relay q's

ok, now i think i want to start fiddiling with bigger electronics, like water pumps, and light bulbs, and such, and i need to get my facts right before i go on purchasing things.

relays confuse me becuase of their size.

ko, i want to control a 120 v water pump, and i need a relay. Do i buy a 5v relay because the arduino outputs 5v? or do i buy like a 120 v relay, because i am switching 120 v?

now radio shack sells relays and they go like this: many relays are small, but most of the smallest are 5v relays with very small prongs to connect to. the larger relays are 24v, and higher, and they have enormous prongs ,which i can guess you would connect the rather large 120 v wires to. Can the 5v relays handle 120v going through this tiny prongs?

thanks to anybody who answers/ explains this to me!

What you need is a relay where one side, can be controlled by 5V and the other side can handle switching 120V

see this for hookup info :

EDIT: you also must make shure that the relay you buy can handle the current the pump draws, not only the voltage.


a solid state relay is also very good, as it doesn't burn out over time- no moving parts. You can also connect it straight to the Arudino. Search the forum with Google for "solid state ralay" and you will find numerous posts on the detiails.


What you are looking for is a relay that is 120v AC and can handle ~5-20 amps on the business side and 5v DC at ~5-15 ma on the coil side.


let's start:

A basic relay is divided into two parts: 1) A magnetic coil (a rectangle with diagonal line on MikMo link drawing), with 2 terminals (1 and 16 on link drawing). This coil works with some voltage. To work with Arduino, you can use 5v. to connect it directly (if coil current comsumption doesn't exceed 40mA - I don't know if exist a relay with less than 40mA of comsumption), or any other voltage, using the example showed by MikMo on link. You can use a 9v. relay using 9v. source from Arduino Vin, for example, or any other voltage relay with external DC source. This is the first parameter of relay: [u]coil[/u] voltage and current comsumption. This says what do you need to make coil works.

2) A simple switch with 3 poles: - 1 central terminal (called common) - P1 on drawing; - 1 "normally oppened" terminal (called NO) - S1 on drawing; - 1 "normally closed" terminal (called NC) - O1 on drawing. These 3 poles can support some voltage and current. This is the second parameter: [u]Poles[/u] voltage and current limits. This says what you can turn on and off with the relay.

It works like the switch in your room, that turns on the light, but with a contact on the "off" position too. But this switch positions cannot be changed by hand: the coil changes it magnetically for you. So this switch have 2 positions: - "Common" short circuit with "NC" when relay coil is not energyzed; - "Common" short circuit with "NO" when relay coil is energyzed;

So you need a relay with low DC coil voltage (5, 9, 12, 24volts) and with medium AC switch voltage (120 volts in your case) and the switch current depends of your water pump current comsumption (peak value - when motor starts).

I hope this clarify your relay question. It's a very simple component. Don't be frightened! :)

if coil current comsumption doesn't exceed 40mA - I don't know if exist a relay with less than 40mA of comsumption

Almost all Solid State Relays (SSR) have really low coil current requirements. I believe 5-30ma is common.

Also a bit more info. If you want to have a bit more control on the lights consider getting a non-zero-crossing SSR. These things switch fast enough to do Pulse Width Modulation (PWM) on mains level voltage and current. Pretty cool! :D

Almost all Solid State Relays (SSR) have really low coil current requirements.

Solid State Relays don't have coils. The circuit that makes it work is called "control".

Big93, I'm posting relays and Solid State Relays links from wikipedia, that explain how relay works, types and differences.

lol soooooooooooo many responses, thank you all very much, i have all the info i need, and will ever need! :) if i have a q for you guys to see if a relay is good, ill post back

thnks again