Question about 4-20mA loop resistance

I normally see a maximum loop resistance specified, but for some reason this Honeywell STT350 temperature transmitter specifies a minimum resistance of 250 ohms.

Datasheet here:
http://hpsweb.honeywell.com/NR/rdonlyres/977AC3C2-13E2-4CC7-BA8A-8B1EC16D21FB/33084/34ST3332A.pdf

Scroll down to chart on bottom of page 33 of 66. (Honeywell page 25.)

I would like to use it with a Panel Meter with a 5 ohm input impedance. (page 45 of 52, Newport page 41)

Datasheet here:
http://www.newportus.com/Pdf/M4132n.pdf

I am using a 24vdc power supply.

Can anybody clarify for me why Honeywell has a minimum resistance requirement?

The Honeywell spec is normal. 250 Ohms converts to 0-5 volts.
I suspect a typo in the Newport manual, but see the same spec in several iSeries manuals at Omega. A 5 ohm load at 20 mA would convert to 0-100 millivolts. Doesn't make sense. I'm sure that should say 500 Ohm load. Converts to 0-10 volts internal. Contact Newport and/or Omega tech support. That's a serious typo.
If I'm wrong, then use the Newport in voltage mode, and add a precision resistor across the input. 500 Ohm for 0-10 volts.

I normally see a maximum loop resistance specified, but for some reason this Honeywell STT350 temperature transmitter specifies a minimum resistance of 250 ohms.

Click to expand...

You left out an important part of that spec. It needs a min. load to support communication. For example Hart communicators needs at least 250 ohms to work properly. See picture below.

Not to be picky but 4 to 20ma through a 250ohm resistor will give you 1 to 5 volts and 2 to 10 volts through a 500 ohm resistor.

Thanks Mickey - I finally got in touch with somebody at Honeywell who basically said what you did. The transmitter was furnished by a vendor and we don't plan to use the communications so the 5 ohms shouldn't be an issue.

learn something every day...

Mickey hit the nail on the head, the loop resistance is needed to produce enough impedance for HART or DE communications to function.

Once the device is configured, the loop needs only enough resistance for the receiver (analog input on a PLC/DCS/PAC/controller/recorder/indicator) to function properly.

The transmitter will fire up and function perfectly with only a fraction of an ohm of copper wire loop resistance to connect to a DC power supply, but you won't be able to establish HART or DE communications with that low loop resistance.