COMPONENTS
Relays
| Date Published: | |
| Last Modified: |
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.
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'.
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.
Like this page? Upvote with shurikens!