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I just tried to use two different ones of these RTD transmitters with a pt100 probe (3 wire) with no success.

I am connecting the transmitter directly to a 4-20mA input on a PLC, as shown in the following circuit diagram. Using 24VDC supply.

However, the unit always flows around 32mA through the current loop, no matter what temperature the probe is, or whether the probe is even connected. This exact same thing has happened with two of these transmitters. I assumed the first one was bad, but then when the second one did the same thing, I started to wonder what I had done wrong.

Any advice appreciated.

Diagram: enter image description here

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    \$\begingroup\$ 32mA suggests a faulty transmitter, or perhaps incorrect supply polarity. \$\endgroup\$ – Jasen Jan 20 at 0:11
  • \$\begingroup\$ @Jasen, thanks. As stated, I ordered one, tried it, then returned it for a replacement and tried that one too. I can confirm that the + side of the PSU is connected directly to the + on the transmitter and the - side goes directly to the PLC's analog input, which then leads to ground as expected. I've used other transmitters like the Minco units with absolutely no problems in the same type of setup. \$\endgroup\$ – Ryan Griggs Jan 20 at 0:46
  • \$\begingroup\$ I wonder what the supplier said. Perhaps ESD was a contributing factor and what did you use for a collector assuming polarity was correct. Measure the current using lower supply voltages and report back. \$\endgroup\$ – Tony Stewart EE75 Jan 20 at 1:53
  • \$\begingroup\$ 32mA is typical of what a transmitter will output with an open sensor. Connect a 100 ohm resistor directly across the terminals A to B and short B to B' and measure the current (preferably with a handheld meter and 24V power supply. Check the resistor first. Just those four things and no other connections. If it still measures 32mA the transmitter is faulty. Maybe they got smashed up in shipping and the terminal block connections are bad. \$\endgroup\$ – Spehro Pefhany Jan 20 at 3:27
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    \$\begingroup\$ @SpehroPefhany you're absolutely right. The pt100 probe is no good: it reads "open circuit" between the two sides. I inserted another 2-wire probe, shorting the two "B" terminals together, and everything works like a charm. Thanks for pointing this out! \$\endgroup\$ – Ryan Griggs Jan 20 at 15:01
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32mA is typical of what a transmitter will output with an open sensor. Connect a 100 ohm resistor directly across the terminals A to B and short B to B' and measure the current (preferably with a handheld meter and 24V power supply. Check the resistor first. Just those four things and no other connections.

If it still measures 32mA the transmitter is faulty. Maybe they got smashed up in shipping and the terminal block connections are bad (seems unlikely two would be like that). Or maybe the sensor is bad. RTD sensors are prone to failure at the element junctions.

Divide and conquer.

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