Wednesday, August 10, 2016

DIY Hot Box for Drying Leather

I decided to build a hot box to speed up the drying time between processes during leatherwork.


Dwight and some other kindly leather working souls are engaged in a thread over at the leatherworkers.net forum and it finally inspired me to research and create one of these little time saving devices of my own.

The following will provide you my results and insights into the process.  Bear in mind that this is a work in progress and things are subject to change as the hot box moves from concept to reality.

I'll update photos as construction proceeds.


LED Digital 110V Temperature Controller MH1210F

150W 110V Ceramic Heat Emitter

110v AC to 12v DC Transformer (2 Amp)

Pulse Width Modulators (Fan Speed Control)



Hot Box Wiring Schematic w/MH1210F Thermostat and Optional PWM Fan Controls

Hot Box Dimensions Front
Note: Width dimensions of opening are to accommodate lighting panel grid.

Hot Box Dimensions Back

Cut sheet for a 24" x 90 panel (modified cabinet depth to 11 3/4" to use what I had on hand)

The following photos show the plywood assembly.  Note the holes cut into the face panel for the controls.



Used a length of 1/4" Pex toilet fill line cut into 2" lengths as stand-offs.  Tapped the plywood and mounted the CPU fans with 4" 8-32 round head screws.  Had a couple of old porcelain medium base lamp holders.  Ceramic heat lamps installed.

View of the sub-top with holes cut for wiring.

110V AC to 12V DC transformer wired up.

Doors cut and mounted.

3/8" plywood back panel glued and nailed.  Note access panel for wiring.

Pulse Width Modulators installed and tested with fan... works like a charm.  Here you can see the thermostat being wired.  I used 18 gauge stranded wire throughout.

Single gange cut-in box installed which will house the timer.  Colored tape was used to mark the hots, neutrals, and grounds.



Here, 10-24 tee nuts are epoxied onto the back of the cabinet.

Came up with an idea for my pegs using 10-24 all thread cut to 6" lengths, covered them in heat shrink tubing, then installed brake bleeder caps on the ends.  NOTE: I had tried to epoxy wood dowels sections to the heads of flat head screws, but the glue failed.

Shelves cut to size... Can't remove without flexing the shelf or removing the door.  I may add some extensions to the shelf cleats to solve this problem.

 The holy grail has been reached... 140º F...!!!


Some testing results:
  • Changed to all-thread pegs... working great now.
  • Shelves can't be removed without flexing or removing doors.  Will glue some extensions to the shelf cleats.
  • Thermostat read out is consistent with oven thermometer.
  • 120º F - 15 minutes
  • 130º F - 22 minutes
  • 140º F - 30 minutes
  • Thermostat interval 140º to 137º - 2 minutes
  • Reheat to 140º cut off - 3 minutes
I may change the cycle interval from 2 degrees to 5 degrees to reduce wear and tear on the thermostat.


Hope this helps,
Mike

7 comments:

  1. which amp model transformer did you use?

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  2. Thanks for posting this info Michael. This will be my next project for the shop for sure. What is the top switch with two dials? I don't see that listed in your parts list. Thanks.

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  3. Added the PWM switches to the list. These are used to control the speed of each cooling fan. I wanted to be able to control the fan speed independently from the heat lamps. Too much air flow will negate the heat being generated by the heat lamps. These controls also have an off position which will allow either fan to be turned off if not needed.

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  4. Michael, thanks so much for taking the time to post this tutorial. I am currently in the process of building one of these cabinets, using your list and wiring diagram as a guide. Not being an electrician by trade, I have found myself at a stand-still. I have the cabinet wired but can't seem to get it to start. Can you offer more clarification (at your convenience, of course) regarding how you wired the timer? I have neutral wires bundled together meeting the supply pigtail...three power supply wires (#1 & #3 from the thermostat and one from the 12v transformer) that I assume connect to the red load wire on the timer and the hot wire from the timer connected to the black on the pigtail...green grounds connected as well. Still no power. Perhaps another timer option? Any help you can proffer would be most appreciated. Thanks!

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    1. You don't mention what you are not getting power to (the thermostat, the transformer, the lamps, or all three). I'd isolate one component at a time. Can you get the timer to work and are you getting 120 volts out of it. Then introduce the 12vdc transformer and check for 12vdc output. Connect that to fans and see if they are working. Then introduce the PWMs and, again, get the fans working. Once that is done and all working. Once that is done, run power from the timer directly to the lamps bypassing the thermostat. Once the lights are working, introduce the thermostat being extra careful to get the correct wires to their designated terminals on the thermostat (it is easy to mix them up). Again, the best way to troubleshoot is to get one section working at a time. I often connect everything on the bench and run the system before installing the components into the cabinet.

      Hope that helps.

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    2. Agreed. I should have been more clear...plugged it all up and nothing received power. On a hunch, I tried separating the neutrals in the cabinet from the neutral in the power supply cable...that got the lights to working on the timer, at least...nothing else. I will check the entire system today, per your advice, and see what comes of it. If/when I can get it working, I will post it on LW.net in the same thread as your build. To reiterate, I appreciate your help. Best to you, sir!

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