Best way to add PID to thermoform machine?

Question

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.

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.

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

Answer 1

Figure 1. Wiring diagram supplied.

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

^{simulate this circuit}

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

Answer 2

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.

^{simulate this circuit – Schematic created using CircuitLab}