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I'm inexperienced at electrical engineering. I'm attempting to add a PID controller to an inexpensive, small thermoforming machine. Here's a link to the machine.

https://www.amazon.com/Excellent-Quality-Forming-Machine-Equipmemt/dp/B0067FFW6E

I made a schematic of the internal circuitry of the machine. It's not a complex machine, there is no temperature control, there are 2 rocker switches (ON /OFF only)- one for the heater and one for the vacuum pump. The machine is rated at 600W, I'm running it on 120 V. The heater and the vacuum can operate independently or simultaneously.

enter image description here

The PID controller I am hoping to use is fairly common, the Inkbird ITC-106VH:

https://www.amazon.com/gp/product/B01N1ZUGUZ/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

My hope is to wire the PID so the machine operates the same way it does now. I'll turn on the heater rocker switch, which will power the PID and the SRR. My hope is that I wire it so that this doesn't interfere with the vacuum switch / pump. I need the heater and vacuum to be able to operate independently with the PID inline.

Here is a schematic of the wiring I think will work.

enter image description here

Does this seem like the best way to approach this circuit? Is it even rational? Thanks very much for any insight or help.

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    \$\begingroup\$ The pid controller needs to be fed with the temperature, you're missing a sensor. \$\endgroup\$ – Ron Beyer Aug 18 '20 at 0:20
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    \$\begingroup\$ Yes, I know, I didn't add it to the post because I wanted to keep the post as straightforward as possible. The sensor will be wired to the PID controller, and I'll be building a mount that will keep it under the heater element. \$\endgroup\$ – GradeSchool Aug 18 '20 at 0:22
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Vacuum forming is a bit of a special plastics application. Usually you're using IR heating of the plastic (that calrod in the lid) and it's difficult to get a temperature reading of the plastic directly.

You might do better to measure the droop in the plastic or something of that ilk.

If your mains voltage is steady the power output of the heater will be steady.

Edit:

I'm not too sure about your original wiring diagram. I guess the switches are illuminated and there are some lamp pins, but it looks like they're switching the line on one switch and the neutral on the other which isn't kosher.

But anyway, I think what you want to do is take the two wires that go to the heater and connect the temperature controller, SSR and heater to those two wires only (plus ground if there's a spot on the controller for it, and ground on the SSR heatsink).

So the two dark gray wires power the controller, one side of it goes through the SSR to one side of the heater, the other side directly to the heater. The +/- drive for the SSR and the sensor also need to be connected (both with the correct polarity).

Remember that the controller can only reduce the heat from the heater element, so the sizing of the heater will be considerably underpowered most likely. You probably would do better to replace the heater with a more powerful one, and add an overtemperature cutoff thermal fuse so when (not if) the SSR fails on it won't cause a fire.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Yes, I've used more advanced types of thermoforming machines in the past. The problem with this machine is that I need to lower and control the heater temp to a very reliable degree. The thermoforming application I am using requires precise temperature, and the machine has no temperature control. \$\endgroup\$ – GradeSchool Aug 18 '20 at 13:06
  • \$\begingroup\$ Your answer gives me a roadmap for how to proceed. I upvoted you, but since I am new here, it doesn't register publically. All parts are ordered and will be here in two days. I splurged and got a better PT100 thermistor. A couple of notes: the heater is so powerful it's actually overheating the plastic. The plastic I'm using is HIPS which forms perfectly around 160 C.And yes- the first diagram I posted is actually how this is wired (the toggle switches do have illumination) and with my limited experience it seems like a funky, maybe poorly made circuit for a commercial product. Thank you. \$\endgroup\$ – GradeSchool Aug 18 '20 at 16:39
  • \$\begingroup\$ Thanks for your help- it's working. My first PID controller setup, it's holding + - 1 degree C. Next I'll figure out how to tune it. \$\endgroup\$ – GradeSchool Aug 20 '20 at 1:04
  • \$\begingroup\$ Good. It should have autotune, so that’s not so hard. \$\endgroup\$ – Spehro Pefhany Aug 20 '20 at 1:14
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enter image description here

Figure 1. Wiring diagram supplied.

The first thing to do is convert the wiring diagram into a schematic.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. The schematic version of Figure 1. Note the weird switching of L on one circuit and N on the other. (L and N weren't specified on the wiring diagram so my selection is arbitrary.)

Since you're modifying the circuit I recommend that you switch from the same side for consistency and less confusion.

schematic

simulate this circuit

Figure 3. Temperature control added. Replace CR1 with SSR as suits.

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  • \$\begingroup\$ Thank you. I received the PID controller today. Will attempt wiring tomorrow and update.Also thanks so much for introducing me to Circuit Lab. \$\endgroup\$ – GradeSchool Aug 19 '20 at 0:17
  • \$\begingroup\$ You're welcome. Note that when you use the CircuitLab button on the editor toolbar and "Save and Insert" on the editor an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. If you go directly to CircuitLab.com you will need a paid account. \$\endgroup\$ – Transistor Aug 19 '20 at 6:34
  • \$\begingroup\$ Got it working. Thanks for your help. I had to bypass the internal heater toggle switch, but after troubleshooting I realized it was a bad solder, and now it works great. Next I need to learn how to tune it properly, but thanks again for your help! \$\endgroup\$ – GradeSchool Aug 20 '20 at 1:04

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