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  • Infinite Heat Contoller

    I build Heat Treat Ovens and planning on building abetter hew one.

    I need a controller that will allow me to control the heat. The ones in the past were built to be certain hets and heated to that point.

    If anyone has any thing that would work, I would be interested.

    Thanks,

    Jerry

  • #2
    You can rob a oven control out of an old electric oven. Appliance dealers often have old stoves waiting for the junk man to come and get them and would probably let you take one off for little or nothing. You'll need to know the current draw of your heat treatment oven to make sure that the control will handle the wattage. If you want to roll your own, you could build a phase-contolled triac dimmer circuit. I could find you a schematic diagram if you want. thanks--Mike.

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    • #3
      Jerry: do you want to control the rate of heat rise or the max temp or time at temps?

      The triac,Quadrac makes a nice way to control rate of rise of temp. (not very precise temp rise though), a thermocouple and inverting op amp make a very inexpensive system to set temps. From there on (more complex) I look into "clock circuits" to set time/ temp/ rise functions/
      Steve

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      • #4
        I never thought about using an oven controller, now I will look up the local appliance guys.

        My unit has a 20 amp, 110 draw, but can be split into two seperate sets of elements.

        Many thanks,

        Jerry

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        • #5
          Any more suggests will be appreciated, this is to limit the ceiling of the temperature, not the rise of it. I want to keep this simple.

          I do have an Omega handheld thermometer, and have ordered a long probe for it.

          Thanks,

          Jerry

          Comment


          • #6
            Jerry
            You can buy microprocessor controls to do this. Real handy for tempering - something that should be done immediately after heat treating to reduce stress failure.

            Omega can sell you what ever you need. Just tell them the problem.

            Comment


            • #7
              Jerry, will an oven control give the temps you need? I suspect an Op Amp, relay, (less than 10 dollars at radio shack)and home made thermocouple would get you to the 2,500 degree mark. a Stove burner element can give a slow heat rise or fall with little extra complexity (just add two wires in the circuit).

              Steve

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              • #8
                I also have a need for a temp controller for a plastic injection molding machine. It is an older one with a BIG controller box and it no longer controls the upper limit. Good advice to get a op-amp and relay for $10, but there are all these little metal spikes coming out of those little black things that I think you solder wires onto, but then where do the wires go? If you can't put a wrench on it, I can't fix it! Whatever does the work in these things is invisible but you can sometimes feel it. Kind of like a ghost! Surely at least Black Magic. Maybe if a circuit diagram was available I could build one. Any chance?

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                • #9
                  Jerry:
                  An home oven controller will not go to high enough temp for heat treating (only about 550 deg F.) Unless you are only going to heat soak after heat treating.
                  Mc Master Carr has temperature controllers for as low as $96 that take a thermocouple input. The low cost one has a dial to set temp but does not have an active display. It is a nice little 1/8 din package that has an octal socket for connections. They also sell an octal socket with screw terminals. If you want a digital display you have to spend $200 or more. You can also buy thermocouple wire from them. You need to twist and fuse the tip of the wires together to make a thermocouple or buy encased ones from OMEGA. The OMEGA catalog has lots of information on which thermocouple type to pick for the temperature range you need.

                  Shavingmaker:
                  I suspect that what you are seeing are the thermocouple wires coming out of the barrel heating assembly although you may be seeing the heating coils. The same information from above applies to you except your machine probably limits at 700 deg F. Mc Master Carr also sells clamp on barrel and nozzel heaters for molding machines for $10 and up. Molding machines usually have a few heat zones to control the melting zone and nozzel temperature. It is very hard to help you without having a diagram of what you have but I would think it would be cheaper to diagnose the problem and fix it than to start over with a complete temperature control system because of the different heat zones.

                  Does the machine have seperate controls for barrel temp and nozzel temp?

                  ------------------
                  Dick
                  Dick

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                  • #10
                    I would like to have as simple as possible. I don't need to connect to a thermocouple. But this is a 20 amp type of things, so many of the simple rheostats won't work.

                    If it can be built from Radio Shack parts, I can do that, I just need a schematic.

                    A dial that controls the current flow is what I am looking for. I have an Omega electronic thermometer.

                    I do appriciate all of the other recommendations, but I can't invest a lot in the HT oven.

                    Thanks,

                    Jerry

                    Comment


                    • #11
                      As I plan on two elements, I can handle them as two circuits, I can use heavy duty ceiling/chandle power controllers? I know they can be found in the 15 to 20 amp range.

                      Any comments of suggestions about doing it this way.

                      Jerry

                      Comment


                      • #12
                        Jerry:
                        I assumed from your first posting that when you said control the temperature you meant feedback control from a temperature sensor.
                        Since you are only interested in a way to adjust the temperature you might consider the oven temperature controls they put on low cost jewlery burn out ovens for lost wax casting. The device has a small heater and a bimetalic strip and senses the current draw of the main heater. The adjustment knob adjusts the temperature of small heater so it switches the power to the main heater in the oven on and off. You can make marks on the dial that are close to the temperature you are interested in. You need to have an instrument to measure the temperature in the oven in order to make your marks. This device is not a true controller since it doesn't measure the temperature of the oven but only the average current to the heater coils. I will look around to see if I can find out where to buy them.

