Like may others here, I've recently decided to incorporate a PID controller into my casting setup. I shopped around and found I could build one for around $50 (I'm an avionics technician by trade, and comfortable with electronics work, plus have been a computer geek for years and am blessed with an abundance of old components) and then wondered how I could save even more, being the cheapskate and tinkerer that I am. So, I found the perfect solution in one of my junk boxes in the shop:
An old computer power supply. This saves me the cost of a housing and components such as switches...One important point is to make sure any components you use are properly rated. For example, my Lee 10 lb. bottom-pour draws 500 watts. 500 watts divided by 115 VAC equals 4.35 amps...Bump it up to 5 for a little padding. So I knew that I'd be working with a max draw of around 5 amps. Driving onward...
Opening and gutting the power supply was simple, just a few screws and clipping some wires. The switch and power receptacle above were rated for 115VAC, 15 amps, so I kept them. That yellow thing is a capacitor, used for filtering/smoothing out the AC power. It can't hurt and might be helpful with the PID controller so I kept it in.
The chassis is already ventilated...a plus. I marked spaces for the PID controller and the outlet for the pot, and used a Dremel cut-off wheel and a hole saw. I used a block of wood as backing support before cutting the round hole, but a couple of the slots were still bent out of shape by the hole saw...Easily fixed with pliers, tapping between 2 wood blocks and a light touch with the cut-off wheel. A quick test-fit...
and some fine-tuning with the cuts, and things fit perfectly. I had to cut off the 'ears' on the electrical outlet so it would fit vertically inside the chassis, it still had holes in what was left of the ears so with the included screws it mounted to the case just fine.
I laid out the components inside...Controller, outlet, solid state relay, and just for kicks, a fan. I bought a 40-amp SSR because it was cheap, even if it's overkill. With only 5 amps through it, it shouldn't generate much heat. I didn't want to spend $6 on a heatsink, plus the metal chassis acts as a heatsink itself.
I'm using the MyPin TA4-SNR PID controller...some older models had incorrect wiring diagrams but this one was correct. That said, I had an idea of using the fan that came with the power supply chassis, and connecting that to the triggering output with the SSR so that when the relay was 'on', the fan would be too. That fan drew too much current for the TA4-SNR's liking, though, and it would malfunction or shut down altogether. So, I dug around in the junk box and found an old video card fan with a copper heatsink. It fit perfectly, drew very little current and the heatsink attached nicely to the chassis after drilling two small holes for the mounting studs. A little more thermal heatsink compound and it was in, plus it's close to the SSR so it may help with heat reduction, besides providing some air movement in and through the chassis. Anyway...proceeded with the rest of the wiring; for the incoming power receptacle, switches and the outlet for the pot I used 14-gauge solid copper wire. 14-gauge is more than adequate for 5 amps, 115 V AC, plus it's a very short wiring run. One foot of 3-conductor Romex was more than enough.
And there ya go. The TA4-SNR was $22 on Amazon, thermocouple/probe was $5, SSR was $5, and the outlet was $3 for a total cost of $35...Free shipping with my Prime membership. Everything else I had lying around the shop, and I repurposed the housing, switches and round grommet from the computer power supply. I have some rubber feet in a drawer somewhere which will attach to the bottom and provide some more airflow underneath
On a side note, the TA4-SNR isn't the most robust controller, it's very sensitive to things like the fan I used originally which drew too much current. But it's cheap to replace; this one works fine for me but if I had to do the project over again there are other controllers listed in threads here which may be more expensive but are sturdier. It has a built-in alarm indicator on the front panel and does its job well. I'd also go with heavier, maybe 12-gauge wire in case I upgraded to a heftier casting pot but the idea here was to see how cheaply I could do a build making use of what I already had. There's plenty of room in the case for an AC/DC rectifier, transformer, etc for those who want buzzers or indicator lights...or maybe power an old car stereo, USB port, whatever...so when the mood strikes I'll likely be doing some more hacking with this project.
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, but still managed to get it done for about 70 bucks. 
Originally Posted by Gunor
