Thermocouple rectifier?

[TConnolly]

In this particular app the customer has a heated/cooled mold with a TC probe in the mold. Electrical contact is made with probe pads when the mold is clamped. Sometimes the operator can put the probe in upside down, and the part will be scrap. The customer needs a way to rectify the voltage from the TC so that it does not matter which way the probe is placed and he would prefer to not have to take time to align the positon of the probe and mold. He wants a "slap it in and push go" solution for max throughput.

I considered bringing the mV signal into the PLC and doing the polarity adjustments in software and linearizing it myself, but I'm looking for a better way.

Rectifiers usually cause a slight voltage drop and I think would be unsuitable for use with thermocouples, or am I up in the night? Has anyone heard of such a device? Any ideas?

[jimbo3123]

Originally Posted by Alaric

In this particular app the customer has a heated/cooled mold with a TC probe in the mold. Electrical contact is made with probe pads when the mold is clamped. Sometimes the operator can put the probe in upside down, and the part will be scrap. The customer needs a way to rectify the voltage from the TC so that it does not matter which way the probe is placed and he would prefer to not have to take time to align the positon of the probe and mold. He wants a "slap it in and push go" solution for max throughput.

I considered bringing the mV signal into the PLC and doing the polarity adjustments in software and linearizing it myself, but I'm looking for a better way.

Rectifiers usually cause a slight voltage drop and I think would be unsuitable for use with thermocouples, or am I up in the night? Has anyone heard of such a device? Any ideas?

Thermocouples rely on having special wiring so that there aren't other bi-metalic junctions to throw-off the reading. (That's why thermocouple connectors and wires have a specific polarity/alignment and can't be connected backwards) If the sensor is connected backward, you will have a negative voltage that is ALSO affected by this.

I'm afraid that this problem can't be corrected at the panel. You need to ensure that the operator aligns the mold so that the device is connected properly. I would look into modifying the mold to make it asymetrical (only able to go one way) if possible, and clearly labled if that isn't an option.

[CroCop]

Can you go to a non contact thermocouple?
Is it the + - leads that are getting flipped, or is it the probe being put in completely upside down?

[rdrast]

Maybe you should switch to RTD's.

[CroCop]

Originally Posted by rdrast

Maybe you should switch to RTD's.

Not with the temps he runs.

[randylud]

Thermocouple rectifier? Sorry, no such animal. Rectification is the process of changing AC to DC. The milivolt signal from a TC is already DC and therefore can not be rectified, no need. Perhaps you are thinking of some other polarity sensing situation, but rectifier is the totally wrong idea.

[TConnolly]

I guess I should have been more specific. A bridge rectifier is what I was thinking of. A bridge rectifier can not only used to change AC to DC, but it can also assure that the polarity of a DC signal is always the same. Its done all of the time. The problem I see here is the voltage drop across the rectifier, not that its already a DC signal. Thanks for the input though.

Jimbo,
This is an S type TC probe (expen$ive) so the extension leads are both copper (platinum's seebeck coefficient is 0) so it shouldn't matter which way its connected except at the instrument itself. Thats one reason a S type probe was selected. I really wish it was as simple as chaning the molds, but in this app that will cost several hundred thousand dollars, and that is out of the question.

Ron's suggestion for an RTD is very clever since RTDs don't care about polarity and it wouldn't matter which pad the third lead was on, or better yet, 4 leads could be used. Unfortunately I don't know of any RTD that has a wide enough temperature range.

[randylud]

I figured you meant something else, that is why I posted my reply. The diodes in the bridge or any other for that matter will require about .6 volts to put them into forward biased condition. The TC will not produce that kind of voltage and as you said, the drop would kill your accuracy.

[danw]

I'm still confusted about the real problem, but it seems the T/C gets installed somehow or oterh so that when it is connected it's reverse polarity on the input.

I have connected a single T/C to more than one input dozens of times, sometimes on two different devices (controller and high limit).

Years ago there could be problems with cold junction compensation and T/C break detection. Improvements in analog input technology have made those issures largely disappear.

Why not take whatever leads you have coming out and conect two sets of extension leads to those two T/C wires.

Run one set of extension leads to input A and wire it for standard polarity.

Run the other set of extension leads to input B and wire it for reverse polarity.

One input will read OK, the other won't. Pick the right one.

All it costs you is a econd AI input.

Dan

[Terry Woods]

One method that I can think of is to replace the T/C with a Thermistor-type detector, and of course, the appropriate controller.

