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THE 4-20mA CURRENT LOOPThe 4-2OmA current loop has been with us for so long
that it's become rather taken for granted in the industrial
and process sectors alike. Its popularity comes from its
ease of use and its performance. However, just because
something is that ubiquitous doesn't mean we're all
necessarily getting the best out of our current loops.A big benefit of the current loop is its simple wiring just
the two wires. The supply voltage and measuring current
are supplied over the same two wires. Zero offset of the
base current (ie. 4mA) makes cable break detection simple:
if the current suddenly drops to zero, you have a cable break.
In addition, the current signal is immune to any stray electrical
interference, and a current signal can be transmitted over
long distances.
Typical wiring for current output transducer.
You can think of the current loop itself as being analogous
to a water system. You have a hose pipe (the wires) and
a source tap (the power supply). You have a spray gun
that regulates the flow (the transducer). You can have
other equipment on the line, but it all has to be connected
together in a ring Ioop. The more holes (devices) you have
on the hose pipe, the higher the pressure will be required
from the tap. Relating all that back to the current loop,
you see a power supply, a transducer and one or more
pieces of instrumentation all connected together in a ring.You'll often hear things referred to as being either active
or passive. Some instruments have an active output which
includes both the control of the current in the loop as well
as provide the supply voltage. This is typically specified
as being a 4-20mA output into 10-750 Ohms, or something
similar. A passive input would be a simple resistor input that
has a voltage drop to be factored into the equation once
the supply voltage is chosen. This is typically specified as
a 4-20mA input into 10 Ohm.Working out the power supply requirement is a simple matter
of adding up all the units in the loop at maximum current
of 20mA. As an example, suppose you have a sensor
'regulator' which requires minimum 12V DC and instrumentation
of 10 Ohm input:10 Ohm x 20mA = 0.2V
So, for this circuit, a 12.2V minimum supply is required, the
sensor's maximum voltage might be specified at 30V, so a
24V supply would be all the circuit requirements with spare
capacity to boot.In order to measure the current loop it is necessary to break
the loop and insert a current meter into it. You can also
measure the voltage across the various components by in
the loop, such as the voltage out of the power supply, the
voltage over a sensor, and the voltages over the various
pieces of instrumentation. This information will give you a
good picture of what is happening within the loop.
Multi-instrument 4-20mA current loop with panel meter,
chart recorder, computers, etc.A question which is sometimes asked is whether it is possible
to use single power supply over several loops. This is possible,
but you have to ensure that the power supply can give enough
current to meet the needs of multiple loops. It is also the
case that the current loops will have the same zero negative
reference, which can cause a ground loop. In addition,
interference from one loop can affect all the other loops
driven from the one supply.This article is printed with the kind permission of
Morten Moller, who runs an internet support and
consultancy business and can be contacted at
morten@askmorten.co.uk
His website is at www.askmorten.co.uk
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