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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?

current loop design

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  • \$\begingroup\$ Do you have any control over the voltage supplied to the loop (indicated as "4V to 28V" on the schematic)? \$\endgroup\$ – The Photon Jan 17 at 5:07
  • \$\begingroup\$ @ThePhoton When I test, I apply only 24V. I didn't tested other level voltage yet. I have TVS SMAJ30CA only \$\endgroup\$ – BEWTY Rodbanyang Jan 17 at 5:39
  • \$\begingroup\$ The lower you can make this, the less power your BJT will consume, and the less it will heat up. \$\endgroup\$ – The Photon Jan 17 at 5:42
  • \$\begingroup\$ Note however, 4 V might not actually work. I'd stick with 5 V or so as a minimum if you want to actually be able to get 20 mA output while keeping the ADP1720 in regulation. \$\endgroup\$ – The Photon Jan 17 at 5:45
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    \$\begingroup\$ Not much you can do then. Whatever transistor you use there will be producing ~400 mW heat at 20 mA output. Only thing you can do is mechanically find a way to keep that heat away from the thermocouple. But without any idea about your physical/mechanical setup there's no way we can advise on that. \$\endgroup\$ – The Photon Jan 17 at 6:14

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