Building my 120vac oven

R

Randy3000

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Jun 3, 2017
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I ordered the pid, ssr, and thermocouple from Auber instruments. I received them today and have it all wired up! I have the k23 bricks and heating element on the way. I have a welder and plenty of metal and hinges to get the oven built when the stuff gets here.

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The first light switch turns the pid on, and the second switch gives the ac power to the ssr.

In hindsight i should of went with the 6" thermocouple instead if the 12", i didnt realize the mount was the way it is. I used a 16awg extension cord with a ground for the power cable and to make all the connections.

My breaker is 15amp, and so i got 15amp light switches and a 25amp rated ssr. The heating element is 13amp. Oven chamber will be 4×4.5×9 and used for folder parts.

I want to start using stainless steel and figured i might as well do it myself since sending it out for heat treat is expensive over time, and i am impatient. I will have less than $300 in this project.
 
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Sounds great.
I will be first to point out 16awg will work, its on the lighter side of adequate.
Typical ”S” type cable 16awg with 2 conductors energised continuious load should max. 10A.
Further, Continuious load of 15A circuit should not exceed 80%(12A).
Yes. Your proposed oven will work, just be aware its cutting into the edge of typical safety margins.
 
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Your build is pushing very close to the max on most components. I would have used 30 amp switches and heavier wiring ... and used a 20 amp breaker circuit.

One treason that you should change the power carrying wire gauge is the ovens have a continuous draw for hours, and undersize wires can melt.

Shorten your TC to the needed length so the tip sticks into the chamber about 2 inches. Cutting down the longer leads will not affect the reading, as only the junction at the end creates the current. Make sure you are using type K TC wires to connect the TC to the PID. Regular wire will give false readings. TC wire is polarized, so make sure the connections are + to+ and - to -.

Make sure your 25 amp SSR is on a good heat sink. It will likely get fried carrying 13 amps without one.

Last comment, while in theory your oven may be able to hit 2000°F for stainless HT, in practicality, it may be very slow getting there, if at all. The gauge on most 13 amp jewelry kilns with a chamber size comparable to yours goes to 2000°, but it is hard to get them past 1800°.
 
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Ah, all that is easy to fix, thanks guys! I do have a 20amp breaker for the washing machine which is in a small room connected to the garage, cant run both at the same time but oh well. I can change out the ssr powering switch to 30 amp, and run 12awg wire (i guess that is good enough?) for it, the power cord, and the heating element. The thermocouple has multiple varying length ceramic shield sections, which i have wondered about, and now it is clear it is for cutting to length.

If it cant do stainless, i will use w2. Really what is important to me is stop using an uncontrolled forge for heat treating.
 
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I read that 16awg wire dissipates 1 watt per foot and should not get too warm. You know there is a lot of conflicting info out there, and it is hard to know who to listen to. I trust you guys though, you have been spot on with everything!

Edit
I got a 30amp switch and 12awg wiring in place. 16awg wire i had said it was rated for 13amp, so i went with it. Safety is important though and i realize an oven is a bit unusual appliance. It seems to me they should build safety margins into the ratings they print on packaging.

I got the heating element today! Just waiting on the bricks now.
 
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I have read that at some point. A lot of the glass and pottery kiln writings use chamber size a lot bigger than what i am wanting, but otherwise it contained some good info. There also isnt a lot of well documented 120vac ovens out there, which is why i made this thread so hopefully others can tell me before i burn my house down!

I just finished up the box. I used this ridiculous hand held face shield thing that came with my harbor freight welder. The switch box fits where the eye lens was, and the pid fits where the mouth opening was. It works. The frane is wood and it is screwed down tight and solid. The rest of the box will get sheet metal and i will weld a piece of mesh in an opening on the back for air.

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I am leaving the sides off for now so i can monitor how hot everything gets when it is running full speed.
 
The SSR still needs a heat sink. Even bolting it to an aluminum plate is better than nothing. The amp rating is the max capacity. The heat generated at any capacity is a separate thing. Your 30 amp unit will survive better running at 13 amps load, but there still will be some heat created. Personally, I consider a heat sink a requirement. Many good quality SSRs come with the heat sink manufactured into the unit. A heat sink for an SSR only costs $6 with free shipping on ebay. https://www.ebay.com/itm/10A-40A-Al...520419?hash=item51f7ba1ce3:g:E6gAAOSw~y9ZC91R
 
Oh i have a heat sink, and the thermal compound. One question about that, how the heck do i mount it?

The heat sink has the fins on one side and ssr on the other. There are 4 corner screws, but they arent long enough to go through the fins all the way. Do i make a cutout in some sheet metal, stick the ssr through where it is inside the box and the fins are outside, and then run the screws from the inside through the sheet metal and threading into the heat sink? It will work, but are the fins supposed to be sticking out like that? It makes sense kind of, they will be exposed to air and cool off easier.
 
