Loop powered 4-20mA: where the volts go, and why the device stays dark

The call I get most often about a loop powered 4-20mA device is not that it reads wrong. It is that it does nothing. Dark display, no current on the clamp, somebody already writing a requisition for a new transmitter. Most of the time that transmitter is fine and the loop has no volts left to turn it on. So here is where a 24 volt supply actually goes, out of the manuals, plus the arithmetic that tells you before you order anything.

What loop powered 4-20mA actually means

Two wires. That is the whole idea. The pair carrying the 4-20mA signal also carries the power the instrument runs on, and the instrument sets how much current flows through itself to represent the measurement. Nothing else feeds it.

Which is why the bottom of the range is 4mA and not zero. The electronics have to keep drawing something to stay alive, and 0mA then means a broken wire instead of an empty tank.

Terminology is a mess here, and it decides how you wire the thing. Loop powered is the US shop term. On a European drawing the same device is a 2-wire passive input: passive because it does not source current, it only limits what a supply pushes through it. Active is the other side, a PLC analog output or a 4-wire instrument. Put two active devices on one pair and they fight.

Where the volts go

Every device sitting in series on a loop powered 4-20mA circuit takes a bite out of the supply, and the transmitter lives on the leftovers.

Emerson puts a hard floor under this in the Rosemount 3051 reference manual, publication 00809-0100-4007: the transmitter operates on 10.5 to 42.4 Vdc measured at the terminals of the transmitter. Not at the power supply. At the terminals, after everything else has taken its share. That floor also comes as a formula:

Maximum loop resistance = 43.5 x (power supply voltage minus 10.5)

Supply voltage	Maximum total loop resistance	Left over after a 250 ohm analog input card
12 Vdc	65 ohms	nothing, a 250 ohm card alone will not work
16.6 Vdc	265 ohms	15 ohms
24 Vdc	587 ohms	337 ohms
42.4 Vdc	1387 ohms	1137 ohms

 

Calculated from the load limitation formula in Rosemount publication 00809-0100-4007. The 265 ohm row is why that manual sets a 16.6 V minimum for a 250 ohm loop.

So a 24 volt supply buys 587 ohms of budget. Sounds like plenty until you start filling it in.

In the loop	Takes at 20mA	Source
Rosemount 3051 transmitter, minimum at its own terminals	10.5 V	00809-0100-4007
PLC analog input, 250 ohm sense resistor	5.0 V	1769-IN067B-EN-P
Loop powered 4-20mA panel display, plain current version	up to 1.0 V	BA01203O
Same display, HART version	up to 1.9 V	BA01203O
Same display again, backlight running off the loop	plus 2.9 V	BA01203O
BEKA BA3xx indicator	under 1.2 V	AG300
BEKA BA3xx with its backlight on the loop	up to 5.0 V	AG300
HART resistor at its 230 ohm minimum	4.6 V	BA01203O

 

Look at the two backlight rows. A display running its backlight off the loop is not a 1 volt device anymore, it is closer to 5. That one line at the bottom of a data sheet has cost more than one panel a supply upgrade it never needed.

On a loop powered 4-20mA circuit the transmitter is last in line. Everything ahead of it spends part of the supply first.

The 250 ohm problem, and why HART makes it tighter

HART does not relax any of this. It puts a floor under the loop resistance, which is a fixed voltage cost you do not get to skip.

The Rosemount manual is blunt about the instrument side: a minimum loop resistance of 250 ohms is required to talk to a Field Communicator, and a loop with 250 ohms in it needs at least 16.6 V. Endress+Hauser gives the protocol side in publication BA01203O: total load of the current loop, cable included, between 230 and 600 ohms, and below 230 ohms the digital signal is heavily damped or short circuited outright.

So resistance costs volts, and 250 ohms at 20mA is 5 volts gone. One way to get it for free: if the loop already runs through a PLC analog input with a 250 ohm sense resistor, that resistor is your HART resistance. Do not add a second one behind it. Two in series is 500 ohms and 10 volts, and you get a loop that reads beautifully at 4mA and falls apart near the top of the span. Good at low current, garbage at high current, that pattern almost always means the budget runs out above midscale.

