Allen-Bradley 1771 – Rockwell Automation
Provide output isolation.Isolation helps assure that a failure on one output circuit does not pull down another output circuit, thus increasing overall system integrity. This isolation also protects the backplane logic from line transients on the output circuits.
Increase flexibility in applied voltages.You can apply either ac or dc voltages to relay contact output circuits. The range of these voltages can be broad (within the module’s operating specifications) without impacting the module’s performance.
Sink or source power.Contacts on relays permit dc outputs to be sinking or sourcing as required by the devices being controlled. Solid-state outputs require specific compatibility with the load device as to current sinking or sourcing.
Switch analog signals through relays.You can use analog module inputs for different devices by switching circuits through relays. Analog signals are typically low voltage (+10V dc) and low current (4-20 mA). Contact resistance can be critical, and should be accounted for in low-impedance circuits.
When using relay contact output modules, you must consider the following:
Relays have a finite number of operations during their life. This varies depending on the current loads and voltages you apply relative to the relay’s design specifications. A relay that is always operating at the same load conditions has a very predicatable life. A low power load delivers longer contact life when the minimum load conditions are met. Varying load conditions can drastically shorten contact life. Do not operate a contact at low current or voltage conditions after operating the same relay under high power conditions. Operate at low power first, then at high power.
Electromechanical relays typically have a longer life than dry-reed relays. When selecting a relay type, we recommend that you consider the number of operations expected over time and the load placed on the relay.
The switching frequency of a relay is limited by its mechanical
characteristics. To obtain maximum relay life, do not exceed the
maximum switching frequency of the module.