Long story short I'm creating my own hot runner controller for work because the QTC-15 is the bane of my existence.
Part of the upgrade is idiot proofing the connection of the heater and thermocouple to the controller. If you connect them backwards the connections in the heater act like a thermocouple and doesn't raise any errors, but the thermocouple explodes when 240V ac is passed over it.
I can't just test for temperature because the heater acts as a bad thermocouple.
My current idea is to isolate the thermocouple and use a circuit to measure its resistance before power up of the heater. Heaters are typically 50-200 ohms and thermocouples are typically 0.5-5 ohms. However, is there a better way to test for a real thermocouple than just measuring its resistance? Does doing so damage it? Thanks
You have a real problem, control and power should not be using the same connectors. They should all be in one polarized or two seperate but different connectors. I used thermocouple connectors for the type J that we used and power connectors (plugs) designed for the heat involved. My solution is change the wiring and save the additional work of testing, someday it will be skipped and boom! From what I remember it plugs into a chassis where your connections are made to the devices on your extruder so changing that should be relatively easy.
Yes, the connectors are different on the mainframes for thermocouple and heaters. I have never plugged them in backwards and blown a manifold tc. However, I am not the only one who is connecting this equipment, and both qualified and unqualified employees mess with the wiring and cause this issue. There are instances where proper connections are not on molds and alligator clips are used. (Some may say that its not up to code I suppose) I can't change the system as a summer intern so I need to reinforce and idiot-proof the controller I am working on.
Good Luck. It will probably only be a matter of time before somebody gets zapped or worse. Just hope OSHA if in the US does not pay a visit. The only other thing I can think of is use color coding on the wires, heat shrink may work, you probably can just put it on and the ambient will shrink it for you.
During my career my department head would often tell me, "Ron, this needs to be foolproof". My response was always the same "Mike it's impossible to make anything foolproof because fools are ingenious people".
That said the thermocouple connector should be just that, a connector designed for use with a thermocouple. A heater power connector likewise should be a connector designed for its intended application.
Measuring the resistance of a thermocouple junction will not harm the thermocouple but consider a thermocouple is a mV (milli-Volt) source with a polarity output. I have a Type K thermocouple laying here at about 70 degrees F. I measure one way I get about 11.2 ohms. If I reverse my meter leads I get about 11.6 Ohms. That is at a room temperature junction. Start increasing temperature at the T/C junction and the spread in numbers continues to grow. We are placing a mV source across an ohmmeter input.
Finally they do make connectors that can house both T/C alloy and Cu pins and sockets but they become expensive. You want a keyed connector which can only go on one way. ITT Cannon is one manufacturer and I do not recommend running heater element power and T/C output close to each other.