current loop


I’d like to explain about current loop which usually used in instrument and process control systems, remember… CURRENT LOOP not CURRENT LOVE, hahahahaha…stt… dont curcol on the blog please. hihi.


1. what do you know about 4-20mA?

2. why we transmits a current not voltage?

3. how to measure a current loop?

4. how to generate a simple current loop from a microcontroller?

5. how its use? 4-wire? 2-wire?

6. what systems that applicable for this?


lets study one by one, I’m  not smart, but maybe I’m one step ahead from you.


4-20mA :

First appearing in the 1950’s with the advent of electricaland electronic controls, the 4-20mA signal standard reigns
as one of the most popular mediums for signal transmission and electronic control in industrial environments nearly
60 years later, at that year Indonesia still playing sharp bamboo.

Prior to the widespread adoption of electrical and electronic controls, buildings often used pneumatic control
systems. Large and powerful compressors drove 3psi to 15psi pneumatic signals throughout a plant and these
pneumatic lines connected to pneumatically controlled valves and pneumatically controlling valves in order to drive
proportional controls and actuators throughout the building, all powered from compressed air. Air pressure at 3psi
served as the “live-zero” and 15psi represented 100%. In this way, the more modern 4-20mA signal standard
emulated the earlier 3-15psi pneumatic controls. Any pressure below 3psi was considered “dead zero” and an alarm
condition. Some installations still use pneumatic control today. Modern I/P converters (current-to-pressure
transducers) are available to convert the 4-20mA control loops to common pneumatic ranges, such as 3-15psi, 1-18psi, 3-27psi, and 6-30 psi.

now, lets see this triangle | relation between R.I.V R(Resistance)/I(Current)/V(Voltage).

Current Loop

From the triangle, I can quickly identify the three common components of a current loop, how they are wired together,
and even the direction of current flow. For my mental model, each side of the triangle represents a component of the
current loop. The vertices of the triangle represent a wired connection between these components. For reference, I
also place a positive/plus sign on the “peak” of the triangle. I will also use the first three letters of “TRIangle” to
identify the principle components. Accepting the convention that current flow will move from the supply positive to
and return to the supply negative, I see that current moves counter-clockwise in my symbolic current loop.

Thats enough for phase one, I’ll continue later…


current loop

One thought on “current loop

  1. Heinrich Baumann says:

    Good explanation.
    I would like to add one more thing. In modern process control equipment many controllers are active. This means there is no fixed resistor to evaluate the signal. Many PLC systems have a constante voltage of about 1Volt at the input. Still able to evaluate the applied 4-20mA signal with 16 bit accuracy.
    The 4-20 mA system is arround for 60 nyears but still evaluating.
    This improvement in signal evaulation also means the current loop systems can not be tested with a simple potentiometer. The use of more sophisticated test equipment is neccesary.

    Best regards,

    Heinrich Baumann

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