Current signal
systems
It is possible through the use of electronic
amplifiers to design a circuit outputting a constant amount
of current rather than a constant amount of voltage. This
collection of components is collectively known as a
current source, and its symbol looks like this:
A current source generates as much or as
little voltage as needed across its leads to produce a
constant amount of current through it. This is just the
opposite of a voltage source (an ideal battery), which will
output as much or as little current as demanded by the
external circuit in maintaining its output voltage constant.
Following the "conventional flow" symbology typical of
electronic devices, the arrow points against the
direction of electron motion. Apologies for this confusing
notation: another legacy of Benjamin Franklin's false
assumption of electron flow!
Current sources can be built as variable
devices, just like voltage sources, and they can be designed
to produce very precise amounts of current. If a transmitter
device were to be constructed with a variable current source
instead of a variable voltage source, we could design an
instrumentation signal system based on current instead of
voltage:
The internal workings of the transmitter's
current source need not be a concern at this point, only the
fact that its output varies in response to changes in the
float position, just like the potentiometer setup in the
voltage signal system varied voltage output according to
float position.
Notice now how the indicator is an ammeter
rather than a voltmeter (the scale calibrated in inches,
feet, or meters of water in the tank, as always). Because
the circuit is a series configuration (accounting for the
cable resistances), current will be precisely equal
through all components. With or without cable resistance,
the current at the indicator is exactly the same as the
current at the transmitter, and therefore there is no error
incurred as there might be with a voltage signal system.
This assurance of zero signal degradation is a decided
advantage of current signal systems over voltage signal
systems.
The most common current signal standard in
modern use is the 4 to 20 milliamp (4-20 mA) loop,
with 4 milliamps representing 0 percent of measurement, 20
milliamps representing 100 percent, 12 milliamps
representing 50 percent, and so on. A convenient feature of
the 4-20 mA standard is its ease of signal conversion to 1-5
volt indicating instruments. A simple 250 ohm precision
resistor connected in series with the circuit will produce 1
volt of drop at 4 milliamps, 5 volts of drop at 20
milliamps, etc:
----------------------------------------
| Percent of | 4-20 mA |
1-5 V |
| measurement | signal |
signal |
----------------------------------------
| 0
| 4.0 mA | 1.0 V
|
----------------------------------------
| 10 |
5.6 mA | 1.4 V |
----------------------------------------
| 20 |
7.2 mA | 1.8 V |
----------------------------------------
| 25 |
8.0 mA | 2.0 V |
----------------------------------------
| 30 |
8.8 mA | 2.2 V |
----------------------------------------
| 40 |
10.4 mA | 2.6 V |
----------------------------------------
| 50 |
12.0 mA | 3.0 V |
----------------------------------------
| 60 |
13.6 mA | 3.4 V |
----------------------------------------
| 70 |
15.2 mA | 3.8 V |
----------------------------------------
| 75 |
16.0 mA | 4.0 V |
---------------------------------------
| 80 |
16.8 mA | 4.2 V |
----------------------------------------
| 90 |
18.4 mA | 4.6 V |
----------------------------------------
| 100 |
20.0 mA | 5.0 V |
----------------------------------------
The current loop scale of 4-20 milliamps has
not always been the standard for current instruments:
for a while there was also a 10-50 milliamp standard, but
that standard has since been obsoleted. One reason for the
eventual supremacy of the 4-20 milliamp loop was safety:
with lower circuit voltages and lower current levels than in
10-50 mA system designs, there was less chance for personal
shock injury and/or the generation of sparks capable of
igniting flammable atmospheres in certain industrial
environments.
-
REVIEW:
-
A current source is a device
(usually constructed of several electronic components)
that outputs a constant amount of current through a
circuit, much like a voltage source (ideal battery)
outputting a constant amount of voltage to a circuit.
-
A current "loop" instrumentation circuit
relies on the series circuit principle of current being
equal through all components to insure no signal error due
to wiring resistance.
-
The most common analog current signal
standard in modern use is the "4 to 20 milliamp current
loop."
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