comp input on S7-300 analog module

Hello guys,
can someone explain me what exactly is a comp input on S7-300 analog module? I never used it and in most cases we simply short circuit it. In which cases that comp input is needed. I suspect it's needed when using thermocouple, but don't know why.

Thanks

It is indeed a compensation input and only used in conjunction with thermocouples. As you know the thermocouple is based on the effect that the junction of two different metals generates a very small voltage depending on the temperature of the junction. This is however also true for the terminals on the input card. In other words, the connections on the input card also act as thermocouples. By placing an extra, equally configured junction at the place of the connection, you can take that effect into account and compensate the needed measurements for the error occuring in the connector.

That's also the reason why you always have to use special connectors if you are prolonging the leads of a thermocouple: the wires must be of the same material all the way otherwise you're placing extra thermocouples in your wiring, which will also generate errors.

Kind regards,

Thanks for reply jvdcande.
I understand your answer but still there is one thing unclear to me. If I use thermocouple, I still short circuit comp input right?
If not, what is connected to comp input then?
Thanks

Pandiani said:

Thanks for reply jvdcande.
I understand your answer but still there is one thing unclear to me. If I use thermocouple, I still short circuit comp input right?
If not, what is connected to comp input then?
Thanks

Click to expand...

I mean if campensation cable is used to connect thermocouple with analog input card, that piece of same compensation cable is used to short circuit COMP input.
Can you confirm this?

Pandiani,

I cannot confirm because I never actually used a thermocouple input card. So far I have always used converters near the sensors and depending on the distance used 4-20mA input cards or -10..+10V input cards. The info I gave you was derived from the books I have on control systems. Maybe someone else an confirm?

Kind regards,

if you jumper 10 and 11- (use standard copper wire)this uses the internal compensation for a thermocouple.

if you need external compensation- wire 10 and 11 to your compensation unit.

Please, can you explain this little further?
What if thermocouple is connected to compensation cable, and that cable I want to connect to analog input card, shoud I use copper wire to short circuit COMP (for internal compensation) or compensation cable?