Thermocouple input

I want to connect a 2 wire thermocouple to a converting module (maybe Weidmuller or Entrelec) with relay output and also directly to a plc thermocouple input module (Koyo), question is can I run the same thermocouple signal to both devices and do I need to wire the signal in series (like a 4-20mA loop) or parallel (voltage)? Thanks.

Steady as she goes!

You have to be careful doing what you describe. A thermocouple is a voltage output device but it only provides millivolts worth of signal at the terminals. You have to parallel those connections to provide the same signal to two different inputs. The distance between them is critical as is the type of wiring you use. If you are not careful you will degrade the signal to the point that it is not usable. I personally would not recommend what you suggest with out a signal converter/isolator, but there are some here who will probably give you a little different slant on things.

Yes, you can.

No, you can't.

It depends entirely on the devices you are connecting the TC to. If it's a critical application, I'd suggest using a dual TC, one to each device. If you cannot, or don't want to, you can try connecting up both, as long as you have a way of verifying proper operation.

One big problem with TC's used in parallel is that some signal conditioners actually configure the TC as part of a bridge on the input, in which case, any external connection will cause the readings to be invalid.

The best solution for you overall, may just be to use one TC, and a stand-alone TC to 0-10V or 4-20mA signal conditioning module, and then parallel the outputs of that to your other devices.

better yet

Use a T/C to 4-20mA converter, and send the 4-20 to the PLC.
The 4-20mA to a PLC will be less expensive...and, with the
savings, you could then use a current-powered alarm relay (STATUS Instruments makes one), and insert it in the current loop for the relay function.

Done, and done.

(Just make sure your signal conditioner can UP or DOWN scale on a broken T/C input for the correct response.)

Why a converting module with a relay output?? Let the PLC do the math or limits or whatever and control the relay.

If the converting module is some sort of a override to the PLC, like an overtemp switch, etc, you should use a seperate thermocouple anyhow--one sensor for one instrument--both the sensor and instrument are backed up.

Randylud was correct about connecting the inputs in parallel. I have done this with a few devices during a controls changeover. ie moving from individual temp controllers to a plc control. I only left this configuration for debug purposes and then did a full changeover. It will depend greatly on the 2 controllers and the length of wire.
With my situation I had a very slow moving temperature deviation. This allowed me to take a reading with one device. Then wire in the second and look for variations in the reading. I normally noticed a deviation around +/- 3 degrees. This was seen as acceptable for our process so we ran this way for some time.

Basically my 2 cents say try it but I have to agree that it is not the correct way to do it.

yes you can connect in parrallel and yes it will work in most cases.
if you use the relays as safety do not connect but use another thermocouple wire. that is cheapest way.
watch the grounding a little but for the rest it should work fine.

What I'm measuring is the temperature on 4 3 cylinder air compressors, one thermocouple for each cylinder (12 tc's total). I want to display the temp on a operator interface and also use the input for AUTO operating mode via the plc. In MANUAL mode I need to bypass the plc and have a relay de-energize the starter when a cylinder high temperature condition occurs. Oh yeah, it needs to be "cost effective and reliable"...

I don't understand why the PLC couldn't control the "drop out" relay in the manual mode as well as the auto mode functions. I must be missing something, here.

John, the requirement for our compressor is to be able to operate if the plc fails

John, the requirement for our compressor is to be able to operate if the plc fails

Click to expand...

But not if the T/C fails?

I'd still go with dual TC's here, the cost of an extra T/C isn't all that much, and will help completely isolate the PLC and manual control system.

Example, what if, oh, say... the PLC fails because a surge caused a fault in the input channel of the thermocouple module, shorting the T/C inputs to ground?. That will also kill your manual mode.