So ive had smoker envy (Bradley envy) for a while now, every time I smoke some fish in my homemade smoker I dream about being able to set it and forget it. However with my current set up I dont feel confident that the smoker will behave itself if I left it to its own devices. Currently I use an electric element at the base of a 44gallon drum with a stainless steel dishwasher perched on top. A lump of wood sits on this element which provides both the heat and the smoke (smoker described with more detail in http://www.fishing.net.nz/asp_forums/home-made-smoker_topic15875_page2.html) However occasionally the wood will catch fire or if the day is cold the smoker wont be hot enough etc etc which requires a bit of tinkering with the elements setting and wood etc to get "just right".
So I have decided its finally time to install a digital control for the temperature of my smoker and the smoke will be produced independently via a DIY smoke generator (not built yet but based on the awesomely helpful thread http://www.fishing.net.nz/asp_forums/diy-smoke-generator_topic62918.html)
Rather than going down the thermostat route I wanted something a bit more sophisticated for my control unit so began investigating PID controllers, which offer much improved control over temperature etc. I picked up most of this stuff from ebay pretty cheap. However I did purchase the incorrect PID controller which needed some minor "modifications" to suit my needs.
My PID controller is a REX-C100 unit (think this is a Chinese knock off job):
This one has a relay output which is not
what I needed - I needed one with a voltage pulse output to drive an external solid state relay (SSR). Ah well - it was a minor effort to "fix" this - as I will describe later on for those that are interested.
The solid state relay (SSR) was likewise purchased from ebay - these are much better than mechanical relays as they have much longer life times.
The general idea is that the PID controller will sample the temperature and control the SSR which in turn is connected to the heating element in my smoker. Pretty straight forward. But why stop there? - Instead of integrating the whole thing into my smoker permanently I decided to build a stand alone PID controller that could be detachable and able to control a variety of processes (like keeping my bedroom at a nice 24 degrees by attaching it to my heater during winter for example)
So I purchased two panel mount mains sockets from Jaycar (the input one is fused) and some banana plugs for interchanging temperature probes and set about installing them into an aluminum project box (also available from Jaycar)
The input mains socket provides 240V to the PID input and also attaches to the output socket via the SSR (which switches live on or off) Only the Live wire is switched by the SSR - the earth and neutral wires connect directly to the output socket.
The Temperature probe is attached to the front of the unit via banana plug sockets which allow quick swapping of probes. The PID delivers a voltage pulse of +12V DC to the SSR control terminals which activates/deactivates the Live connection to the output socket.
The whole thing is earthed and fused so if something does come un-stuck inside this aluminum housing it will ground out and blow the input fuse.
As stated above, the PID controller I brought was the wrong type and needed some minor modifications - Basically it has (had) an internal mechanical relay that would switch 240V AC PID connections were such that it was expecting the Live terminal for the output socket to be connected directly to the PID controller. I simply took the internal relay out and jumpered the drive voltage of this mechanical relay to the output connections on the PID controller. As shown below:
Find output Relay - de-solder and attach jumpers from the Relay control to the Relay Output - now you can run a SSR - yay.
I will update once my element is installed in the smoker - Im going to remove the 44 gallon drum and just use the dishwasher body as that will be less internal volume to heat.