(summary) This is just a formatted-edited email thread exported from Eudora from my mailbox using the content concentrator. The circuit above was made by TF a student, This thread for circuit above has been put online to share information. The part under "From: delabs" are my writing. there may be some mistakes and broken links.
1) Reg diagram STC1000A :
Is the reference voltage REF used for cold temperature compensation ?
2) What is the function of q1 and q2 transistors ? Are they also BEL100N ?
3) What is the function of VR2 ?
4) What is the function of P2 , P1 ?
5) How can I calculate the voltage at the output IC2A?
6) Is is possible for you to send me a circuit explanantion which will help me to understand all the circuit operations ?
The circuit stc1000a.pdf is here name is "temperature controller, j and k input, pdf"
REF is pin no 13 in header of the connector CON1 of board STC1000A. The CON1 of STC1000A is plugged into CON1 of STC1000B main board. it goes to Stable Reference LM336 2.5 IC2 of STC1000B, the entire STC1000x series is a set. RT1 is for cold junction compensation, see how to build it in dact0006.pdf .
Q1 and Q2 are BC547 small signal NPN, The act like crude attenuators with the resistor networks. This circuit is to linearize the output, study other circuits in my page you will understand. When the analog voltage becomes a bit more than 1% off from a line the switch can be put on. You can even use things like CD4066 and other analog switches but it will increment the cost. IC2D is just a differential opamp circuit see theory.
This amp switches the transistors to linearize output at an area determined by VR2. P1 and P2 are for 'zero' cold junction and 'full scale' calibration, these are given in the theory section. IC2A is a buffer there is nothing to calculate, read the theory here you have to study many of the links i post and also many other links to other sites, that will help you more.
Read the Full Page - Design of a Thermocouple Amplifier Full