The PLC card does not feed the loop

Anybody coming off a DCS gets caught by this one. On most DCS analog inputs the card hands out the 24 volts itself, you land two wires, done. PLC I/O usually does not. Rockwell puts it in plain language in the 1769-IF8 installation instructions, publication 1769-IN067B-EN-P: "The 1769-IF8 module does not provide loop power for analog inputs. Use a power supply that matches the input transmitter specifications." What the card gives you is a 250 ohm window to measure across. The volts are your problem.

Publication 1769-IN067B-EN-P is explicit: the 1769-IF8 does not provide loop power. Its 250 ohm input is a measuring window, not a supply.

Same trap on the SLC 500 side with the combination cards. A 1746-NIO4I has current outputs on it, which makes it look like it ought to be able to push a loop, and it will not power an input for you.

Sizing the supply is the easy part. Plan 24 Vdc and 25mA a channel, since a loop powered 4-20mA device cannot draw more than its top of range plus a little over-range headroom. Eight channels comes in under 5 W. Capacity is almost never what fails. Voltage is.

Devices that speak 4-20mA but are not loop powered

The confusing ones output a standard current signal and still need their own feed. Mag meters, Coriolis meters, most analyzers, mass flow controllers with a setpoint input. The electronics behind that output need far more than 20mA at 12 volts can deliver, so they get a separate 24 Vdc or 120 Vac connection plus a separate pair for the signal.

How to tell	Loop powered 4-20mA	Separately powered
Terminals	two, signal and power share the pair	four or more, in separate power and signal groups
Data sheet line to look for	voltage drop, or burden voltage	power consumption in W or VA
Supply spec reads like	10.5 to 42.4 Vdc at the device terminals	24 Vdc, or 100 to 240 Vac input
Loads it can run	LCD, HART electronics, a small backlight	large graphic display, relay coils, heated sensor
Land only the signal pair and	it runs	it stays dead

 

That data sheet line is the fastest check there is. A loop powered 4-20mA device specifies a voltage drop, sometimes called burden voltage, measured in volts. Turn to a separately powered device and the same slot on the sheet reads power consumption, in watts or VA. Different quantity, different animal.

Measuring a loop powered 4-20mA circuit without tripping the process

A 4-20mA clamp meter is what I reach for on anything running. It reads loop current without opening the circuit, and on a live line it is the only method I will use. Costs real money. Pays for itself the first time you do not have to break a loop in production.

Where the loop lands on a card with a 250 ohm input, put the meter across that resistance instead and read 1 to 5 volts. Nothing gets opened and you can do it at the terminal strip.

Ammeter in series comes last. Current is measured in series, always, which means breaking the loop and putting the meter in the gap. Fine on a bench. On a live loop the control system sees a dead transmitter for as long as your fingers take. And never put an ammeter across a loop instead of in it. That shorts the pair and you go find a fuse.

Three ways to read a loop powered 4-20mA circuit. Only the last one opens it, which is why it is last.

FAQ

What is meant by loop powered?

A device that takes all of its operating power from the same two wires carrying its 4-20mA signal. No separate supply terminals, no third wire. It runs on current the loop is already pushing through it.

How do I tell if something is loop powered 4-20mA?

Count the terminals and read one line on the data sheet. Two terminals plus a specified voltage drop or burden voltage means loop powered 4-20mA. Separate power terminals plus a power consumption figure in watts or VA means it is not.

Is loop powered 4-20mA active or passive?

Passive. A loop powered 4-20mA device does not source current, it regulates what a separate supply pushes through it. Active describes the side providing that current, normally a PLC analog output or a 4-wire instrument.

What is a loop powered 4-20mA signal?

Just current: 4mA at the bottom of the range, 20mA at the top, proportional in between. Loop powered describes where the device gets its power, not the signal format. The reading means the same thing whichever kind of device produced it.

Before you order the replacement

Measure the volts at the device terminals first. A loop powered 4-20mA transmitter reading 8 V at its own terminals is not broken, it is starved, and swapping it gets you two dead transmitters instead of one. If the volts are there and it still will not come up, then it is time to look at parts. IQElectro stocks tested HART transmitters, analog input and output cards, and isolators, including the older SLC 500 and Compact I/O modules that are getting hard to find new.