Latching Relays, How They Work

Latching relays are electrically operated switches that change state (switch) with a pulsed voltage input to an actuator coil. Once they have changed state, they remain in that state without continuous application of the actuator voltage. Subsequent return to the original state is caused by additional voltage inputs as described below.

Jennings Technology's latching vacuum relays may be either Single Pole Single Throw (SPST) or Single Pole Double Throw (SPDT). The change of state for an SPST relay is between open and closed conditions, like a light switch. A SPDT relay switches a voltage between two paths, like choosing between lighting a red light or a green light. Typically vacuum relays are only used in high voltage or high frequency applications. The Common Terminal is switched back and forth between two different terminals, the Normally Open (NO) Terminal and the Normally Closed (NC) Terminal. The minimum duration for the actuating pulse is usually slightly longer than the high voltage transfer time and is specified as "Operate Time." The relay remains in that state until it receives a second input, or release, for a similar period. The minimum time period required to do this is specified as "release time." These times vary from 1 to 18 ms, depending on the model.

The actuators on Jennings Technology's latching relays are bistable magnetic latching actuators. They feature 2 coils, C1 & C2. C1 is used to close the actuator armature and transfer the high voltage contacts from one state to the other with a pulsed control voltage. A small permanent magnet is used to hold or latch the movable contact in the new position and no continuing voltage is required. This reduces power usage, relay coil heating, and insures that the relay does not change state in the event of an interruption of control power. There are two methods to return the relay to its original state. An intermittent pulse of the same polarity as that applied to C1 may be applied to C2. This will overcome the influence of the permanent magnet and a spring will return the moveable contact to the original position. Alternately, if you have bi-polar voltage source available, a pulse of the opposite polarity can be applied to C1 with the same effect as applying a pulse to C2.

Latching Relays provide stabile high insulation resistance for sensing applications and low non-oxidizing contact resistance of value in dry circuits. They are used in applications where the power used by the coil and coil heating are important. If the relay is to be switched infrequently, you may want to use a latching relay. If it is constantly changing state, it may be simpler to use a non-latching relay. Another consideration is, what do you want the relay to do when actuator power is interrupted? (Perhaps, caused by the loss of a power supply.) If you want the relay to remain as it was, use a latching relay. If you want it to return to its original state, use a non-latching relay.

The latching relays listed below are currently available from Jennings Technology. Detailed information is available on To find them, look under Products, Select Relays and choose the desired category. All relay types in that category, latching and non-latching, will be shown with summary specifications. Complete specifications may be seen by clicking on the blue hyperlink part number. On that data sheet, there is a button to connect you with a downloadable PDF Data Sheet, which contains an outline drawing.

# of Poles

60 Hz Voltage

60 Hz Current

Model Number


SPST 3.6 Kv 12 A RMS RF44-26S  
SPST 3.6 Kv 12 A RMS RF46-26S Actuating Time
< 1 ms
SPST 5 Kv 12 A RMS RF45-26S 4 ms operate time
SPST 7 Kv 12 A RMS RF43-26S  
SPST 8 Kv 35 A RMS RF47-26S  
SPST 8 Kv 35 A RMS RF72-26S 1.5 - 4 ms operate time, Ground Isolated
SPST 12 Kv 35 A RMS RF73-26S 4 ms operate time
SPDT 3.6 Kv 12 A RMS RF60-26S  
SPDT 7 Kv 10 A RMS RF63-26S  

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