It always bugs me a little bit when people refer to their temperature controllers for ferm chambers as "PID" since you aren't using the PID, you're using a basic on/off control. But that's just a little OCD coming out
Originally Posted by Uziyahu
First, does anyone know of a cheap DC powered heat source that I could wire to the SSR to control the heating function. The SSR runs DC up to 24v and 20mA. I know it's designed to control a relay, but I'd like to avoid buying one.
20mA at 24V is less than half a watt, there's no way you can get enough heating power with that. I'd buy an SSR and have it switch a few outlets so you can plug in, say, a light bulb and a small fan simultaneously. I have a similar box that I use to keep fermentors/bottle warm, and even with no proper insulation I can keep a fermentor at 75-80 degrees (ambient temp ~55) using a regular 60W light bulb that's cycled off a good portion of the time.
I'm an engineer and part of my job is setting up test stands in an electrochemical research lab, I haven't read through the manual for your controller but I've worked with lots of the lab grade Omegas.
In general I would avoid using the alarm outputs to control. Especially with a refrigerator, it will cause too-frequent cycling that'll kill the compressor. The alarm outputs I've used are either latched or unlatched (user selectable), if it's latched you have to reset manually and if it's unlatched the alarm shuts down as soon as the PV is within the "safe" range. You can't set a deadband on the alarm, so say you set the high limit to 70 degrees, at 70.0 the compressor will kick on, turn off at 69.9, turn back on a second later at 70.0, etc. The compressor won't last long doing that.
I'm not sure if your controller has two outputs and if they're both SSR outputs, if they are you can use two relays and run output 1 as cooling, output 2 as heating, etc. and use it as a dual stage on/off output. That way you can set proper deadbands on the on/off and give the two outputs slightly different setpoints so they aren't fighting each other.
Let me know if you need more help! I can check out the manual if you need me to. Either way this looks like a nice setup
I need to buy some insulation and make mine a proper heating box, I've been OK so far using a swamp cooler for cooling.