuit functions as the coupling between the input and output circuits. In an electromechanical relay, the coil accomplishes this function.

3-Output Circuit

The Output Circuit is the portion of the relay that switches on the load and performs the same function as the mechanical contacts of an electromechanical relay. Solid-state relays, however, normally have only one output contact.

Relay Style Comparison

The decision to use an electromechanical or solid-state relay depends on an application's electrical requirements, cost constraints and life expectancy. Although solid-state relays have become very popular, electromechanical relays remain common. Many of the functions performed by power-hungry heavy-duty equipment need the switching capabilities of electromechanical relays.

1-Although a Solid-State Relay (SSR) accomplishes the same result as an Electromechanical Relay (EMR), its physical structure and the way it functions are different. An SSR switches current using non-moving electronic devices such as silicon controlled rectifiers.

These differences in the two types of relay systems result in advantages and disadvantages with each system.

2-Because an SSR does not have to either energize a coil or open contacts, less voltage is required to "turn" an SSR on or off. Similarly, an SSR turns on and turns off faster because there are no physical parts to move.

3-Although the absence of contacts and moving parts means that SSRs are not subject to arcing and do not wear out .

4- contacts on EMRs can be replaced, whereas the entire SSR must be replaced when any part becomes defective.

5-Because of the construction of SSRs, there is residual electrical resistance and/or current leakage whether switches are open or closed. The small voltage drops that are created are not usually a problem; however, EMRs provide a cleaner ON or OFF condition because of the relatively large distance between contacts, which acts as a form of "insulation."

●Electromechanical Relays

The chart below summarize the advantages and limitations of both relay styles.

●Solid-State Relays