What are the uses of latching relay?

Latching relays are a type of relay that can maintain their state (on or off) even after the power is removed. This is because they use a mechanical latch or permanent magnet to hold the contacts in place. Latching relays are especially useful in applications where power must be conserved, such as in battery-operated devices, or where it is desirable to have a relay stay in one position if power is interrupted. For example, a latching relay could be used to control the door lock on a car. Once the door is locked, the relay can be de-energized, but the door will remain locked until the relay is energized again.

Mechanical Latching Relays

Mechanical latching relays use a locking mechanism to hold their contacts in their last set position until com-manded to change state, usually by energizing a second coil. Figure below shows a two-coil mechanical latching relay. The latch coil requires only a single pulse of current to set the latch and hold the relay in the latched position. Similarly, the unlatch or release coil is momentarily ener-gized to disengage the mechanical latch and return the relay to the unlatched position.

How a two-coil mechanical latching relay works:

• Latching relays have no normal position for their contacts. In the unlatched state, the contacts are open.
• Pressing the on button energizes the latch coil, which closes the contacts and turns on the pilot light.
• The relay coil does not need to be continuously energized to keep the contacts closed.
• Pressing the off button energizes the unlatch coil, which opens the contacts and turns off the pilot light.
• If power is lost, the relay will remain in its original latched or unlatched state when power is restored.

Latching relays are useful in applications where power must be conserved or where the relay needs to stay in one position if power is interrupted. For example, a latching relay could be used to control a door lock or a security system.

Magnetic Latching Relays

Magnetic latching relays are single-coil relays that use a permanent magnet to hold their contacts in the closed position. A momentary pulse of voltage applied to the coil in the correct polarity will energize the relay and close the contacts. A momentary pulse of voltage applied to the coil in the reverse polarity will de-energize the relay and open the contacts.

Magnetic latching relays are useful in applications where power must be conserved, such as in battery-operated devices, or where it is desirable to have a relay stay in one position if power is interrupted. For example, a magnetic latching relay could be used to control the door lock on a car. Once the door is locked, the relay can be de-energized, but the door will remain locked until the relay is energized again.

Latching Relay Applications

Advantages of latching relays in electrical circuit design:

• Remembering events: Latching relays can be used to remember when a particular event takes place and prevent certain functions from occurring after that event. For example, if a part runs out on an assembly line, the latching relay can be used to shut down the process. The process can then only be restarted after the latching relay is manually reset.
• Power failure recovery: Latching relays can be used to ensure that circuit continuity is maintained during power failures. This is important in automatic processing equipment, where a sequence of operations must continue from the point of interruption after power is restored.
• Power conservation: Latching relays can be used to conserve power in battery-operated devices. For example, a latching relay can be used to power an alarm circuit. The latching relay will only consume power when the alarm is triggered.

Alternating Relays

Alternating relays (also known as impulse relays) are a type of latching relay that switches between two loads with each pulse. They are used in applications where it is important to equalize the run time of two loads.

Alternating relays require a continuous input voltage and a control switch. Each time the control switch is opened, the output contacts will change states. This means that alternating relays can be used to switch between two loads without the need for a timer or other control circuit.

Alternating relays also have a toggle switch that can be used to lock the relay to one of the two loads. This feature can be used to prevent one of the loads from running if it is not needed, or to ensure that a particular load always runs first.

In certain pumping applications, two identical pumps are used for the same job. A standby unit is made available in case the first pump fails. However, a completely idle pump might deteriorate and provide no safety margin. Alternating relays prevent this by assuring that both pumps get equal run time. The operation of the circuit can be summarized as follows:

• In the off state, the float switch is open, the alternating relay is in the load A position, and both loads (M1 and M2) are off.
• When the float switch closes, it energizes the first load (M1) and remote PL1 to indicate that pump motor 1 is running. The circuit remains in this state as long as the float switch remains closed.
• When the float switch opens, the first load (M1) is turned off and the alternating relay toggles to the load B position.
• When the float switch closes again, it energizes the second load (M2) and remote PL2 to indicate that pump motor 2 is running.
• When the float switch opens, the second load (M2) is turned off, the alternating relay toggles back to the load A position, and the process can be repeated again.

DPDT crossed-wired alternating relays are used in applications where additional capacity may be required in addition to normal alternating operation. These relays have the ability to alternate the loads of a dual systemduring normal operation or operate both when demand is high. Figure below shows the cross-wired contact version of an alternating relay used in a dual pumping circuit. The operation of the circuit can be summarized as follows:

• The selector switch located on the relay allows selection of the alternation mode or either load for continuous operation.
• LEDs indicate the status of the output relay.
• With the alternation mode selected, if the level in the tank never reaches the high level only the lead float switch cycles and normal alternating operation will occur.
• When both the lead and lag float switches close simultaneously, because of a heavy flow into the tank, both pumps A and B will be energized.
• This system saves on energy because only one pump is operating most of the time; yet the system has the capacity to handle twice the load.