4-20mA Loop Examples

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Application Note AN2301
SPLIT LOOPS WITHOUT A LOOP SPLITTER
Reduce costs using a low-cost single loop isolator
Users often specify dual-output 4-20mA loop splitters in their system applications. Common reasons include:
• The need for an output boost to drive several loads at once.
• The need to run independent isolated current loops to two different systems.
These needs often can be met with a simple, low-cost single loop isolator. Here’s how to save money!
Application 1: Boosting and splitting a transmitter’s output.
Many (not all) readouts on a 4-20mA loop use 250 ohm dropping resistors on their inputs; that is, they drop 5 volts at
20mA. Typical 4-20mA transmitters may drive 600, 800, 1000 or at most, 1200 ohms. This limits you to between two and
four 250 ohm loads on a loop.
Application 1 uses a JH4300C plug-in isolator to drive
five loads. In the example, the source transmitter needs
to drive only the local display plus the JH4300C’s input.
In fact, since the JH4300C uses only a 62 ohm input
resistor, you could put at least one more load in series
with the display.
The JH4300C’s output, which can drive 1200 ohms (24V),
drives the remaining four loads. Note that this
arrangement provides two isolated loops, so that the output
may be grounded separately from the source transmitter
if necessary. You might, for example, choose to ground
the controller’s input to minimize noise and common
mode pickup.
_
+
_
+
OUTPUT
INPUT
JH4300C
ISOLATOR
Application 2: Boosting and splitting a 2-wire transmitter’s output.
Similar in principle to Application 1, this example illustrates the use of a JH5050 isolator with a 2-wire (loop-powered)
transmitter. (The JH5050 is similar to the JH4300C but in a DIN-rail-mount housing.) The application includes an
intrinsic safety barrier for hazardous
(explosive) area protection; however, the
concepts are equally valid without the
barrier. Typically, the loop is driven from
a 24 volt dc supply.
+
INPUT
_
+
OUTPUT
_
JH5050 ISOLATOR
The 2-wire transmitter drops 10 to 12
volts, the readout 5 volts, the JH4300C
1-1/4 volts and the barrier a few more.
The use of a barrier forces the loop to be
grounded.
Again, the isolator provides a second,
isolated loop capable of driving up to 24V
of additional loads.
When are dual-output loop splitters required?
Primarily, when both loops must be isolated and the source transmitter’s output is not. This is most obvious in the above
2-wire application where the safety barrier forces the input loop to be grounded. If there is a need for grounding elsewhere
in the same loop (for example, if one of the readout device’s inputs must be grounded), then a dual-output isolator must be
used. Even without a barrier, the minus 24V supply is grounded in many systems.
The input transmitter itself can sometimes cause problems if its input is not isolated. The most common example is a
nonisolated transmitter used with a grounded thermocouple. If the input loop is also grounded anywhere else in the
system, unwanted currents will flow and the reading will be incorrect.
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Application 3: Isolators with built-in loop excitation.
This application is conceptually identical to Application 2 but uses
an isolator having a built-in 24V loop supply.
Models JH4305 (plug-in) and JH5040 (DIN-rail mount) include
the isolator and the 24V supply all in one instrument. The input
loop current flows from the positive supply to negative through the
System 1 input, barrier (optional), transmitter
and the 62 ohm resistor. Like the JH4300C and
JH5050, these isolators can power up to 1200
ohms of external load.
HOW ISOLATORS WORK
Models JH4300C and JH5050, like many isolators,
convert the input to pulses which are optically coupled
to the output. The output circuit converts the pulses
back to DC, proportional to the input.
The 4-20mA input flows through a 62 ohm resistor,
which converts it to a voltage input (1/4 to 1-1/4 volts)
to an integrator. The integrator works in conjunction
with the DC-to-pulse conversion circuitry to produces
pulses whose duty cycle (width) are proportional to
the input. The pulses drive an LED opto-isolator.
Output circuitry, powered from a separate transformer winding, amplifies the isolator’s output pulses to a precise 5-volt
level and filters them to create a voltage proportional to the input. The output amplifier converts the voltage to 4-20mA.
Zero and span trimpots allow precise adjustment of the output, or may be used to fine-tune overall system characteristics.
ISOLATORS AND TRANSMITTERS FROM JH TECHNOLOGY
PLUG-IN
JH4300C 4-20mA Isolator
JH4305 Isolator with Loop
Excitation Supply
DIN RAIL
JH5050 4-20mA Isolator
JH5040 Isolator with Loop
Excitation Supply
A full range of other input styles is available
FIELD MOUNT
FT5000 DC Input Isolator
FT5040 Isolator with Loop
Excitation Supply
Sarasota, FL USA
(800) 808-0300 / (941) 927-0300 / Fax (941) 925-8774
www.jhtechnology.com
e-mail [email protected]
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