Project: SMT Hotplate

Thread Starter

Wendy

Joined Mar 24, 2008
22,436
3 Testa.jpg

I am not going to call this a completed project until I receive and install a new thermocouple (TC). The old one is a Teflon insulated unit. Teflon emits toxic gasses around 145°C so once I saw it had hit 200°C I shut it down until I have the right TC for it. Truth is, I was a little surprised it worked. But as a proof of concept it did its job. Most of the hard work is finished. Hope my other TC is OK. Walmart is cheaper than Amazon, but a 3 week lead time is an enthusiasm damper.

First step is to get the sheet metal. I got a .01” x 12” x 12” sheet metal and used the following pattern to cut 6 metal spacers shown in the .png(see attached) file (print this pattern using M$ Paint and the individual sheet is to scale). I have a centigrade thermometer but the Fahrenheit unit was more convenient to use. So after cutting and drilling the spacers to spec I started assembly. You will also need a separate unnotched top plate of a bit thicker material that won’t bend (I had extra steel from a previous project I used).
0 Stacking hardware a.jpg




I bought 4 silicon 12V 25W heaters (calculates at 5.76Ω ± ?) I wired them as shown.
Schematic.png


Do yourself a favor and crimp the lugs on before starting the stacking phase and if you are real adventurous cut the lead lengths too (I’m chicken and used the entire wire length). I labeled each lug with the resistor number. After the spacer plates are stacked on top of the top plate which does not have the notch for the heater pads internal connections (the thermocouple should be sandwiched between the top plate and the first spacer. Then stack the four heater pads between spacer plates as shown. Put a spacer plate on the bottom of the stack .
2 Partial Assy a.jpg
The nut will create a dead space between the wooden base (.png attached)and the soon to be 200°C hotplate, which will definitely be needed. Now stack the total assembly as shown and drill the holes for wiring the lugs I used a 3/32” drill bit then label each hole with the terminal number (the lugs are hiding my labels).
1 Wiring a.jpg

I found a small pack of lugs after I ordered a 100 pack, just as well since I ruined as many as I used. Hey, doing this with one hand ain’t easy, but I got er done. Then use the wrap around box(drawing attached) to give your hands a place to rest while solder your SMT components. Next the all important proof of concept test. This is my power controller.
4 Power Supply.jpg
When the hotplate hit 203°C (398°F) I shut it down. No burnt odor or charring was noted. When the parts come in I will repost this as a Completed Project.
 

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Thread Starter

Wendy

Joined Mar 24, 2008
22,436
Bill of Materials
Qty Description
5 20 Ga Spacer Plates (see drawings)
1 Top Plate
2 #6/32 x 1¾” pan head Screws, will use 2" in rebuild
4 #6/32 nuts
2 #6 Washers
2 #6 split ring lock washers
2 6½” x 1¼“ x 1” front wooden frame (see drawings)
2 2” x 1¼” x 4”*
1 ¾” x 4” x 4.6” Wooden Base (I used plywood)
4 #6 x ¾” pan head wood screws
4 1” square ¼” high rubber feet (required!)
4 25W 12V Silicon pad heaters
18 #6 crimp on lug ring terminals
Wire to carry up to 4 amps 20 Ga or better.
1 24V 4A Power Supply (variable preferred)
1 Type K thermocouple and meter (optional, but recommended)
 

atferrari

Joined Jan 6, 2004
4,332
No heating profile? Just going from minimum all the way up to desired maximum?

Maybe rather crude for components and PCB, I guess.
 

Thread Starter

Wendy

Joined Mar 24, 2008
22,436
No controller,just a variable power supply w/ a temp sensor. With the sensor you can a) Always add a controller later, b) figure what voltage gets what temperature.

Odd thing is since it is not polarized you could use 24VAC transformer w/ a light dimmer. KISS.
 

Thread Starter

Wendy

Joined Mar 24, 2008
22,436
I will rework every drawing when I do the rebuild. This thread allows me to present the project to allow revisions.
 
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