Of course... that might not get much money for you.

Alternatively... you could talk them into modifying the mold and the T/C so that they have a keyway which would insure correct installation when assembled... thus forcing correct installation.

That won't get much money for you either. That might be a deal-killer... however, a little bit of clever thinking goes a long way! How about simply inserting a pin into the T/C connector and a matching hole in the mold? Do they have a macine shop?

Another option would be to modify the T/C connector in some way so that there is an additional set of contacts. That additional set of contacts, on the Mold side, would be connected to a diode (internal in the connector). The diode would be oriented to indicate proper installation. An external source would be applied, through a load, to those additional contacts. The source could be a simple 5V-DC. That circuit could then be run through a 5V PLC Input... or, through an opto-isolator which controls a 24V-DC source, or a 120V-AC source, to a PLC Input.

If the PLC "sees the light" from the opto-isolator... then the PLC sends a signal to the temperature controller indicating that the probe is correctly oriented. Until the PLC "sees the light"... the controller is prevented from operating.

You could make a few bucks on that one... however... modifying the Mold and/or the T/C might be a deal-killer. Although... if thought through... it could be a minor modification.

I've got yet another idea... one that doesn't involve any modification to the T/C or the mold... but I'll have to do a bit of thinkin' (MGD) to realize it!

Hint: Milli-Voltage Controlled Opto-Isolator (as opposed to current controlled). This would function, or not function, in a polarity sensitive manner.

[rsdoran]

I believe Alaric is actually thinking of a Wheatstone bridge system like a Thermistor would use. A thermistor nor RTD will usually go to the 2500 deg F range which is what I assume Alaric needs.

I am cofused how a probe could be put in upside down, the connector should only be able to connect one way using proper polarity, at least those I am familiar with.

Is there a special reason the probe has to be put in a certain way physically??

May just want to go to www.omega.com and use their configurator to see if you can find a different type probe that will work in the application.

If all of what I said is bullcrap then it may be easier to take it to the plc. Using a signal conditioner to change the signal to 4-20ma may offer a cheaper option than purchasing a T/C card.

[geniusintraining]

You may want to contact these guys

http://www.statesvilleprocess.com/profile.htm

I have all of my RTD's PT100, T/C's and other stuff made by them, they are up by Randy in NC, They may be able to come up with a 'well style' that may be robust enough for your applacation, their prices are good also.

[TConnolly]

Originally Posted by rsdoran

I am cofused how a probe could be put in upside down, the connector should only be able to connect one way using proper polarity, at least those I am familiar with.

This is a custom TC with contact pads rather than connector pins. When the mold is clamped contact is made with the pads. Normally + is to the right, - is to the left. If its in upside down, then the pads will be reversed, + on left, - on right.

[keithkyll]

Dan's suggestion of 2 Conditioner Modules is probably the cheapest way out of this. Parallel the 0-10 volt outputs with two Schottky diodes (.2 volt drop). The highest voltage output will prevail. Compensate for the .2V drop with an offset in the PLC. Keep the diodes at a stable temp to avoid drift.

1) A full-wave bridge with zero voltage drop is done with 4 op-amps. This would get too complicated unless you can find a commercial unit. I don't know a source.

2) Detect a low temp. If low, latch in a polarity reversing relay or a shutdown alarm.

3) Read temp. Fire heater for a few seconds. Read temp again. It will be lower if T/C is reversed. Energize reversing relay or alarm.

[randylud]

Originally Posted by keithkyll

Dan's suggestion of 2 Conditioner Modules is probably the cheapest way out of this. Parallel the 0-10 volt outputs with two Schottky diodes (.2 volt drop). The highest voltage output will prevail. Compensate for the .2V drop with an offset in the PLC. Keep the diodes at a stable temp to avoid drift.

1) A full-wave bridge with zero voltage drop is done with 4 op-amps. This would get too complicated unless you can find a commercial unit. I don't know a source.

2) Detect a low temp. If low, latch in a polarity reversing relay or a shutdown alarm.

3) Read temp. Fire heater for a few seconds. Read temp again. It will be lower if T/C is reversed. Energize reversing relay or alarm.

Just a point here about voltage drops. The .2 volt drop mentioned above is 200mV. Depending on the TC, the available voltage from a typical "J" type is in the range of 70mV maximum at the highest temperature it can measure. That is far less than the .2V needed to forward bias a diode. The problem here is not going to be solved by using diodes. Check out this link on TC output voltages. http://www.omega.com/temperature/Z/pdf/z203.pdf