DIN Rail Mounted Electrical Relays

DIN Rail Mounted Relays

Function of Relays

Relays use an input voltage (on one circuit) to open or close one or more contacts (on a separate circuit). This allows the input circuit to operate at a different voltage than the output circuit. Or, it may be used to isolate the two circuits to prevent electromagnetic surges to be transmitted from one side to the other. This second application is called “signal isolation”.

Types of Contacts

The secondary (or switched) circuits are controlled by contacts. These contacts are usually a pair of gold-plated or silver-plated copper discs that connect and disconnect to allow the secondary circuit to be interrupted or completed. These are referred to as “mechanical” contacts. Contacts of this type may exhibit a property called “contact bounce” where the contacts may close, then open briefly, and then close again. Because mechanical relays must physically move the contacts from open to closed or closed to open, there is a time delay between the input request and the output response. The larger the contact size is the large the delay will generally be. An alternative to these types of contacts are semiconductor contacts.

Relay Construction Styles

Several options are available for relay construction styles:

• A miniature relay in a completely enclosed housing that looks like a terminal block
• A miniature relay that can be plugged into a housing that looks like a terminal block
• A completely enclosed relay with mounting feet for DIN rail mounting
• A relay that is permanently mounted on a printed circuit board. This PCB has mounting feet for DIN rail mounting
• A relay that can be plugged into a printed circuit board. This PCB has mounting feet for DIN rail mounting.

Pluggable versions offer the flexibility of changing the electromechanical nature of the relay after the equipment has been installed. This also provides easier maintenance in the event of relay failure.

Contact Designations

There is a universally accepted set of abbreviations for the number and function of relay output contacts. These are:

• (S) = Single
• (D) = Double
• (T) = Throw
• (P) = Pole
• (NO) = Normally Open
• (NC) = Normally Closed

If powering the input circuit will cause the output contacts to close, the output contacts will be referred to as “NO or Normally Open”. This is the state of the contacts when there is no power applied to the input circuit. Normally Closed contacts are just the opposite.

If a relay has one pair of output contacts, they will be “Single Throw”. Is possible, however, to connect a single output circuit to one contact by default and change it to another contact when the input is powered. This is known as “Double Throw” or a change-over circuit.

There is virtually no limit on the number of output contacts that a relay may have. But, using the above abbreviations, you can easily determine the number and types of output contacts that have been specified for a relay. For example, the designation, SPDT, can be read as “Single-Pole, Double Throw”. This means that there is one contact (the single pole) and it is a change-over contact (the double throw).

Contact Rating

The output contacts will have a voltage and current rating. Very high voltages will cause the electricity to arc across the contacts and burn (or oxidize) them. Contacts rated for very high voltages will generally have gold-plated contacts, the contact size will be much larger, and the contact housing may even be maintained in a vacuum to reduce oxidation.

The output contact current rating is related to the size of the conductors that used in the construction of the relay. The designer should choose a relay with output contacts that have sufficient voltage and current rating for the application.