                        ------------------
                        Dick
                        Dick

                        Comment


                        • #13
                          Jerry, I made my first trip to the mall in years, with you on my mind. greater love hath no man and all that BS.

                          Sears has, on display, thermocouples Type K. The insulation they have is good for 1500 degrees (memory serving correctly), but you can sperate the wires in the hot area and need no insulation/ Sears asks near 30 dolllars, which is about 5 to ten times what hey should be. A good thermocouple is just two wires made of the right materials twisted together (usualy brazed, but sometimes not. Braze is just for longer life etc.) so that gives you a sensor of tempratue. Thermocouples put out a voltage with sufficient current to move a cheap meter. calibration can bedone with the cones pottery kilns use. So for say thirty five bucks you have a calibrated thermometer. But being cheap, I would make the thremocouple and it would cost (if the materials can be bought in small lots, five dollars or less for couple, check the surlpus houses. Ultimate simplicity would be a just electric oil pressure switch with a sensors screw. The sensor screw is a switch that closes Or opens at a certain meter reading (the actual setting is adjustable). So when the couple gets hot. the needle moves up until it touches the snesor screw. the completed circuit can control a relay, and the relay contoroling the heater, horn etc heater on or off, as you desire. the An op amp and relay is cheaper still. but you can't save much when the meter thermocople rig is so cheap.

                          You put the heater, the relay contacts, the stove burner controller all in series. If you are willing to use max heat rather than slower heating, yo udon't need the burner controller.
                          Tell us (1) voltage you intend to use (220. 110 or other), max temp you wish to reach (if yo ugo too high the heater will burn out, so will your sensor (thermocouple). do you want to just turn the heater on and let it rip or do you want to bring it at an adjustable rate, of what material will your heating elemets be made. will you use one or more elements?

                          Steve

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                          • #14
                            Jerry et al: being a natural born cheapskate and cobbler par excellant, I just went to shop. Ambient temp here (tuesday ) is 67 degress F. Took a length of wire used to tie re-bar, cleaned it, brazed a length of nichrome (safety wire) to it and made a thermocouple. At room tmep output is 0v. With wire in mouth it put out .0002 volts (.2mv). at critical temp of iron wire (no longer magnetic) it put out 3.1 Mv.
                            Then tried copper, at room temp 0 mv (as expected). Oral temp was near .2Mv (it flashed between .1 and .2 Mv. highest temp recorded was 2.4 Mv at which point the copper failed (Melted right in two for you practical guys). These voltages are high enough to drive a op-amp, which can then drive a transistor or sensitive relay and the relay can drive any thing the contacts can stand.

                            Radio shack (which i seldom visit any more) has a reference manual showing where the pins go for opamp. My manual is 1991 edition, page evelven shows pin outs for type 741 op amp used as a comparator. THey also have a little book that I just bought for this project (99 cents) showing several circuits where photo cells drive the op-amp and a meter. There is also a "comparator circuit (which I have used before and their circuit will work) where you trigger the out put when two voltages are equal. In one circuit, I used a thermistor on one input and a potientometer for the twmp control. The circuit held temp with in a tenth degree and was adjustable from ambient to about 600 Degrees F. The thermocouple would do the same thing.

                            As far as thremocouple go- gas fired heaters use a 25 MV (if memory serves) thermocouple to directly drive a relay to operate a gas valve as flame sensors. Those are realiable commercial units.

                            Beware: there are at lease two types of flame sensors. one is the thermocouple, the other depends on a flame touching a wire. the flame is conductive so a small current goes through the flame and "holds" a relay open to allow gas to flow. The flame currnet type is of no use in temp controls. Both types are inserted in the pilot flame so I have no idea of temps they will stand. Should be fairly high though.

                            Regardless of the temp control used, you should have a loop of wire inside the oven, in series with the heating element that will melt at a safe temp should your system "stick" closed and try to melt the oven.

                            Now that you can control the temp, yo need to "calibrate" the controls. The kiln types have "cone" that are set to melt (sag) at very precise temps (if the charts are to be belived). Put a thee cones of near same temp in oven. one cone below desired temp, one at desired temp, one higher than desired. When low temp cone sags you are below the temp so watch it closely, when middletemp cone melts, adujust your pot so the relay turns power off and mark the setting. the thing again and again until you have your set points marked. Just remember, ambient temp vary there fore, unless you have a "electrical" cold reference "thermocouple" the settings will only be accurate by the variation in ambient.

                            I have probaly told more than I really know, made some mistakes, or not been clear. I invite disagreements. so jump my frame!!!!!
                            Steve

                            Comment


                            • #15
                              Steve,
                              I use the cones as a verification on my single heat ovens. The place that makes and sells the cones is not too far from me.
                              This multi temp oven should be affordable by everyone, that is why I am going thru all of this trouble.
                              I am using the book Electric Kilns, by Harry Fraser as the main guide for this, as I have in the past. It's a wonderful book.

                              Jerry

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