Using test diodes to measure loop current

Another way to measure a 4-20 mA signal without interrupting it involves the use of a rectifying
diode, originally installed in the loop circuit when it was commissioned. A “test” diode may be
placed anywhere in series within the loop in such a way that it will be forward-biased. During
normal operation, the diode will drop approximately 0.7 volts, as is typical for any silicon rectifying
diode when forward biased. The following schematic diagram shows such a diode installed in a
2-wire transmitter loop circuit:

If someone connects a milliammeter in parallel with this diode, however, the very low input
resistance of the ammeters “shorts past” the diode and prevents any substantial voltage drop from
forming across it. Without the necessary forward voltage drop, the diode effectively turns off and
conducts 0 mA, leaving the entire loop current to pass through the ammeter:

When the milliammeter is disconnected, the requisite 0.7 volt drop appears to turn on the diode,
and all loop current flows through the diode again. At no time is the loop current ever interrupted,
which means a technician may take current measurements this way and never have to worry about
generating false process variable indications, setting off alarms, or upsetting the process.
Such a diode may be installed at the nearest junction box, between terminals on a terminal strip,
or even incorporated into the transmitter itself. Some process transmitters have an extra pair of
terminals labeled “Test” for this exact purpose. A diode is already installed in the transmitter, and
these “test” terminals serve as points to connect the milliammeter across.

Using a clamp-on milliammeter to measure loop current

One better way to measure a 4-20 mA signal without interrupting it is to do so magnetically, using
a clamp-on milliammeter. Modern Hall-effect sensors are sensitive and accurate enough to monitor
the weak magnetic fields created by the passage of small DC currents in wires. Ammeters using
Hall-effect sensors have are completely non-intrusive because they merely clamp around the wire,
with no need to “break” the circuit. An example of a such a clamp-on current meter is the Fluke
model 771, shown in this photograph:

Note how this milliammeter not only registers loop current (3.98 mA as shown in the photograph), but it also converts the milliamp value into a percentage of range, following the 4 to 20 mA signal standard. One disadvantage to be aware of for clamp-on milliammeters is the susceptibility to error from strong external magnetic fields. Steady magnetic fields (from permanent magnets or DC-powered electromagnets) may be compensated for by performing a “zero” adjustment with the instrument held in a similar orientation prior to measuring loop current through a wire.

Troubleshooting Current Loops using standard milliammeter to measure loop current

The most fundamental diagnostic method for troubleshooting 4-20 mA analog current
loops is to measure current and/or voltage at different points in the circuit. Several types of test
instruments are available for this purpose.

Since the signal of interest is represented by an electric current in an instrumentation current “loop”
circuit, the obvious tool to use for troubleshooting is a multimeter capable of accurately measuring
DC milliamperes. Unfortunately, though, there is a major disadvantage to the use of a milliammeter:
the circuit must be “broken” at some point to connect the meter in series with the current, and
this means the current will fall to 0 mA until the meter is connected (then fall to 0 mA when
the meter is removed from the circuit). Interrupting the current means interrupting the flow of
information conveyed by that current, be it a process measurement or a command signal to a final
control element. This will have adverse effects on a control system unless certain preparatory steps
are taken.
Before “breaking the loop” to connect your meter, one must first warn all appropriate personnel
that the signal will be interrupted at least twice, falling to a value of -25% each time. If the signal to
be interrupted is coming from a process transmitter to a controller, the controller should be placed
in Manual mode so it will not cause an upset in the process (by moving the final control element in
response to the sudden loss of PV signal). Also, process alarms should be temporarily disabled so
they do not cause panic. If this current signal also drives process shutdown alarms, these should be
temporarily disabled so that nothing shuts down upon interruption of the signal.
If the current signal to be interrupted is a command signal from a controller to a final control
element, the final control element either needs to be manually overridden so as to hold a fixed setting
while the signal varies, or it needs to be bypasses completely by some other device(s). If the final
control element is a control valve, this typically takes the form of opening a bypass valve and closing
at least one block valve:

Since the manually-operated bypass valve now performs the job that the automatic control valve
used to, a human operator must remain posted at the bypass valve to carefully throttle it and
maintain control of the process.
Block and bypass valves for a large gas flow control valve may be seen in the following photograph:

In consideration of the labor necessary to safely interrupt the current signal to a control valve in
a live process, we see that the seemingly simple task of connecting a milliammeter in series with a
4-20 mA current signal is not as easy as it may first appear.

