Relays

A "relay" is an electrically operated switch.

What is a relay, and why should I care?
- Mechanical Relay
- Solid State Relay
Gallery of assorted relay pictures
Choosing a relay for a project
Where can I buy one?

What is a relay, and why should I care?

Mechanical Relay

A "solenoid" is a coil of wire that, when electricity is sent through it, becomes a magnet. The resulting magnet is attracted to iron objects. Being able to turn a magnet on and off allows you to exert a pull or push from a distance, and under electronic control.

The most common haunt application for a solenoid is to activate an air valve, controlling a pneumatic animated prop.

But the magnetic pull of a solenoid coil can also be used to open or close an electric switch. This configuration is called a "relay".

Relays are pretty handy:

A relay provides isolation from one electrical circuit to another.
A small coil voltage can control a large load. This is usually how computers control the outside world.
A high control voltage can control a low load.

[photo] These are the switching contacts of a simple relay.

When the electromagnet coil is turned off, a spring lifts the shiny diagonal bar up, which makes the "normally closed" contact close, as seen here.

When power is applied, the shiny bar is pulled down. This opens the "normally closed" contact and closes the "normally open" contact.

This relay is used in our line voltage switch and Gemmy fogger control projects. It features a 110 VAC coil and SPDT contacts rated at 16 Amps at 380 VAC. It sells for $.99 (October 2002) from Hosfelt Electronics.

This image shows how our real relay corresponds to the schematic diagram symbol for a relay.

In the picture on the left:

The coil is attached to the bottom two lugs.
The lug in the center of the picture is "common".
The topmost lug is "normally closed".
The one below it is "normally open".

Different relays may have different electrical connections than the one pictured.

Solid State Relay

Mechanical relays use electromagnetism to push and pull the contacts of a mechanical switch. Since pieces of metal are pushed around, mechanical relays often make an audiable "click", and the more power the relay can handle, the larger the contacts, and the bigger the noise they make when they open and close. CLUNK!

"Solid state relays" (SSRs) have no moving parts. Instead of using an electomagnet, it uses light to isolate and control a load.

[photo] This is a "hockey puck" style solid state relay.

In general, solid state relays:

take very little power to operate
switch on and off almost instantly
do not have mechanical contacts that get dirty or wear out
are silent
are capable of switching heavy loads
have SPST, No

There are many different solid state relays available, differing mstly in the power they are capable of switching.

Gallery of assorted relay pictures

This is a relay with a 24VAC coil.

You can attach wires directly to the lugs on the right, or put it in a socket.

[photo] This older relay is made to fit in a vacuum tube socket.

[photo] Here's a box full of assorted relays. They really do come in all shapes and sizes.

This page from the All Electronics catalog (#103, Winter 2003) shows some types of relays.

Choosing a relay for a project

When looking for a relay for use in a project, there are several things to consider:

coil voltage
This indicates how much voltage (and what kind, AC or DC) must be applied to activate the relay.
Make sure that the coil voltage matches the signal you feed into it! If you feed high voltage into a low-voltage relay, the relay will burn out.
contact ratings
This indicates how heavy a load the relay can control.
If you can't find exactly what you want, you can use a relay with a higher rating.
contact geometry
There are many kinds of switches, so there are many kinds of relays. The contact geometry indicates how many poles there are, and how they open and close. Some examples are: single-pole single-throw; single-pole double-throw; and double-pole double-throw.
If you can't find exactly what you need, you can use something with more poles, and simply ignore the unused contacts.

Here are some relays that you might find useful [as of September 2005]:

part number	description	contacts	coil	price
Radio Shack #275-241	12VDC/1A SPDT Micro Relay	SPDT 1A at 125VAC	12VDC, 37.5mA, 320ohm	$4.29
Radio Shack #270-206	12VDC/5A DPDT Plug-In Relay & Socket	DPDT 5A at 125VAC/250VAC/32VDC (resistive)	12VDC, 70mA, 160ohm	$9.39
Radio Shack #270-218	12VDC/10A DPDT Plug-in Relay	DPDT 10A at 125VDC	12VDC, 130mAh, 160ohm	$8.39
Radio Shack #275-249	125VDC/5A DPDT Mini Relay	DPDT 5A at 125VAC	12VDC, 60mA, 200ohm	$5.29
Radio Shack #275-248	125VDC/10A SPDT Mini Relay	SPDT 10A at 125VAC	12VDC, 30mA, 400ohm	$4.29
Radio Shack #275-217	125VAC/10A DPDT Plug-In Relay	DPDT 10A at 125VAC	125VAC, 15mA, 4.5ohm	$8.39

Where can I buy one?

Relays are electronic construction parts. Don't expect to find then at Sears or ordinary stereo shops.

Look in stores that cater to electronic experimenters, such as:

Thank you for visiting. Your comments are welcome.
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