# How do relays and contactors work?



## HMF (Mar 4, 2011)

Many machines, especially old ones, required the use of relays or contactors in order to make their electrical systems to function.

For example, I wanted to use the box switch that originally came with my lathe. However, because the switch buttons do not hold their position, relays were necessary, I was told. Someone was kind enough to do a drawing for me of the relays in operation, and I bought some ice-cube relays from MacMaster-Carr.

Being an idiot when it comes to all things electrical/electronic, I did not even know what a relay or contactor is.

This YouTube video explains both relays and contactors very well, so that even *I* understand their functions:

[video=youtube;RjYyAPcr_7k]http://www.youtube.com/watch?v=RjYyAPcr_7k[/video]


Best,

Nelson


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## pdentrem (Mar 4, 2011)

I agree with Turbinedoctor. 
There are volumes written about electrical controls. I am currently building a camera that requires a shutter. The shutter will be controlled via a relay, soleniod and 2 microswitches. You apply power to the relay and the relay closes the soleniod moves the shutter to the open position and the 1st mircoswitch stops the power. (Soleniods do not like to be powered continuously unless they are build for that and are harder to find in the real world.) Remove the power from the relay and the soleniod reverses and the second mircoswitch does the same as the first. At the same time the camera has to use the same signal and turn on and take the picture between the open and close states of the shutter. AND you have to intergrate with a laptop to control all of this and download the picture as there is no memory in the camera. Tethered camera! I am at the intergrating stage at the moment. Still machining the chassis for the camera and shutter parts.


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## pdentrem (Mar 4, 2011)

turbinedoctor link=topic=1211.msg5958#msg5958 date=1299296149 said:
			
		

> I understand the controls and the machining but as to the camera, I prefer the firearm method, point and shoot.
> 
> Got any pictures of your project you care to share?
> 
> Turbo




Only a couple at the moment. The third one is the face of the chip. I do the P&amp;S as well. Long and short barrelled stuff.

I was turning down the body after supper but had to come in from the cold, I am still reducing the mass of the alum to shed weight. The camera is an 11mp full frame 35mm chip. It is based on a Phase One H10 digital back.


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## ChuckB (Mar 5, 2011)

allthumbz link=topic=1211.msg5915#msg5915 date=1299260540 said:
			
		

> For example, I wanted to use the box switch that originally came with my lathe. However, because the switch buttons do not hold their position, relays were necessary, I was told. Someone was kind enough to do a drawing for me of the relays in operation, and I bought some ice-cube relays from MacMaster-Carr.



What you are describing is a self latching circuit. They are used in the automatic door trade from time to time. I am no artist, but I drew this up after reading your post:




When the "ON" button is pressed, it energizes the relay coil which allows the latching contacts to "take over" for the "ON" button, once it's released. The other set of contacts also changes state to be used for the device you are powering. Once the "OFF" button is pressed, it drops power to the relay coil, unlatching the relay and turning the device you are controlling , off. This is considered a "fail safe" circuit, meaning that in the event of a power outage, the device you are controlling will be de-energized.


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