Loop Powered Transmitters

A loop-powered transmitter gets its operating power from the minimum terminal voltage and
current available at its two terminals. With the typical source voltage being 24 volts DC, and the
maximum voltage dropped across the controller’s 250 ohm resistor being 5 volts DC, the transmitter
should always have at least 19 volts available at its terminals. Given the lower end of the 4-20 mA
signal range, the transmitter should always have at least 4 mA of current to run on. Thus, the
transmitter will always have a certain minimum amount of electrical power available on which to
operate, while regulating current to signal the process measurement.
Internally, the loop-powered transmitter circuitry looks something like this:

All sensing, scaling, and output conditioning circuitry inside the transmitter must be designed
to run on less then 4 mA of DC current, and at a modest terminal voltage. In order to create loop
currents exceeding 4 mA – as the transmitter must do in order to span the entire 4 to 20 milliamp
signal range – the transmitter circuitry uses a transistor to shunt (bypass) extra current from one
terminal to the other as needed to make the total current indicative of the process measurement.
For example, if the transmitter’s internal operating current is only 3.8 mA, and it must regulate loop
current at a value of 16 mA to represent a condition of 75% process measurement, the transistor
will bypass 12.2 mA of current.
Early current-based industrial transmitters were not capable of operating on such low levels
of electrical power, and so used a different current signal standard: 10 to 50 milliamps DC.
Loop power supplies for these transmitters ranged upwards of 90 volts to provide enough power
for the transmitter. Safety concerns made the 10-50 mA standard unsuitable for some industrial
installations, and modern microelectronic circuitry with its reduced power consumption made the
4-20 mA standard practical for nearly all types of process transmitters.

2-wire (“loop-powered”) transmitter current loops

It is possible to convey electrical power and communicate analog information over the same two
wires using 4 to 20 milliamps DC, if we design the transmitter to be loop-powered. A loop-powered
transmitter connects to a process controller in the following manner:

Here, the transmitter is not really a current source in the sense that a 4-wire transmitter is.
Instead, a 2-wire transmitter’s circuitry is designed to act as a current regulator, limiting current in
the series loop to a value representing the process measurement, while relying on a remote source
of power to motivate current to flow. Please note the direction of the arrow in the transmitter’s
dependent current source symbol, and how it relates to the voltage polarity marks. Refer back to the
illustration of a 4-wire transmitter circuit for comparison. The current “source” in this loop-powered
transmitter actually behaves as an electrical load, while the current source in the 4-wire transmitter
functions as a true electrical source.

4 wire Current Loop - Continuation

Typically, process controllers are not equipped to directly accept milliamp input signals, but
rather voltage signals. For this reason we must connect a precision resistor across the input terminals
to convert the 4-20 mA signal into a standardized analog voltage signal that the controller can
understand. A voltage signal range of 1 to 5 volts is standard, although some models of controller
use different voltage ranges and therefore require different precision resistor values. If the voltage
range is 1-5 volts and the current range is 4-20 mA, the precision resistor value must be 250 ohms.
Since this is a digital controller, the input voltage at the controller terminals is interpreted by
an analog-to-digital converter (ADC) circuit, which converts the measured voltage into a digital
number that the controller’s microprocessor can work with.

In some installations, transmitter power is supplied through additional wires in the cable from a
power source located in the same panel as the controller:

The obvious disadvantage of this scheme is the requirement of two more conductors in the cable.
More conductors means the cable will be larger-diameter and more expensive for a given length.
Cables with more conductors will require larger electrical conduit to fit in to, and all field wiring
panels will have to contain more terminal blocks to marshal the additional conductors. If no suitable
electrical power source exists at the transmitter location, though, a 4-wire cable is necessary to service
a 4-wire transmitter.

