I am working on design loop current transmitter- to transmit reading value from thermocouple sensor using below design. I measured co-junction temperature using NTC instead of RTD.
During applying supply and connect load resister 500 ohms at Vloop-, the current flow 4-20mA when input is 0-100 degree. My co-junction reading start to increase from 25 degree to 27 degree.
My problem was when applying supply, BJT bc548 heats up when sensing current. It makes Co-junction temperature increases then my input temperature reading is not accurated.
I try to place NTC near input terminals and all the circuit is inside the small closed case. wherever I tried to place NTC, my co-junction reading still drifting from the heat of BC548.
I wonder Is there a away to reduce BJT heat during sensing the current?