Wednesday, June 15, 2011

Homemade PID Controlled Hotplate

     Hot on the heels of my last hack (no pun intended), is my latest project, a PID (proportional - integral - derivative) controlled hotplate. The inspiration for this project came from this post on MightyOhm.com. I choose to build it because I wanted to increase my knowledge on PID controllers and also wanted to get into soldering surface mount components. I got lucky and got all of the components for this project for under $20. Check out after the break for the build details.


     The PID controller is an Omega CN132 that came with a instruction manual. In the manual there was a circuit diagram for operating the controller with a mechanical or solid state relay. I tested the system with a mechanical relay and eventually bought a cheap 24-480V AC, 25A solid state relay for use in the system. I am using a type K (−200 °C to +1350 °C range) thermocouple as the sensor input.

Circuit diagram for controller use with solid state relay

     I used fiberglass heating strips as my load. I originally only had one 22" strip but it would just barely reach the right temperatures to melt 60/40 solder (190 °C). I got another heater strip, this time 44" long, and placed them in parallel as the load. They are sandwiched in between a 1/2" and a 1/4" aluminum plate. The plates are 4" x 5". The 1/2" plate on top has a hole drilled through in from the side into the center where the thermocouple resides. 5" bolts attach the top heating area to another 1/4" aluminum plate that holds a set of plastic feet. The bolts also act as a heatsink by keeping the heat only at the top plates.





     Since I had the electronics assembled, I knew what size I wanted the box for the controller to be. I drew up some quick specifications and got them cut out on thin aluminum plating. I began by marking out the holes for the controller and power switch and cutting those out. I had never used a spiral saw before, but I definitely wish I knew about it earlier. I've tried cutting holes in metal plating before and it typically ended up with small holes that needed to be filed for a long time to get them right. The spiral saw was quick and painless. Definitely a recommended tool for this kind of work. After all the holes were in place in the metal box's base, I went ahead and started bending the box up. The only problem I ran into is that the short walls got in the way while bending the front and back up, so I cut them at an angle and bent all the sides up just as I planned. After the bending was complete, I threw another set of plastic feet on the bottom and called it finished.



     Next up, I bent the top of the box into its final position and drilled some holes for the screw clips so the top and bottom of the controller box could be attached. After that was finished, I got the idea of applying the circuit diagram inside the controller box cover in case I ever have to open it up and fix anything. That way, I don't have to find the controller manual to figure out the wiring. After that, all I needed to do was wrap the cables connecting the plate to the controller and it was finished.




Finished product