4-Wire Current Loop

DC electric current signals may also be used to communicate process measurement information from
transmitters to controllers, indicators, recorders, alarms, and other input devices. The simplest form
of 4-20 mA measurement loop is one where the transmitter has two terminals for the 4-20 mA signal
wires to connect, and two more terminals where a power source connects. These transmitters are
called “4-wire” or self-powered. The current signal from the transmitter connects to the process
variable input terminals of the controller to complete the loop:

Controller Output - Variable Speed Motor Drive

The input resistance of the motor drive circuit converts the 4-20 mA signal
into an analog voltage signal (typically 1-5 V, but not always). This voltage signal then constitutes
a command to the rest of the drive circuitry, telling it to modulate the power going to the electric
motor in order to drive it at the desired speed:

Controller- An Insight

Inside the controller, a dependent current source provides the 4-20 mA DC current signal to
the I/P transducer. Like all current sources, its purpose is to maintain current in the “loop”
circuit regardless of circuit resistance or any external voltage sources. Unlike a constant current
source, a “dependent” current source (represented by a diamond shape instead of a circle shape)
varies its current value according to the dictates of some external stimulus. In this case, either the
mathematical function of the controller (Automatic mode) or the arbitrary setting of the human
operator (Manual mode) tells the current source how much DC current it should maintain in the
circuit.
For example, if the operator happened to switch the controller into Manual mode and set the
output value at 50%, the proper amount of DC current for this signal percentage would be 12 mA
(exactly half-way between 4 mA and 20 mA). If everything is working properly, the current in the
“loop” circuit to the I/P transducer should remain exactly at 12 mA regardless of slight changes
in wire resistance, I/P coil resistance, or anything else: the current source inside the controller will
“fight” as hard as it has to in order to maintain this set amount of current. This current, as it flows
through the wire coil of the I/P transducer mechanism, creates a magnetic field inside the I/P to
actuate the pneumatic mechanism and produce a 9 PSI pressure signal output to the control valve
(9 PSI being exactly half-way between 3 PSI and 15 PSI in the 3-15 PSI signal standard range).
This should move the control valve to the half-way position.
 Usually, it takes the form of an operational amplifier circuit driven by the voltage output of a DAC (Digital-to-Analog Converter). The DAC converts a binary number (either from the controller’s automatic calculations, or from the human operator’s manual setting) into a small DC voltage, which then commands the op-amp circuit to regulate output current at a proportional value.

Controller output current loops

The simplest form of 4-20 mA current loop is the type used to represent the output of a process
controller, sending a command signal to a final control element. Here, the controller both supplies
the electrical power and regulates the DC current to the final control element, which acts as an
electrical load. To illustrate, consider the example of a controller sending a 4-20 mA signal to an
I/P (current-to-pressure) signal converter, which then pneumatically drives a control valve:

This particular controller has two digital displays, one for process variable (PV) and one for
setpoint (SP), with a bargraph for displaying the output value (Out). One pushbutton provides
the operator with a way to switch between Automatic and Manual modes (A/M), while two other
pushbuttons provide means to decrement and increment either the setpoint value (in Automatic
mode) or the Output value (in Manual mode).

Reverse-Acting I/P transducer signal Calculation

A current-to-pressure transducer is used to convert a 4-20 mA electronic signal into a 3-15 PSI
pneumatic signal. This particular transducer is configured for reverse action instead of direct,
meaning that its pressure output at 4 mA should be 15 PSI and its pressure output at 20 mA should
be 3 PSI. Calculate the necessary current signal value to produce an output pressure of 12.7 PSI.
Reverse-acting instruments are still linear, and therefore still follow the slope-intercept line
formula y = mx + b. The only differences are a negative slope and a different intercept value.
Instead of y = 16x + 4 as is the case for direct-acting instruments, this reverse-acting instrument
follows the linear equation y = −16x + 20:

First, we need to to convert the pressure signal value of 12.7 PSI into a percentage of 3-15 PSI
range. We will manipulate the percentage-pressure formula to solve for x:

Next, we plug in the 12.7 PSI signal value and solve for x:

This tells us that 12.7 PSI represents 80.8 % of the 3-15 PSI signal range. Plugging this percentage
value into our modified (negative-slope) percentage-current formula will tell us how much current is
necessary to generate this 12.7 PSI pneumatic output:

Therefore, a current signal of 7.07 mA is necessary to drive the output of this reverse-acting I/P
transducer to a pressure of 12.7 PSI.

pH Transmitter Calculation

A pH transmitter has a calibrated range of 4 pH to 10 pH, with a 4-20 mA output signal. Calculate the pH sensed by the transmitter if its output signal is 11.3 mA.
First, we must convert the milliamp value into a percentage. Following the same technique we
used for the control valve problem:

Next, we take this percentage value and translate it into a pH value, given the transmitter’s
measurement span of 6 pH (10 pH − 4 pH)and offset of 4 pH:

Therefore, the transmitter’s 11.3 mA output signal reflects a measured pH value of 8.56 pH.

