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New latching relay driver (LRD)

Latching (bi-stable) relays have the great advantage to maintain their contact position indefinitely without any power being sent to the coil. They save a lot of energy compared to standard contactors that can consume up to 0.4 A to maintain their contacts closed (10 Ah per day / 300 Ah per month). But they need to be commanded by pulses on two different inputs.

The LRD makes it easy to use a power saving latching relay.

  • converts standard output from a BMS, a relay or a switch into the proper signals to drive a dual-coil latching relay
  • it is reliable: built with discreet components (no micro-controller)
  • it resists humidity: the components are coated with an epoxy resin
  • the spring clamp terminals are coated with an anticorrosion molecular compound

What is a latching (bi-stable) relay?

In a latching relay the position of the contacts (power circuit) is maintained by a magnet. It just needs a short pulse on one of the coils to change the position of the contacts. Once the contacts are in the desired position the drive power to the coil can be removed.

When the “ON coil” is powered the power circuit is closed

When the “OFF coil” is powered the power circuit is opened

The key characteristics of a latching relay:

  • maximum switching current (the maximum current in the power circuit)
  • coil voltage (the range of voltage to apply to the coils to change the state of the contacts)
  • maximum coil current (the maximum current drawn by the coil when powered)
  • coil suppression (a circuit to avoid high voltage spike when the power is removed from a coil)
  • coil pulse duration (ms)
  • high / low side drive (whether the coil is controlled on the positive or the negative terminal)

The LRD drives the coils on the high side, which is compatible with most latching relays on the market.

Examples of latching relays

LRD connections

LRD top view
  • Power
    • power supply should be sourced directly from the battery (before the relay) or from a separate battery (starter battery)
    • the voltage of the power supply (vcc) should match the coil voltage of the relay (9 – 30 volt)
    • should be protected with a fuse rated to match the relay coils
  • BMS
    • input for the driving command (BMS output or the output of other command source like a relay)
  • Input for an override switch
    • use a switch with 3 positions to set the relay ON, OFF, or have it set by the driving command
  • Outputs
  • ON
    • a pulse is sent on that output when the driving command transitions to vcc or when the override switch is flipped to the ON position
close the relay: trace 1 is the input (driving command)
trace 2 is the ON output / trace 3 is the OFF output
  • OFF
    • a pulse is sent on that output when the driving command is disconnected from vcc or when the override switch is flipped to the OFF position
open the relay: trace 1 is the input (driving command)
trace 2 is the ON output / trace 3 is the OFF output


  1. Hi Jens,
    I had the same question a few years ago and I called Blue Sea support to understand.
    Actually the Blue Sea manual say: “Amperage Operating Current when changing state <7A (40 ms)"

    As you can see on the wiring diagram https://d2pyqm2yd3fw2i.cloudfront.net/files/resources/wiring_diagram/7700_7702.png , you need to have two fuses. The 7A switching current is taken from the supply that is protected by a 10A fuse.

    The Latching Relay Driver takes the place of the “Control Switch” and is protected by the 2A fuse – You can use the LRD to drive the Blue Sea 77xx relay. I did some test with early LRD prototypes.

    Here is a link to the wiring diagram: https://www.taoperf.com/wp-content/uploads/2021/12/LRD_1.drawio-1.pdf

  2. Hello Phil
    I just bought a Blue Sea ML-RBS to control the Under Voltage Protection of my Lithium Battery.
    I want to command this relay with the Tao LRD.
    In terms of current to run the coil of the Blue Sea ML-RBS I cannot find matching information.
    The specification in the manual of Blue Sea shows: “Amperage Operating Current when changing state max command current 1A

    The maximum output for the LRD given in the specification “up to 5 A”

    What is the correct current for commanding the Blue Sea relay?
    Can I use the Tao LRD for the Blue Sea relay with out damaging anything?
    Did you test the Tao LRD on this Blue Sea 77xx relays?

    Thanks for your help


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