Magnetic Latching Relay Guide: Coil Drive, Key Specs and Meter Selection

Hold a 100 A non-latching power relay closed at 12 V and it will draw roughly 2 W and run warm for as long as the meter stays energized. A magnetic latching relay deletes that current: the coil fires for a pulse of 30 to 100 ms, a permanent magnet takes over, and steady-state coil power drops to 0 W.

That single difference is why bistable designs now occupy the load-switch position in smart electricity meters, DIN rail meters, charging pile PDUs, over and under-voltage protectors and three-phase smart capacitors. It is also why so many field failures trace back to the drive circuit rather than to the contacts themselves.

How a Magnetic Latching Relay Actually Works

A magnetic latching relay is a bistable electromechanical switch. A short current pulse moves the armature, a permanent magnet holds the contacts in the new position, and the relay stays there after the drive signal disappears.

Compare that with a standard non-latching relay, which needs a constant coil voltage to stay closed. Two holding mechanisms are sold under the latching label. Magnetic latching relays hold with a permanent magnet, which works in any mounting orientation and never needs a release force. Mechanical latching relays hold with a spring and detent; they appear in low-power signalling circuits but rarely in meter load switches, where the holding force has to survive vibration and a decade of thermal cycling.

Set and reset: two coils or one reversed coil

Changing state takes one of two arrangements. A dual-coil relay has separate set and reset windings. A single-coil relay uses one winding with reversed current direction. In both cases the drive signal is a pulse, not a level, and three system-level consequences follow.

• No holding current, so the coil never adds a continuous thermal load inside a sealed meter housing.
• State survives a power failure, so a load switch returns from a blackout in the position it was left in.
• The driver has to deliver a burst of energy, usually from a reservoir capacitor rather than a permanently enabled transistor.

Single-Coil or Dual-Coil: Decide the Drive Circuit First

Dual-coil wins when the board can afford two drivers; single-coil wins when terminal count and package size are the binding constraint. The contact side of the specification is usually identical, so the real decision sits in the control circuit.

Pulse width is a design parameter, not a suggestion. Meter-class coils need roughly 30 to 100 ms at rated coil voltage to complete armature travel. A pulse shorter than about 20 ms risks an incomplete stroke that surfaces months later as intermittent closed-state failures; a pulse longer than about 200 ms adds coil heat without improving contact force.

Which Magnetic Latching Relay Specifications Predict Field Failures

Rated contact current is the number buyers compare first, but contact resistance drift, dielectric margin and pulse energy cause most warranty returns. Review these six items before approving a part number.

Take the single-phase wall-mounted meter case. A 120 A UC3-class part such as the HJE11F120A covers the top of that range, and the number that decides field reliability is not the headline current rating but the resistance band held across a full production batch.

HJE11F-120A Single-Phase Magnetic Latching Relay for High-Power ControlA 120 A UC3-class PCB relay for smart meters and high-power equipment, with magnetic latching, low coil power, and stable batch resistance.View Product →

Matching a Magnetic Latching Relay to the Meter, Pile, or Protector

Match the relay to the load profile, not to the current number alone. A meter may switch twice a day, while a charging pile PDU may carry 120 A for eight hours, and those two duty cycles stress the same component in completely different ways.

DIN rail meters add a mechanical constraint that wall-mounted meters do not have. The relay has to sit behind a 2P, 4P or 7P rail enclosure, which is why split-type constructions such as the HJE11B60A exist: the switching element and the shunt CT sit in separate mouldings so the assembly stacks into a standard rail footprint.

HJE11B-60A Split-Type Combined Latching Relay for DIN Rail MeteringA 60 A split/modular latching relay for DIN rail meters, separating switching element and shunt CT to fit standard 2P to 7P enclosures.View Product →

Charging Piles and Protection Devices: Where the Duty Cycle Changes

Charging piles and protection devices load a latching relay thermally rather than mechanically, and that flips the specification priority.

Inside an AC charging pile, the PDU relay carries 120 A at 250 VAC for hours at a stretch in a sealed enclosure where summer ambient can reach 60 C. Continuous-duty margin, terminal cross-section and copper area on the board matter more than switching life, because the relay may only operate a few times a day. The HJE11F120A250VAC is built for exactly that PDU position, with the 250 VAC rating stated in the part number rather than inferred from a family datasheet.

HJE11F-120A-250VAC Latching Relay for AC Charging Pile PDUA 120 A 250 VAC PCB relay for charging pile PDUs, offering continuous-duty margin and high-density power distribution in sealed enclosures.View Product →

DC piles push the problem in another direction. The HJE12A150A48VDC carries 150 A at 48 VDC, where arc extinction and terminal heating need explicit attention instead of a copy of an AC layout. Over and under-voltage protectors and three-phase smart capacitors then behave like meters with faster trigger logic. The same 60 A to 150 A, 250 VAC platform reappears across all three families, which is why component reuse between them is practical rather than optimistic.

Sourcing, Certification and Delivery Risk for Volume Programs

The relay is the highest-current component on most meter and pile assemblies, so supplier qualification is a program-level decision rather than a purchasing line item.

Huajin Technology Jiaxing Co., Ltd. builds its volume around magnetic latching relays and publishes ISO 9001-2015, CCC, State Grid and TUV credentials for the parts it supplies into metering programs. The company operates from a facility in Yucheng, Haiyan County, Jiaxing, with a municipal-level high-tech R&D centre and eight senior R&D engineers behind the customised variants.

Before releasing a purchase order, confirm five things.

• Certification documents issued for the exact part number, not for the product family
• Batch-level contact resistance and millivolt drop records
• A pulse-drive validation on your own driver board, not on a bench supply
• Thermal cycling results at the ambient your enclosure actually reaches
• Committed lead time against the annual capacity envelope

Selection support is worth using early. The DIN rail meter relay selection guide walks through split-type versus integrated formats, and the product matrix published by Huajin Technology Jiaxing Co., Ltd. maps part numbers to enclosure formats, which shortens the gap between a mechanical design freeze and a qualified relay.

Frequently Asked Questions

Does a magnetic latching relay consume power while it is latched?

No. Steady-state coil power is 0 W. Current flows only during the set or reset pulse, typically 30 to 100 ms, so the coil contributes no continuous heat load inside a sealed enclosure.

How long should the set or reset pulse be?

Most meter-class coils complete armature travel in 30 to 100 ms at rated coil voltage. Pulses shorter than about 20 ms risk an incomplete stroke, while pulses longer than about 200 ms add coil heat without increasing contact force.

Can a magnetic latching relay replace a standard relay in an existing design?

Only if the drive circuit changes. The relay needs a pulse driver, an H-bridge for a single-coil part or two low-side drivers for a dual-coil part, plus a reservoir capacitor. The contact side usually drops in unchanged when terminal geometry matches.

What current rating does a three-phase DIN rail meter need?

Match the meter basic current and the standard it must satisfy. Three-phase DIN rail latching relays are commonly specified between 80 A and 120 A at 250 VAC in split-type and integrated-with-shunt-CT formats, with the 25th edition State Grid variant at 120 A.