Relay Terminology Guide

SPST Switch

SPST Switch: A Single-Pole-Single-Throw relay (also referred to as a 1P1T) has one input port and one output port. Typically, this relay is configured as a ‘Failsafe’ device and the RF path remains open when actuating voltage is not supplied.  

SPDT Switch

SPDT Switch: A single-pole-two-throw relay (also referred to as a 1P2T) has one input port that is common to two output ports.

DPDT Switch

DPDT Switch: A double-pole-double-throw relay (also referred to as a 2P2T) has two input ports that are common to two separate output ports. (Same as two SPDT switches). These can be configured to actuate together or independently. 

2P2T (5 Port)

2P2T (5 Port): This relay provides 5 ports that are in line. It differs from a DPDT by sharing one common output port with the two input ports. These relays are configured with ‘SMA’ connectors and can also be provided with internal terminations for the unused output ports. (Ports 1 and 5) 

Multi-Throw Switch

Multi-Throw Switch: A relay with one input port that is common to three or more output ports. Relcomm Technologies multi-throw relays are available with up to 12 output ports. (Also referred to as a 1PxT where ‘x’ is the number of output ports). All standard multi-throw ‘failsafe type’ relays are configured with normally open RF paths.

Normally open Switch

Normally open Switch:  A continuous actuating voltage must be supplied to maintain the selected RF path connection and will return to open when voltage is removed. This function can be made available in most ‘Failsafe’ relay types.

Normally open with failsafe to position 1

Normally open with failsafe to position 1: Position 1 RF path is always connected until actuating voltage is applied to select another RF path. Upon removal of voltage, the relay will return to NC position. This function is only available with multi-position ‘Failsafe’ relays.

Transfer Switch

Transfer Switch: A four port relay made up with four RF paths. This relay consists of two independent RF paths that are transferred simultaneously between output ports and is similar in action to a DPDT except the two output ports are commonly shared with each input port. 

Circuit Insertion Switch

Circuit Insertion Switch: Configured the same as a transfer relay with one of the internal RF switch paths removed. The relay is then used like a SPDT relay and allows for an active or passive component to be inserted externally between ports 2 and 4 which is then included through the switch path.

Termination Switches

Termination Switches: Un-selected RF ports are switched to an internally matched resistive load opposed to an otherwise open circuit. (See individual outline drawing data sheets for power rating and VSWR specifications).

Manual Switch

Manual Switch: The RF path connection of this type of switch is manually selected by a mechanically linked rotary knob or toggle handle.

Indicator Circuit

Indicator Circuit: A set of internal contacts that is set up to provide a circuit closure with the selected RF path. These can be used for external monitoring or for controlling auxiliary equipment. (See individual outline drawing data sheets for load ratings).

Suppression Diodes

Suppression Diodes: Used to suppress transient voltages that are generated by the relays internal drive coils collapsing electromagnetic field when actuating voltages are removed. This is known as ‘back EMF’. These voltage spikes can generate many times the input voltage causing system noise and possible damage to sensitive components. Relcomm relays that are equipped with logic interface and other types of circuitry incorporate suppression and polarity protection diodes to prevent circuit damage.

Failsafe: This configuration returns to a pre-selected RF path with no applied voltage. A continuous actuating voltage must be supplied to maintain other RF path connections. This function is used primarily with SPDT and transfer type relays.

Latching: After applying voltage to select the RF path, voltage can then be removed and the relay will remain in the selected RF path. This RF port will reset to open only after applying voltage to select a different RF path.

Pulse Latching: An external 50 – 250 millisecond pulse at the proper voltage level is recommended to select the RF path. This function is also available with TTL & BCD logic interface.

Latching with self cutoff: Once the selected RF path is completed, a timing circuit within the relay automatically turns off voltage to the drive coil. This function is also available with TTL & BCD logic interface.

TTL drive options: Allows the use of logic inputs to select the RF port. Typically, these are set up to require standard high level logic to activate. Application requires logic input to each relay position. (SP4T requires 4 logic lines). This option is available with most relay configurations Failsafe and Latching. All TTL configured relays come with drive coil suppression diodes. (See individual outline drawing data sheets for logic information and truth tables).

BCD Decoder: RF path closures are enabled by binary logic levels. This option is available for multi-position relays to minimize the number of logic input lines. (See individual outline drawing data sheets for logic information and truth tables).