A Technical Guide to Latching Relays

General

Latching relays are classified as mechanical and magnetic latching systems. It uses a magnetic H-armature system with the magnets inset into the high grade plastic of the H-armature. Rotation of the H-armature round its anchor point allows a short switching time and it is not sensitive to vibration of shock Compared to mono-stable relay, a latching relay only requires power to change the contact position, with the obvious advantages.

• No power consumption in the hold position.
• No additional heating caused by the driving coil.
• Maintenance of the contact  position in case  of power failure.
• Higher contact force and lower operating power.
• High resistance  to vibration and shock

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§  Figure 1: Driving a double coil polarized latching relay

§  How to control

§  The driving coils of the latching relays are equipped with three connectors (centre-tap-ped). This enables the user to choose between a single coil and a double coil configuration.

•   The position of the relay depends on the current through the driving coil. Figure 1 shows the circuit with a double coil using two transistors, one for each switching position

The centre tapped  Pin is connected to a DC supply, but also a capacitor discharge can be used. Without utilization of the centre tapped Pin the coil can be driven as a single coil relay through a Triac as shown in Figure 2.

The power for the double coil can be taken from a storage capacitor or a  battery, the single coil in Figure 2 can only be driven when the AC voltage from the mains is present. Therefore the single coil only requires half the power of a double coil.

The  latching relay does not require a large power supply, because of its low energy consumption. Even battery driven applications are possible, depending on how often the relay is switched on and off.

About Current sensoring

Using a latching relay in energy management  also requires a current Sensor. Today four major different current sensing technologies are available:

• Current Transformer
• Hall Sensor based
• Rogowski Coil
• Shunt resistor

As a first step Gruner added a Shunt resistor to the 704 and 720 relays. A strip of Manganin™ is integrated in one of the load terminals. Two connections are located close  to the resistor on the copper terminal. These must only carry the current Sensor signal. A third connection is reserved for the reference signal of the electronics to avoid RF-sensitivity.

Lamp load

Another application for latching relays is lamp load switching. Because of high inrush current the 707L and 704L relays were adjusted to meet this requirement. They switch lamp load up to 4800W of 200µF parallel compensation. A strong connection between the H-armature and contact spring allows the relay to break micro welded contacts caused by the high inrush currents. 

No semiconductor-based disconnection device is able to withstand the harsh operating conditions such as

• high dielectric strength
• high overload capacity
• high fault currents during switching without expensive protection.

    

Gruner Latching Relays from ITP

ITP India launched Gruner latching relays in the year 2000 to fulfil the needs of its valued customers in the Energy meter segment. For 50 years, the name Gruner has stood worldwide for innovative relay technology.

Superior design principles, centuries of experience in co-development, permanent investment in the latest production and QS technologies as well as high vertical integration - all of this has turned Gruner into a reliable and efficient partner for numerous leading companies all over the world

All Gruner latching relays meet the IEC 61036 and IEC 61037 requirements