Randy
First, if you let us know what part of the third rock you live on, someone might get with you and give you some hands on help.

There are many types of heat sinks, with out a photo it is impossible to give details. Heat sinks should be mounted with the fins vertically to allow the hot air to rise and the cool air to flow in from the bottom. Of course if you use a fan to move the air you can mount them in whatever position meets the airflow. The flat part of the SSR is coated with the thermal compound and fitted flat against the heat sink. Usually secured with four screws. The screws should be snug enouh to firmly hold the SSR without distorting the heat sink or the SSR base. MOST SSRs are electrically isolated from the flat surface which contacts the heat sink, but I suggest you confirm this. Otherwise the heat sink could have a voltage on it.

Looking at your photo I noticed the wiring running from the SSR to the controller (red and yellow in a yellow sleeve) looks to the type K thermocouple wire. If it is, I suggest you replace it with hook up wire.

Just a note: Type K thermocouple wire polarity, red is negative and yellow is postive.
Jim A.
 
After it was mentioned i could cut down the thermicouple, i did sone reading on how it works. I read that the red and yellow wire for thermocouple are made from different metals. So are the two halves of the thermocouple, and that is how the thing works. After reading that, i did change out the wiring from the pid and ssr to use 16awg copper wire.

I will post a photo of what i mean about mounting the heat sink to the box when i get home.
 
Here is a pic of the ssr and heat sink. A mounting screw is laying in front of the heat sink. It is clearly shorter than the fins.
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So when i put the sheet metal sides on the box, i was thinking of cutting out a hole a size between the ssr and the top of the heat sink. Then i would place the ssr through the bottom of the box through the hole. Then the screws would down from the top, inside the box, and go through the sheet metal and thread into the heat sink. That way the ssr is in the box and the fins are aticking out from the bottom. Is this dumb?
 
cut a hole in one of the sides or the back of the box which the SSR will just fit throuh (you may have to disconnect the wires until after mounting) slip the SSR in the hole with the heat sink on the outside of the box with the fins running vertically. This way as the heat from the SSR travels through the heat sink and rises, it flows out the top and draws in room temp air from the bottom.

Putting the heat sink in the bottom means the heat has to travel down to get to the heat sink (no problem) but once it starts to radiate off the heat sink it has to build up a layer of hot air which will slowly spread out and flow off the ends of the fins. This creates lousy air circulation and minimizes heat dissipation.
Jim A.
 
OK, The heat sink you have is the type for the back of a VFD or other speed control. The four corner holes are used to mount the heat sink and VFD to the cabinet they are installed in. It will still work for your control, as it is far larger than necessary.

Here is how to do it:
It looks like you have used two machine screws to attach the SSR to the heat sink. Good, that is the normal way.
To attach the sink and SSR to the cabinet, drill two 1/4"holes to match the ones currently holding the SSR to the sink. This should be on the top or back of the cabinet. Don't make any cutout as you mentioned.
Apply a blob of heat sink compound on the inside and set the SSR on the compound. Place the machine screws through the SSR and cabinet, apply heat sink compound to the outside and screw the SSR to the sink. The metal in the cabinet will conduct the heat to the sink just fine.Put screws in the four corner holes of the sink to hold it tight to the cabinet. If you have to change an SSR sometime, just unscrew the two machine screws and everything else will stay in place.
 
Ok, i will put it on the top with fins outside the cabinet like stacey said. That would dissipate the most heat i think. I havent used any thermal compound since i am just dry fitting everything right now, so i have plenty to put it on both sides of the cabinet.
 
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Yes, I should have mentioned dry fitting. It makes life much simpler.
Put everything together and check it all out. It will be fine to run under power for a while without the compound. Then, after all changes and adjustments have been made, apply the compound and make it permanent.
 
My neighbor gratiously left this microwave out by the road. I am going to rebuild the electrical box for the oven using metal housing from this. The housing is all i intended to take off of this, but then i saw this fan.... anything else i can salvage without creating a mini chernobyl in my garage?

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So apparently there is a lot of cool stuff inside a microwave. And stuff that can kill you. I got a bunch of sheet metal, steel mesh that i can use to protect connections for heating element, lexan, resistors, huge transformer, ridiculous capacitor of death, a 120vac 6amp double motor (buffing and polishing maybe?), a fan, 2 really strong circular magnets, a bunch of small wires, and most importantly the interior hull of the microwave will work to make a partial outer hull for my oven!

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I dont see a decimal point. Maybe 6A means something else. I dont know.

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It has red, yellow, blue, white, and black wire. It will take some research to learn how to wire this.... does anyone here know?

Edit
Apparently the wires mean
White = Common
Black = High Speed
Blue = Medium High Speed
Yellow = Medium Low Speed
Red = Low Speed
Which could be useful. I wonder if it will have the torque needed for something knife shoppy.
 
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