Keith’s Hotplate and PID Controller Teardown Pics
May 18th, 2009 by Jeff
Keith of Keith’s Electronics Blog made a PID-Controlled Soldering Hotplate based on the one I fabricated earlier this year. He’s already using it to build the stepper controller PCB for the MakerBot CupCake CNC!
He also posted a bunch of teardown photos (like the one shown below) of the CD101 PID Controller from Sure Electronics. I suspect the CD101 is a cheap knockoff of an RKC PID controller since I can’t find the part number on RKC’s website, even though the front panel clearly says RKC on it. I guess at $40 you can’t ask too many questions, the price is right…
Copycat PID-Controlled Solder Hotplate « Keith’s Electronics Blog.
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Jeff, welcome back from your cross-country trip!
I hope I gave you appropriate and adequate attribution in my blog post. I was pleased to see that the Make: blog entry included my first paragraph that linked to your original project — so hopefully you got some hits off the publicity too.
Let me know if there’s anything you feel was attributed inappropriately or inadequately and I’ll do my best to make amends. And thanks again for the idea and all the input!
Keith – Thank you very much for taking the time to include proper attribution, most people don’t. As long as my name and a link to the original page is in there somewhere I am happy!
I think you are only the second person to make a hotplate based on mine. It’s great to see the other designs – in the end I’ll probably incorporate the improvements everyone else makes to make mine even better.
Thanks!
Jeff
I’m trying to figure out how to create a custom sized hot plate for my encaustic work. The wax needs to be kept below 200 degrees, would something like this set up for for me?
Thank you!
Shelly – I don’t see why not. Assuming you mean 200 degrees Fahrenheit, you could probably get by with a lower wattage heater unless your hot plate is huge. The PID controllers will accept any temperature, so you would just dial it in for 200F instead of 400.
I would like to know what sort of clearance hole was necessary for the heater in the hotplate. I saw the spec diameter of the heater was 1/2 inch. Will the thermal expansion of the heater itself cause any problems or does the plate expansion compensate for that of the heater?
What would be the consequences of increasing the plate size to 4″ X 6″ be. Will the heater have enough oomph to handle the increase in the thermal mass?
Any enlightenment will be appreciated.
Vic
Victor,
Very good question, because this is something that is pretty critical in making a hotplate that works well and doesn’t damage the cartridge heater.
The basic rule is that the hole for the heater should be drilled to the stated diameter of the heater. Ie. a 1/2″ heater requires a nominally 1/2″ hole. It turns out that heaters are manufactured slightly undersize to allow for this – a 1/2″ heater is actually 0.492-0.497″ in diameter. This allows a slip fit into the hole while still providing good thermal contact when the heater expands as it warms up.
In general this works, but in certain situations you may need to actually do the math to find the optimum hole size for a given heater. Omega.com has a good article explaining how hole size (fit) corresponds with allowable watt density of the heater for a given operating temperature. The graph here allows you to pick a hole size based on your other requirements, or vice versa. I think generally a 0.10-0.15″ hole ID – heater OD is recommended.
The choice of wattage depends on a lot of things, such as size of the hotplate, desired operating temperature, tradeoff of warmup time vs. overshoot, thermal losses due to standoffs, air convection, etc. I used 500W for my plate because it’s a common power rating for 110V 1/2″x4″ cartridge heaters. It was mostly a guess, perhaps also guided by the specs of some commercial hotplates I saw. I was prepared to buy a different heater if 500W wasn’t right – fortunately they are fairly cheap and easy to swap out!
If you don’t have very good control over the finished size of the hole or if you just want a more conservative design, I would recommend avoiding the highest wattage spec for a given size of heater. Ie. if a 1/2″ diameter 3″ long cartridge heater is available in 250, 500, and 750W, I would avoid 750W as the highest watt density requires the closest hole fit to avoid damaging the heater.
Hope this helps!
Jeff
PS: If you haven’t seen it, my heater is very similar to Keith’s shown above.
Jeff,
Thanks for taking the time to answer my questions. Just the info I needed.
Regards
Vic