What is a Changeover Relay Switch?

Monday, 3 November 2014  | 

  This post covers the changeover switch relays covered in the range of auto electrical products at Arc Components Limited. The relays supplied in-store are configured to DIN 72552 terminal standardisation and examples can be found in the switch relays section.

  What is a changeover relay?
 The most common changeover function relays are produced to switch a single feed line (usually positive - position 30) between two active functions in a circuit, the relay is switched manually via a toggle/push switch or remotely through the circuit. A simple 'single line changeover switch' is achieved by switching a single line input from terminal position 30, at rest in the normally closed terminal position 87a (indicated by the 'thick set' black lines on the diagrams below), to the 87 position. When the coil is energised the line switches from terminal 87a to terminal 87, remaining in the second function until the power to the energised coil (via 85 and 86, which can be swapped in some applications) is switched off, returning the circuit to the normally closed (NC - 87a) position. A typical explanation of this function is to associate it with a headlight dipped beam, when momentary switching to full beam and then back to the dipped position (with both output positions 'by design' being active). In practice it would be correct to say that changeover relays have been used (sometimes by vehicle manufacturers) to achieve a normally open or normally closed make and break action, by leaving a line out position undesignated.

  There are many configurations of a changeover relay used for automotive electrical circuit switching, each allowing a variation on the switch operation, versions include normally closed (NC or active circuit), latching and coil suppression. One of these, the latching relay allow a mechanical (or in some cases an electronic) function that retains the switched position even when power to the coil is cut, useful for conserving power, the line switch is reactivated (usually with a momentary push switch) only when the coil is energised again. Typical builds are:

  • Standard line changeover relay: as described, switching input 30 between 87a (NC) and 87.
  • Changeover relays with diode: used for suppressing coil voltage spikes, 85 and 86 terminals become polarity sensitive; 86 must remain a positive designation if the diode tracks according to the diagram example below. 
  • Changeover relays with resistor: used for a more reliable suppressing of coil voltage spikes, 85 and 86 can be swapped for coil energising in some instances as the terminals do not become polarity sensitive.
  • Mechanical latching relay: allowing switching circuits and the position remains 'made' until the coil is energised again.

Relay Coil - Positive and Negative Coil Energising by DIN 72552 Definition.

Terminal or ContactIdentification
85Relay coil negative.
86Relay coil positive.


Relay Contacts - Line Feed or Switching by DIN 72552 Definition.

Terminal or ContactIdentification
87Common contact.
87aNormally closed contact.
87bNormally open contact.
88Common contact 2.
88aNormally closed contact 2.
88bNormally open contact 2.
30Feed or line in, positive.


Examples of Changeover Relays:

Standard Changeover ( 87a, NC)Changeover Relay with DiodeChangeover Relay with Resistor
Standard Changeover Switch RelayChangeover Switch Relay with DiodeChangeover Switch Relay with Resistor
Mechanical Latching Relay  
Mechanical Latching Relay  

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