Type J thermocouple connected to Siemens analog input card, measuring temperature inside an injection mould. TC wires in close proximity to heater wires in same wireway. When heaters short out, TC cables carry surge (spike) into AI card and damage card. Can anybody recommend some kind of interface, e.g. opto isolator that will protect AI card?

crowther. steven at gmail. com

 

I'd suggest a Phoenix MINI MCR-SL-TC-UI module:
input J or K thermocouple, output 0..10V or 4..20mA

ordercode for a pre-configured 0..10V output: 2864448/J/0/<temprange>/OUT03/A

 

We use temperature transmitters between thermocouples and 4-20ma AI card with siemens plcs. You can can use Sitrans transmitter if you want an all siemens solution or you can choose between dozens of other producers.

 

Replacing the thermocouple with an ungrounded style will solve your problem.

The thermocouple is almost certainly in a metallic sheath.

The really cheap style has no insulation between the sheath and the T/C. The T/C junction will touch the sheath. That is definitely a grounded T/C, a path for the high voltage to damage your AI card.

The other style is mineral insulated, where the T/C is insulated by, typcially, magnesium oxide, MgO, a white powder. But, When you buy an MgO style, you have to pay extra (typically 2 USD) to buy the 'ungrounded' style, where care is taken in the manufacturing so that the T/C junction is insulated and therefore isolated from the metallic sheath.

Replace the thermocouple.

Issac