Modern automobiles utilize a mixture of relay types, including traditional, latching, and solid-state relays. Each type has unique characteristics, and it is important to have a basic understanding of the differences for successful and efficient relay and relay circuit analysis.

In this article we will focus on latching relays.

OVERVIEW

Latching relays are similar to traditional relays in that both utilize coil(s) and mechanical switches. Unlike traditional relays which need a constant current to remain activated, latching relays only use a single pulse to latch or unlatch. This strategy saves energy and eliminates the heat resulting from the constant current needed to keep the relay activated.

• A latching relay is referred to as a bistable relay.
• Traditional relays are referred to as monostable or single-sided stable.
• Solid-state relays, as the name implies, only use solid-state electronics and have high-speed switching capabilities.
• Latching relays are available in one and two coil configurations.
• Single coil latching relays are typically controlled by reversing polarity to the coil.
• Two coil latching relays use one coil to latch and one to unlatch.
• A unique characteristic of latching relays is that they will maintain the last position even when not powered - similar to a vehicle door lock system.
• Latching relays, before initial activation, are naturally in the reset state.
• Rough handling can result in the relay changing from one state to the other.

GENERAL IDENTIFICATION

Most latching relays use the same body style and ISO terminal arrangement as traditional relays, including ISO Mini, Micro, and 280. But there can be important differences in the assignment of terminals. To ensure safe circuit testing, it is imperative to understand the differences.

To begin, it is important to identify when a relay is a latching or traditional type. Some steps for identification of latching include:

1.  Read the circuit operational description found in your diagnostic information. Does it mention a latching relay?
2. Examine any markings on the actual relay. Most relays have an internal schematic printed on the side of the relay.
3. Always check the circuit wiring diagram. For example, if the circuit diagram shows a relay symbol with two coils, it is latching.

The following diagrams are examples to illustrate latching relays. Any given symbol may have different mixtures of the variations shown here.

• A coil maybe represented by a standard coil symbol or a simple box. 
• A solid box typically represents the coil that is used for the reset action.  
• A box with a diagonal line through it represents a coil with a single winding and is not specific to a latching relay. The diagram on the above uses two boxes with opposing diagonal lines to indicate two separate coils.    
• R stands for reset and S for set. 
• The coils of a dual coil relay may share a power, as in the diagram on the left and switch each coil to ground to latch and unlatch. Or the coils can share a ground and be switched to power to latch and unlatch.
• The last two symbols show single coil relays which rely on reverse polarity for latch and unlatch.

CONSIDERATIONS AND CONNECTION

As stated earlier, more often than not, latching relays utilize the same body style as non-latching relays. The main concern is the terminal assignments/designations.

Single Coil Latching Relay

Single coil latching relays, frequently have the same pin-out arrangement as traditional relays. What is different is the control circuit for the coil. The latching relay's single coil is controlled by a circuit that alternates the polarity of inputs to latch and unlatch.

• If it is a four pin latching relay, do all of the terminal assignment/designations of the diagram on the relay housing and the vehicle circuit diagram match each other along with those listed in the chart below? If so, you can use a standard four pin uActivate cable that matches the ISO body style and terminals of the relay.
• The uActivate® 85/86 coil control LED will alternate between red and green indicating polarity of the coil control pulses.
  

Double Coil Latching Relay

In the left-most relay diagram above is a five-pin relay and the terminal assignment/designations use the DIN 7265 single digit standard.

As we know, a traditional five-pin relay has dual contacts 3 (30) switching between 5 (87) and 4 (87a). In this dual coil latching relay diagram it only has one contact and the terminal for the traditional normally closed contact 4 (87a) is now is assigned as 6. In this case, 6 is supplying power to both coils and in other cases maybe supplying ground to both coils.

(DIN 7265 standard defines 6 as 'open to the discretion of the relay manufacturer'. )

• Do not use a standard 5-pin uActivate cable with a dual coil latching relay! If pin 6 supplies ground to both coils and the uActivate is switched to the 87a position a direct short between the power of pin 3(30) and the pin 6 ground will result. 
• As long as the terminal assignments/designations meet the standards in the ISO and DIN chart, a standard 4-pin cable that matches the relay body style can be used. A 4-pin cable will ignore terminal 6.  
• The uActivate 85/86 coil control LED will not respond to system latch and unlatch commands. 

• The most effective connection is accomplished with the uActivate Universal Cable (uCable). 
• Connect uCable red wire marked 30 to relay socket terminal 3 and green wire marked 87 to relay socket terminal 5.
• Connect uCable yellow wire marked 85 to relay socket terminal 6 and orange wire marked 86 to pin 1 or 2 - both set and reset command pulses can be viewed but only one at a time.

Refer to your User Guide for more information.