What is a loop-powered panel meter ?


Thread Starter

K.S. Neoh

Dear all :

Can anybody point me to literature that will explain the operational principles of a Loop-Powered panel meter ?

I have also read about "4-20mA" . Is this some kind of standard in the automation or communications community ?

Please excuse the newbieness. I guess everyone has to start at some point in time. Thanks for your patience.

[email protected]

Scott Faulkner

This is a std. in the power electrical industry. The 4-20mA refers to the input signal for the analog or digital ammeter, which is a device for displaying current flow. The meter can be calibrate for varying outputs, i.e., 20mA or full range could equal 100 amperes.
Does not take much explaining:
Firstly, when sending analog signals over any distance, current is traditionaly preffered to voltage because you do not introduce errors as a result of wire resistance. Allthougth modern electronics can easily avoid losses by
means of high input impedence, this also increases noise suscetibility.

Why 4 to 20mA?

Instead of using e.g. 0 - 20mA to represent a range of values, you use 4 to 20mA.

For example, a 'percentage display' would read 0 when there is 4mA at the input,and 100 when there is 20mA.

The advantage is
A: You can detect when the connection is open circuit (and thus display 'Error' rather than 0% error), and
B: if the device (display or transducer) consumes less than 4mA it may be powered by the loop current.

Note that the voltage source from which the current will be drawn may be on either side depending on cofiguration/requirements/capabilities. For
example a PLC reading from a remote 4-20mA sensor will source current on the input via the transducer (for example a 24V source will be connected to the input via the transducer in series, whilst a PLC powering a remote diplay will source a current from the output to power it.

There are also limits impossed by voltage limits, line powered units will require that a minimum voltage be maintained at 4mA, whilst outputs have a maximum output voltage.
Also, inputs will have an input resistance, manufacturers specifications detail these limits and usually give example calculations for thier apparatus.

Bruce Durdle

Mr K.S.Neoh and "S2"

Many analogue transmitters are effectively constant-current siources, with the current passed being modulated in accordance with the value of the parameter (pressure, temperature, flow, etfc) being measured. On this scale, 4 mA represents the Lower Range Value or "0%" of the measured signal range, with 20 mA being 100%. An open circuit results in 0 mA and can be clearly identified as a fault.

It is also possible for such an instrument to be powered by the current in the line - provided that 4 mA is enough to supply the internal electrionics. A 4-20 mA loop powered instrument requires only 2 wires and is typically powered from a 24 V DC supply, with the supply positive being fused at around 50 mA, connected to the transmitter and then the circuit completed by passing the return current through a 250 ohm resistor to the negative side of the supply (which is usually earthed).

Transmitters typically require about 12-14 V to operate, so provided the voltage drop in the circuit conductors is less than 5 v this poses no problem.

It is also possible to connect a meter in series with the line, again recognising voltage drop restrictions. Again, a meter may be analogue, but
there are digital displays which also take their power from the signal loop.

Hope this helps,

Jeffrey W. Eggenberger

A 4-20ma Current loop is a standard used in Industrial Automation and Controls. The reason for using the current loop is that it is not varied by line length and the resulting voltage drops. There are other standards used as well, but mostly I see the 4-20ma being used.

Jeffrey W. Eggenberger
Electrician: Industrial, Commercial, and Residential
A Loop powered indicator works without any separate power supply to switch it on. It normally works on the input signal 4 to 20 mA.
Loop powered indicator normally connect in a loop of the process signal. It's voltage drop is less than one volt. It can calibrate in any
engineering unit.

Devendra Pradhan
CRG Manager
Prolific Systems & Technologis pvt. Ltd.