APPS: 4-20ma and loop powered

Thu Oct 1 21:46:00 EDT 2009

==> Automation List post by Rick Irvine...
To view or reply to this thread: http://www.control.com/thread/1026245006#1254441290

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Re: APPS: 4-20ma and loop powered
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Transmitters are called 4 wire or 2 wire.  A 2 wire transmitter only has 2 wires connected to it.  The power to run it comes from the device it is connected to (in your case, an analog input card) which internally measures the current the instrument is drawing from the card. That is why it is important that there be enough voltage reaching the transmitter to allow the transmitter to operate. You will see a specification for the minimum voltage the transmitter needs. This is measured at the transmitter, not the source.

A 4 wire transmitter, has 4 wires, 2 for the 4-20MA output, and 2 for the power to supply the transmitter.  This is usually an AC Voltage. In this case, the transmitter provides the power for the device it is connected to, to measure the 4-20 MA coming from the transmitter.  Sometimes it is a device which simply measures the voltage drop of the 4-20 MA across a resistor. For example 4-20 MA across a 250 Ohm resistor gives a 1-5 VDC signal into the analog input card.  Most input devices have separate terminals to use either a 2 wire, (loop powered) or 4 wire (field powered) input.  But in either case, there will still be only 2 wires connected to the input card.  If you can get a schematic for the internals of the input card you will be able to see how the two different inputs are treated.
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