RELAYS

Relays

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Overview

Relays are traditionally electro-mechanical devices used for switching electronic circuits. There now exists “sold-state relays”, which are purely electronic and contain no moving parts.

Uses:

  • Turing on mains voltage devices from 5-12V circuits, while providing isolation
  • Switching large currents

Relays with built in timers.

Terminology/Parameters

  • The Coil: The windings which turn the relay on when you apply a voltage
  • The Contacts: The output pins when get connected or disconnected when the relay turns on
  • Rated Coil Voltage: The recommended voltage that should be applied to the coil to turn the relay on.
  • Rated Coil Current: The current the relay coil will draw when the rated coil voltage is applied to it.
  • Contact Current Rating: The maximum current that the relay can conduct through the contacts

Solid State

Solid-state relays are relay's that don't have any moving mechanical parts involved in the switching. They use semi-conductor devices to perform the switching instead.

The normal way of doing this is to perform the switching with back-to-back N-channel MOSFETs whose gates are activated by a isolating opto-coupler (LED and receiver).

Below is the typical schematic symbol for a solid-state relay, along with a resistor connected to the input to limit the current through the internal LED.

This is the typical schematic symbol for a solid-state relay, along with a resistor connected to the input to limit the current through the internal LED.

This is the typical schematic symbol for a solid-state relay, along with a resistor connected to the input to limit the current through the internal LED.

Contact Arrangements

Normally Open vs. Normally Closed

Single-pole, single-throw (1A)

Double-pole, single-throw (1C)

The following diagram shows three of the most common contact arrangements for relays.

Three of the most common contact arrangements for relays. Image from 'http://relays.te.com/pnb.asp'.

Three of the most common contact arrangements for relays. Image from 'http://relays.te.com/pnb.asp'.

Inductive Kickback

Protect with diode in anti-parallel. This diode can keep the relay turned on for longer than expected, and this turn-off time is called the relay drop-out time. This slower off-time can decrease the life of the relay due to arcing.

Latching

With a little bit of clever (in design, not in the components themselves) external componentry, a mechanical relay can be made to latch-on, and will only reset once the load has been disconnected.

Common Relay Packages

Most PCB-mount relays have an asymmetric lead configuration so that it cannot be installed incorrectly.

Suppliers

The relay category page on DigiKey. TE's relay product page.


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