Hey, everyone. It's been a while since I've been on this forum and since I've been able to do some projects with my arduino, but I finally have a good reason to bring it out again.
I need to control the temperature of a mini fridge to be in the 60-70F range (started homebrewing beer). Right now, the lowest setting on the built in temperature control of the mini fridge is keeping it at about 44F.
I would like to set up a temperature controller with my arduino using a thermocouple or similar, and a relay to control when the refrigerator gets power.
The mini fridge I bought is an Igloo model fr320 and according to the manual, the specs say that it is rated at 115V~/60Hz/1.3A. I don't really know what kind of relay to buy. I did some research and I found out that someone suggested this one to another guy http://www.allelectronics.com/make-a-store/item/RLY-499/5-VDC-SPDT-6A-RELAY/1.html and it was rated to handle 120V through the contacts and it should be able to be controlled by the arduino.
My main question is: Can anyone help me learn how to buy relays for my needs so that, if they stock them in a nearby electronics store, I will be able to pick one out?
I'm sorry if this seems too basic. I did try to do some research but came up short.
You need a relay whose contacts are rated for 120v AC.
I'm using a Goodsky UDH-SH-112D to control my 230v fridge - it has a 12v coil because I have 12v DC readily available. I can't remember where I bought it but I THINK it was listed as suitable for domestic applicances - and wasn't expensive.
It is unlikely that a relay can be switched directly by the 20mA available from the Arduino pins so you will need to control a transistor with the Arduino and use the transistor to switch the current in the relay coil. For this reason it was just as easy for me to use a relay with a 12v coil.
I think I've bitten off more than I can chew with this project.
I went to Radio Shack to see if they had a relay that I could use, but only found an Arduino kit that had a little relay in it. It said 5v, 3A/120VAC. Will that work? The kit also came with a temperature sensor and a 9v battery clip which would be useful.
Another option that I found: RadioShack.com Official Site - America's Technology Store Would that be useful? I wasn't able to tell whether I could use it. I would like to find something that I can go out and buy today so that I can get started now.
I guess I would also have to buy an extension cord and strip one side to hook up the connections right?
Either of those things would do the job - if your fridge is rated 115V, 1.3A then a 120V 3A relay will control it perfectly well. You only need one relay, so the multiple versions are inappropriate.
Our concern in advising you is safety and in particular, the need to methodically isolate the mains power connections from the rest of the low voltage control system which you are developing. Unless you are adequately experienced in the construction of such equipment which as is so often said here, "if you are asking these questions, you clearly are not", the only advice we can give is to use a "powertail" (Adafruit) which is fully constructed to provide the necessary isolation in a totally safe fashion.
you have to understand that as far as the Arduino is concrened, the relay is nothing but a coil.
a simple PN2222 can deliver 5v power to the relay. there are dozens if not hundreds of photos, schematics and drawings to show the connection.
one only has to understand that the coil is the end of the universe as far as the arduino is concerned.
what that coil does is to isolate the arduino from the other side.
the other side is the contacts. these could be 0,1ma, AC or DC, 5giggawatts.... pretty much anything, it just does not have any connection to the coil.
the contacts side of the relay is nothing more than a switch. or multiple switches.
get the temperature controller because you find it easier, but get some led's and some relays, set up your circuits and light some lights, turn the hair drier on and off. you would be opening up the world for this easy system of control.
Most mains fridges use AC synchronous motors which have huge inrush currents - perhaps up to 1kW for a very short period. And a longer period (until the overload protection cuts in) if the motor stalls at startup which occasionally happens. I suggest using a relay with 10A contacts on a 230v system. I presume that implies 20A contacts on a 120v system.
If one is interfacing with existing hardware, it will be MUCH better to use the existing switching. #1) it was engineered by the manufacturer #2) it operates existing equipment #3) less work is needed to control a signal than to control main power.
I am not a fan of just turning the power on and off. most equipment requires power for controls and other processes.
find out what controls the existing motor and find out what signal that takes. just control that device and your job becomes much easier.
As for any compressor, you much incorporate a duty cycle that is long enough for the internal gasses to reach equilibrium. you should never attempt to start a unit shortly after it has stopped.