Temperature Transmitter Calculation

A pneumatic temperature transmitter is ranged 50 to 140 degrees Fahrenheit and has a 3-15 PSI
output signal. Calculate the pneumatic output pressure if the temperature is 79 degrees Fahrenheit.
First, we convert the temperature value of 79 degrees into a percentage of range based on the
knowledge of the temperature range span (140 degrees − 50 degrees = 90 degrees) and lower-range
value (LRV = 50 degrees). We may do so by manipulating the general formula for any linear
measurement to solve for x:

Next, we take this percentage value and translate it into a pneumatic pressure value using the
formula previously shown:

Therefore, the transmitter should output a PV signal of 6.87 PSI at a temperature of 79o F.

Transfer of PLC Program to Memory Card

Considering ABB PLC and ABB Control Builder V2.1.0.

Flow Transmitter- Calculation

A flow transmitter is ranged 0 to 350 gallons per minute, 4-20 mA output, direct-responding.
Calculate the current signal value at a flow rate of 204 GPM.
First, we convert the flow value of 204 GPM into a percentage of range. This is a simple matter
of division, since the flow measurement range is zero-based:

Next, we take this percentage value and translate it into a milliamp value using the formula

previously shown:

Therefore, the transmitter should output a PV signal of 13.3 mA at a flow rate of 204 GPM.

Controller output to Valve Calculation

An electronic loop controller outputs a signal of 8.55 mA to a direct-responding control valve (where
4 mA is shut and 20 mA is wide open). How far open should the control valve be at this MV signal
level?
We must convert the milliamp signal value into a percentage of valve travel. This means
determining the percentage value of the 8.55 mA signal on the 4-20 mA range. First, we need
to manipulate the percentage-milliamp formula to solve for percentage (x):

Therefore, the control valve should be 28.4 % open when the MV signal is at a value of 8.55 mA.

Relating 4 to 20 mA signals to instrument variables

Calculating the equivalent milliamp value for any given percentage of signal range is quite easy.
Given the linear relationship between signal percentage and milliamps, the equation takes the form
of the standard slope-intercept line equation y = mx + b. Here, y is the equivalent current in
milliamps, x is the desired percentage of signal, m is the span of the 4-20 mA range (16 mA), and
b is the offset value, or the “live zero” of 4 mA:

This equation form is identical to the one used to calculate pneumatic instrument signal pressures
(the 3 to 15 PSI standard):

The same mathematical relationship holds for any linear measurement range. Given a percentage
of range x, the measured variable is equal to:

Signals and Process

DC current signals are also used in control systems to command the positioning of a final control
element, such as a control valve or a variable-speed motor drive (VSD). In these cases, the milliamp
value does not directly represent a process measurement, but rather how the degree to which the
final control element influences the process. Typically (but not always!), 4 milliamps commands a
closed (shut) control valve or a stopped motor, while 20 milliamps commands a wide-open valve or
a motor running at full speed.
Thus, most industrial control systems use at least two different 4-20 mA signals: one to represent
the process variable (PV) and one to represent the command signal to the final control element (the
“manipulated variable” or MV):

The relationship between these two signals depends entirely on the response of the controller.
There is no reason to ever expect the two current signals to be equal, for they represent entirely
different things. In fact, if the controller is reverse-acting, it is entirely normal for the two current
signals to be inversely related: as the PV signal increases going to a reverse-acting controller, the
output signal will decrease. If the controller is placed into “manual” mode by a human operator,
the output signal will have no automatic relation to the PV signal at all, instead being entirely
determined by the operator’s whim.

4-20 mA- Calibration

For example, if we were to calibrate a 4-20 mA temperature transmitter for a measurement range
of 50 to 250 degrees C, we could relate the current and measured temperature values on a graph like
this:

This is not unlike the pneumatic instrument signal standard or 3 to 15 pounds per square
inch (PSI), where a varying air pressure signal represents some process measurement in an analog
(proportional) fashion.