Understanding Reed Relay Terminology


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The relay industry has evolved with a set of its own nomenclature that describes the products available, however, not all of these terms are familiar to users. The following blog post seeks to describe and explain these relay terms.

Form A

Form A describes a relay whereby the contact is a simple switch, which is open or closed, and the un-energised position is the open condition. For a single relay this would also be described as a single pole, single throw (SPST) relay with a normally open (NO) contact.

If the relay has multiple contacts in the same package it would be described as having (for example) 2 Form A contacts (DPST).

Pickering has a range of  Form A Reed Relays, for example our Series 109 that are an ideal choice for high density applications, or our high voltage Series 104 for up to 3kV.

Form B

Form B describes a relay whereby the contact is a simple switch, which is open or closed, and the un-energised position is closed.

If you are interested in Form B relays Pickering have a range of catalogue parts, for example our Series 107 Mini-SIL reed relays that feature the highest grade switches, and are encapsulated in patented mu-metal can.

More information on Pickering Series 107.

Form C (Changeover - break before make)

Form C describes a relay with two contact positions, the normally closed contact and the contact which becomes closed when the relay is energised. For a single relay this would also be known as a changeover switch, or a single pole double throw (SPDT). If the relay has two contacts sets it would be described as 2 Form C contacts, or double pole double throw (DPDT).
For Form C relays take a look at our SIL Series 106, these relays have superb contact resistance stability and ultra high insultation resistance, making them an ideal choice for Automatic Test Equipment (ATE).
More information on Pickering Series 106.

Form D (Changeover - make before break)

Form D is a changeover relay that is designed to make contact with the second contact before releasing from the first contact. These relays are very rarely constructed from reed switches because of implementations issues

Latching Relay

Latching relays have two or more stable positions for the contacts when power is not applied. To change the state of a relay a coil has a voltage transiently applied to it with a defined duration. Latching relays can be used for applications where minimisation of control power (coil current) is critical or where a power failure requires the switch to be left in the state it was set to until power is restored. The latter case needs careful design to avoid transient change instructions as the power fails. The latching mechanism usually relies on a magnet to provide the latching function.

Latching relays are generally not liked in modern software controlled systems because the software may not have direct knowledge of the relay state, particularly at power on. Some latching relays can have extra contacts to provide a direct indication of the contact position. This type of relay is not commonly available in reed relay form.

Safety Relay

Also known as a force guided contact relay this type of relay is designed with two or more
contact sets (poles) and the mechanical design is such that if one contact one pole fails to change position because of a weld the other contact on another pole cannot close the corresponding contact. The mechanical design usually relies on forces being applied to close to the contacts. There are no commercial solutions for a safety relay using reed switches.

For more indepth information about the wonders that are reed relays please see our Reed RelayMate page.