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How I modified an Iron to PID control

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Registered Users
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Jan 6, 2010
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Midland Ontario Canada
How I modified my iron to control temp, to shrink fabric, with a PID Controller . This should cost less than $100. I waited to post this until I used it, I am very happy with it's performance. I have a 30 foot cord from the control box to the iron so the box can sit out of the way and I can move around a wing with the iron. This is a long explanation, for those new to this type of control.
Modifying an Iron is easy, but if not done carefully can be very dangerous. I am not telling you that you will be safe if you do it. If you don't feel comfortable working with electricity doing this, don't. This worked for me, it may not work for you, so don't sue me if it doesn't. You will be working with a controller that can turn power on, maybe when you don't want or expect it to, so be sure it is disconnected from line power unless you want it to be hot! The controllers can be a bit of a pain to set up, mostly because the instructions are minimal, and what you would expect from Chinese translation, but not too bad, and once set up it works very well.
The problem with the old thermostat systems is accuracy. The iron heats up to the set temp, then the thermostat turns the power off. But the element is still hot, so the iron heats up higher until it consumes the heat in the element. Then it has to cool down enough to trigger the thermostat again, way below set temp. So we get heat swings from too high to too low.
In the PID system a thermocouple (TC) senses the temp of the iron and sends the info to the PID Controller. You set the required temp on the controller (SET TEMP) and the controller compares that to the actual temp, and turns power on or off as needed. It also LEARNS how the iron behaves! As the iron gets CLOSE to the set temp the controller shuts the power off, and back on, and off,,, many calculated times, as needed, to creep up on the set temp without passing it. It will also "flick" the power on and off as needed to maintain set temp very accurately as you use the iron.
If you turned an old thermostat or magnetis controller on and off many times a second, thousands of cycles per hour, you would burn it out quickly, and they don't respond quick enough anyway. So we use an SSR (Solid State Relay)
The Controller sends a low voltage signal (power) to a SSR, to turn it on as needed. The SSR is basically a switch. We connect the black power wire (110V) to the SSR just like you would to a switch. So at the PID Controllers command, the SSR turns the power on and off as needed.
The SSR will generate heat, so it needs to be mounted to a heat sink, with heat sink paste to dissipate the heat. The box you install all this into needs to be vented for that reason. I used perforated sides, kept the lid solid in case anything spills on it.
I also installed a main switch (light switch)to turn all power off.
I got an old iron and dissected it. I threw away the thermostat, water tank etc. Kept the cord and its attachment parts, the wire nuts, because they are made for the iron temps.
I found a spot that was thick ( ~1/2") , and drilled not quite through, and tapped it 1/4" NFT, to screw the TC into. I used a TC with stainless braid cover on the wires. TC's , from their name Thermo Couple, Thermo = heat, Couple = two. There are two wires of dissimilar metals, they are different, and they are connected at one end. As the connection heats up, it creates varying amounts of electricity, a very small amount, but enough for the controller to measure. If you introduce a 3rd metal, like solder, you will screw the system up and it won't work. Never try to solder TC wires. I have had good luck splicing them to lengthen by tightly twisting about 1" of wire together and taping. I lengthened mine to about 30 feet. You also have to be sure you are splicing like wires together. TC's are identified by letters, I used "K" TC's, and K wire. It's best to order TC's with the right length wire but you can order extra wire and splice. I had TC's and wire from other projects.
I used the original iron cord because of it's heat rating, and spliced onto it a few feet from the iron. In the iron, after removing the thermostat, you will have to reroute the element wire back to the cord. I also added a ground wire to the iron and spliced it into the long cord. I then packed fiberglass insulation on top of the iron plate, running the TC wire up through and on top of the insulation, and out the back of the iron, and put the top and handle back on. I had to modify the top attachment a bit at the front because the thermostat was part of the mount. You might want to use the thermostat for that but I drilled and tapped one of the thermostat mount holes for the TC.

There are 3 main parts you will need and a few others.
SSR Solid State Relay, I used 40 amp, overkill, but I had it.
Heat sink
PID controller
Thermocouple S (short, K, long leads)
I buy my parts from auberins.com, I have no affiliation with them. You can also find lots of this stuff on ebay.
[FONT=&quot]The controller I used for this is a simple one, for not much more cost, you can get one that you can program heat and time cycles into a memory, not needed here unless you wanted to program a shutoff time for safety in case you forget and leave it on. I used that type to control powder coating and windshield molding ovens. [/FONT]




Iron Schematic smb.jpg


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