Relays greater than 10A at 240V

Hi

Can anyone help? I'm looking for a relay module that I can connect to my Arduino. I see there are plenty of these things about but I need one that can operate a fridge. It is quite an old fridge and has no voltage/current rating label so Id like to play it safe and use a relay capable of 15A 240V (UK supply). Does anyone know where I can get such a relay in the UK. I'm really hoping to find one already PCB mounted with screw fixings for the 240V side and male headers for the DC side a bit like the relay at http://tinyurl.com/c3abwcf but 15A rather than 10A. There seem to be plenty of examples rated for 10A but I have not seen one rated for up to 15A.

Ultimately this is for managing a temperature and humidity controlled fridge converted for making Salami.

Thanks for your help

You have checked Farnell, right? (Sorry, I'm in the states and that is the only UK electronic component supplier that I can think of off the top of my head.)

But to your issue. Since you are hacking the fridge anyway, it might make more sense to hack into the thermostat control and put your relay in there. Won't need such a beefy relay, or you just control a relay that already exists (one less thing to buy).

When writing your code, remember that it isn't a good idea to turn the refrigeration compressor on too soon after turning it off. I don't remember the specifics, but Google should help you understand why and figure out a good delay.

http://www.maplin.co.uk/15a-spdt-miniature-relay-37519#specification

Search Ebay for SSR-25DA or SSR-40DA, they work well with Arduino.

But it is indeed bad for a fridge to switch the mains as Sembazuru wrote. It is even worse for an old fridge.

Perhaps you can connect a servo to the control knob. If it doesn't have a control knob, you have bad luck.

Sembazuru:
You have checked Farnell, right? (Sorry, I'm in the states and that is the only UK electronic component supplier that I can think of off the top of my head.)

But to your issue. Since you are hacking the fridge anyway, it might make more sense to hack into the thermostat control and put your relay in there. Won't need such a beefy relay, or you just control a relay that already exists (one less thing to buy).

When writing your code, remember that it isn't a good idea to turn the refrigeration compressor on too soon after turning it off. I don't remember the specifics, but Google should help you understand why and figure out a good delay.

Thanks for the thoughts on this. I will see if I can hack the thermostate somehow and attach my relay there. Thanks also for the pointer on not turning on and off too often, I had not realised that could be a problem. It can be anywhere in about a 5 degree range so I can allow it to warm and cool a bit so hopefully the compressor will not work too often on a cycle. Thanks again great ideas.

Erdin:
Search Ebay for SSR-25DA or SSR-40DA, they work well with Arduino.

But it is indeed bad for a fridge to switch the mains as Sembazuru wrote. It is even worse for an old fridge.

Perhaps you can connect a servo to the control knob. If it doesn't have a control knob, you have bad luck.

Thanks for this. Am I correct in thinking that for the relays you show I would need a diode in reverse across the relay and a transistor to protect the Arduino. If so can you recommend the correct parts to go with them? Thanks very much for your help.

A normal mechanical relay with a coil requires a flyback diode and a transistor with a resistor.
See the ABC guide: http://www.pighixxx.com/abc-arduino-basic-connections/
Card 3, upper-left.

But the SSR-25DA and SSR-40DA are Solid State Relays (SSR). They can be connected to the Arduino without any other parts.

Okay, you are right.
I was confused with the situation an old fridge was not powered for a long time or has been transportated.

Telecommando:

Erdin:
But it is indeed bad for a fridge to switch the mains as Sembazuru wrote. It is even worse for an old fridge.

Really? Where did you hear that? Because in most older (and many newer) refrigerators, the thermostat interrupts the mains, at least as far as the compressor is concerned.

Domestic Refrigerator Wiring | Hermawan's Blog (Refrigeration and Air Conditioning Systems)
Whirlpool ED22CQXHW Refrigerator Wiring Diagram | Fixitnow.com Samurai Appliance Repair Man
http://www.refrigeratorschematics.info/20111031_whirlpool-refrigerator-combi-integrable-circuit-and-wiring-diagram.html

I know this thread is a couple months old but I'm new to the forum so I don't know yet what quite makes an old thread old yet. But since I do have a bit of experience in this matter I thought I could give a little bit of advice.

Telecommando is correct, most refrigerator control circuits cycle the compressor by switching the mains directly with the tstat. Sembaruzu is also correct, turning on the compressor on and off to quickly is bad for the compressor, probably even worse if your dealing with well used equipment. Putting a time delay into the programming will only make for larger temperature fluctuations, what you really want do is add thermal mass to the refrigerator, the easiest way to do this is just to keep the refrigerator as full as possible. If you are not going to keep it filled with salami, fill the rest of it up with bottles of water.

You will also want to keep your temp probe submerged in a small bottle of water, this will also keep the compressor from short cycling, and better control the temperature of the product in the fridge rather than the air temperature in the fridge.

As far as your relay is concerned, unless you are working with a huge walk in refrigerator, your 10amp relay will be plenty, a typical refrigerator on U.S. 110v current will only pull a few amps when running, it might get over 10amps on start up, but at 240v you'll have plenty of capacity to spare. If your worried find someone that you can borrow an ammeter from or add a 10amp fuse to the circuit, which probably isn't a bad idea anyway. Also if you can't find any name plate data on the fridge you might be able to find some information on the compressor itself.

Hope this helps, and hopefully I didn't spend ten minutes typing out a post on dead thread.

15 amps!

i hope it only requires 15amps briefly to get the compressor going and not to power it :o

if it was purely to get the motor started a 10amp relay might be enough, get a power meter and check out how much power this really needs...