Reality Check for a Temperature Controller

Hey Guys,

Very first post on these forums so be nice :cold_sweat:

I'm attempting to build a temperature controller using a DS18B20 and a power relay. The idea is to use this in a chest freezer to keep the temperature within a set temp range by controlling when the chest freezer is turned on/off. Exact same functionality as a love/ranco controller. The extra features that I want to add to this are beyond the scope of what I'm looking for in this post, so here are the questions:

  1. Beyond the probe and relay, is there something I am missing that will be required? (Module, shield, etc)
  2. I know that I need to make sure that the power relay can handle the load of large appliances. However, almost all relays I am finding on the arduino shops are similar to this one: http://arduino-direct.com/sunshop/index.php?l=product_detail&p=218 which only supports up to 10A. Are there any recommended relays for this particular application?

Just trying to plan this out and am definitely inexperienced, so I apologize in advance if I'm making too many assumptions or incorrect.

TIA

You will need a transistor or circuit to drive the relay since it requires more than 30ma. Keep looking for 20-30A try Ebay.

sbright33: You will need a transistor or circuit to drive the relay since it requires more than 30ma. Keep looking for 20-30A try Ebay.

Thanks!

I've found the following on ebay: http://www.ebay.com/itm/SSR-25-DA-Solid-State-Relay-25A-Output-24-380V-AC-F-PID-/280679548830#ht_4415wt_906 Would something like this work? It's 25 amps, I just don't know if it's the equivalent to what I'm looking for.

Also, where did you get the 30ma figure? How would one get an exact requirement? (Sorry, complete electronic newbie)

Looks great! Nice price too. At first glance it looks like that would work without any intermediate circuit, just hook pin 3 directly to the Arduino. The Arduino is rated at 40ma maximum on a pin.

sbright33: Looks great! Nice price too. At first glance it looks like that would work without any intermediate circuit, just hook pin 3 directly to the Arduino. The Arduino is rated at 40ma maximum on a pin.

Wow, thanks for the quick help. I'm assuming that pin 4 on the relay is for ground?

You know, I forgot to mention that I plan to have two of these relays to control two external devices: 1. Heat (Lamp, or heater) and 2. Cool (Chest freezer).

I was making the assumption that if one relay draws less than 40ma then I could still get away with using 2 relays since they aren't switching at the same time. Am I assuming incorrectly? Do I need to add a transistor?

Pin 4 to ground. You can switch about 40ma PER PIN. With a coil relay there is a spike to energize the coil. How man ma does the SSR take? Then you can answer your question.

sbright33: Pin 4 to ground. You can switch about 40ma PER PIN. With a coil relay there is a spike to energize the coil. How man ma does the SSR take? Then you can answer your question.

I just realized that I wasn't considering the fact that I need to power the board from the power input on the relay. Is this a huge endeavor? Or is this not that much of a big deal?

I don’t understand. Please explain. I’m from Missouri.

sbright33: I don't understand. Please explain. I'm from Missouri.

LOL. OK.

I'm just realizing that I have to power the controller as well as the appliance (Attached to the relay). Instead of using a battery, USB, or separate power source, I'd like to grab the power from the outlet that's on one side of the relay.

So, I would like the outlet to power both the appliance on the relay and the controller that handles everything. Obviously I will need to tap into the power before it gets to the relay and apply some sort of resistance. Just not sure what to do.

I hope I am being clear... XD

Just stop by a store and buy a 5V wall wart or, even better, a β€œUSB charger” that plugs into an outlet. Wire it in parallel with the AC leads to your relay.

Chagrin: Just stop by a store and buy a 5V wall wart or, even better, a "USB charger" that plugs into an outlet. Wire it in parallel with the AC leads to your relay.

Thanks . However, my preference would be to use only one slot from a wall socket.

haputanlas:

Chagrin: Just stop by a store and buy a 5V wall wart or, even better, a "USB charger" that plugs into an outlet. Wire it in parallel with the AC leads to your relay.

Thanks . However, my preference would be to use only one slot from a wall socket.

Adding a socket inline is pretty easy and cheap since the devices your using need to be plugged in anyway (I'm assuming your not going to just cut off there existing plugs) your already going to be putting some after the relays anyways. Baring that your going to have to make or re purpose a power supply I generally pull apart those usb chargers making your own is complex if you want a high efficiency unit. The classic power supply a transformer + bridge rectifier + caps + LDO quickly costs more than the wall wart. Besides I always seem to have an excess of wall warts hanging around.