Thermo-well or Dunkin?

Originally posted by Guy Thomas
Kevin! I see you are using the Omega CN76000 series controller! I bought one of these (not from Omega however) for a bargain price and I am having trouble figuring it out. Do you suppose I could lean on you for a little tech support? The manual for it seems extremely basic on some matters and almost incomprehensible on others. Mine is set up for a 5 volt output, which most people are telling me won't control most solenoid valves (I'd need a switch of some kind in the circuit?)

I'd initially like to just set it up for use as a pyrometer only and then use the controller functions as I understand it better. Will it accept input from a thermocouple and show a temperature reading without any programing? (that is, can I attach a power chord and leads from a thermocouple to it, plug it in and immediately use it to say read the temp in my forge?)
Click to expand...

I am not sure about the output thing. The controller aspect of it is simply a relay, on the back of the controller you attach one half of the circuit to one lead and the second half to the other. All the controller does is turn it on and off, it doesn't supply the juice. I have wired it directly to the solenoid but have learned my lesson and won't ever do it again. The full retail price of that controller is a steep price to pay for knowledge, but it does increase your capacity to learn. You see, if anything goes wrong down stream of the controller, and it is wired directly to the load device, the controller will now become a $325 disposable fuse to protect a $10 part. I was changing some wiring around one day and even though I had shut down the controller I had another live wire in the box and shorted them out... burned all the leads on the back of the controller,(they are just itty bitty little gold PCB leads that carry all that load)it also scrambled the controllers brains. I now wire the controller to a mercury displacement relay and run the juice through that.

You will need to program your controller for input from your particular type of thermocoupler. If you need help I can look things up in my owners manuals. The CN7600 are very good units and still not that expensive considering all that they can do. My desktop PC isn't capable of learning and adapting like those little boxes are.
 
I must agree with Kevin, it is better to power a relay than the final element. A mercury wetted relay is one answer however I like using solid state relays. Mechanical relays may develop a lag or "hang" with time.

There are also small SCR drives available for electrical applications that have the capability of having tighter control than any on/off or time proportioning output type.

I have been in the process control field for longer than I care to admit but there is one thing I have learned. As far as control of a variable (temperature, flow, or pressure)...repeatability is much more important than accuracy.

Craig
 
Whatever you use for a relay, it is VERY important that it fails in the open position, if it fails, my relays will do that. If you have a device that can fail closed, you will have a runaway situation in gas or electric. One of the few horror stories that I know of with the low temp reaching the critical temperatures where they become unstable, came from my friend Brian Lyttle. He started the salts going and then stepped out of the shop to accomplish other work while the things came up to temp, as we all do. The relay device failed closed. He got it in time, but barely, the room was filled with smoke and the salts were glowing (very bad for low temp).
 
Another "gotcha" is if your thermocouple wires short somewhere in the leads. It will indicate ambient temperature at the short and will keep the relay closed because it senses a much lower temperature.

If possible, it is not a bad idea to have "hi-limit" cutout. This would be a separate device (or the other channel if your controller has two) that would cut power if it reached an exceedingly high temperature. It would require another T/C and relay but would be a much higher level of safety.

Craig
 
My newer CN76000 controller has an alarm feature that I can set for high or low temp. It has come in VERY handy, and it only cost $16 more. I have it wired into my security system in the shop so that I can start a heating operation and just go up to the house and have dinner. When the temp is reached the security alarm in the house oes off and tells me that the operation out in the shop is proceeding as planned.

Craig, with your experience perhaps you could advise me with a problem that I have. I have the two controllers and when the high temp cycles it causes a slight surge in the shop power when the circuit opens. The controllers are very sensitive to these types of surges and it will often cause the low temp controller to shut off and restart. I have completely isolated them on seperate circuits and breakers but they still share the main line that comes into the shop. It is just a 110V single pole relay that is causing a controller running the 220v relay to dump. Is there a way to put an end to this nonsense? I worry for the well being of the one that gets dumped all the time.
 
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