Convection oven,PID controller

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slide posted this 4 weeks ago

I don't know if this has been posted before. Do any of you guys have any experience putting a PID on a convection oven? I bought one from a guy named Hatch over on the other site. I have read that you should put a seperate cord on the fan so not to burn it out with it going on and off. Otherwise,do you just plug the oven into the PID and go from there or is there wiring involved? This is a PID that is already built and ready to go. It has a auto tune feature. That is about all I know.

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Squid Boy posted this 4 weeks ago

I use a separate temperature controller to operate a 25 amp SSR that powers the heating coils. Set the fan up to run continuously when the unit is on. Your controller might work if the relay is rated for the heater load. Mine isn't and I use the SSR for that reason. Thanks, Squid

"Squid Pro Quo"

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Tom G posted this 4 weeks ago

Slide,  

I bought a Black and Decker convection oven that had a dial that went to 450 deg..  My intention was to cure powder coated bullets and heat treat WW bullets for more hardness.  I have an industrial grade temperature monitor that uses J thermocouples.  I put it in the oven and fired it up with great expectations. I was disappointed to find the it only reached an honest 420 deg. F.  Not hot enough to heat treat WW's.  I also have an industrial grade temperature controller that my brother sent me years ago. It also runs on J thermocouples. 

This oven is adequate to powder coat but I wanted it to heat treat. As a result, I opened it up and wired the output of the temperature controller in after the oven thermostat so that it controls the heating elements of the toaster oven only. This way the fan runs all the time it's supposed to. Since this thing can draw 12.5 amps I used a three contactor relay to control the on and off of the juice to the heating element. The Temp controller puts out one amp from the control circuit and this is plenty to pull the relay in and out. In order to keep the amp draw through the contacts lower I ganged two of them together to handle the current to the burners.  IOW I have two contactors handling the current instead of just one. 

I don't know if my temp. controller is a PID but it has a set of rollers on the front to set the temperature you desire and it regulates + Or - 3 degrees from the set temperature.  

I monitor the actual temperature where the bullets reside with the Doric trendicator you see in the photo that is registering room temp at the time I took the picture. I made a couple of entry holes in the back of the oven with a couple of brackets to keep the thermocouple probes just above the bullets that are being heated. The T couples are in the airflow from the circulating fan and both agree with each other within a couple of degrees. After knowing the calibration of the temp. controller, the monitor isn't really needed as the temp goes right to the set temperature and you can take that to the bank.  

This setup will very accurately hold the exact temperature you want for heat treating at any reasonable temperature between 450 and 500 degrees. 

In operation, I can see the bullets that I'm pc'ing through the door and the procedure for the Eastwood paint powder is to pre heat the oven to 450 deg. then introduce the bullets. Then when the coating on the bullets goes shiny, bake them for another 20 minutes. This convection oven does heat quite uniformly inside as I can see that all the bullets seem to go to the shiny state at the same time. 

I first tried using a sheet of foil above the bullets to keep them from any direct radiated heat from the top elements. When i did that, I could see the bullets in the outer edges of the group go liquid before the ones in the center. So, I omitted the shade and let them get the heat directly from the upper elements. 

So, yes you can control the heating elements with an external controller but you will have to do it through a relay that can handle the high current load. As long as your PID can put out enough to pull in the big relay you should be fine. 

Tom Gray

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Tom G posted this 4 weeks ago

I forgot to mention that after pre heating at 450 you do the actual cure on the paint at 400 degrees for 20 minutes. 

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mashburn posted this 4 weeks ago

Hello Tom G,

I also bought a Black & Decker  oven rated at 450 degrees. It came in today and the first thing I did was to stick the probe of my pyrometer (I use my pyrometer when doing color case-hardening)  in and set it at 450 and turn it on. It went to 450 quickly and the thermostat was accurate. I could have gotten some more heat if I had wanted. I'm very pleased so farrrrr.

Mashburn

David a. Cogburn

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Tom G posted this 4 weeks ago

I guess I just had some bad luck with the one I got. When I disassembled it I saw that it was not made for for taking back apart.  It has a lot of snap in tabs and looks to me that it is really a throw away item if it doesn't work right. I spent an hour trying to put it back together and finally got it far enough together to work OK after changing the wiring inside. 

I was looking at some of the remarks from others who had bought it and they complained that it wouldn't make 450 even though it was set on 450.  So, it might be quite common to have that problem. You are one of the fortunate ones!!

Tom

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slide posted this 4 weeks ago

I can relate. Today the thermostat on my convection oven started sticking. It got up to 500 F before I knew it. I don't know if you can buy replacement thermostats or not. I am still going to hook it up to the PID when it shows up and see what happens. You are sure right. They are made to throw away and then you can buy another one.

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