| 2.1 Full-duplex
20 mA Circuit
Figure 1 is a full-duplex 20 mA current loop circuit.
Simultaneous two-way communications is possible with
this circuit. Two 20 mA current generators are necessary
with this circuit. It is possible to have one of the
two current generators in one current loop interface
and the other current generator in the other interface.
For example, the original IBM PC serial adapter card
had a current loop interface that contained only one
current generator. When you made a correct connection
to this current loop interface, the second current loop
device would need to provide one current loop generator.
Figure 1: Full-duplex 20 mA Circuit
2.2 The Simplex 20 mA Circuit
Figure 2 is a diagram of a simplex 20 mA current loop
circuit. The fundamental elements of a 20 mA current
loop are a current source, a current switch, and a current
detector. The transmitter is the current switch and
the receiver is the current detector. The interface
that contains the current source is called the active
unit and all other units are referred to as passive
units. Figure 3 is a diagram of the levels in an RS-232
interface and how they relate to the presence and absence
of current in a 20 mA current loop circuit. In a 20
mA loop the current flows when the loop is idle (no
data being transmitted). In a simplex type circuit a
number of transmitters and receivers are put in series
in a current loop. As long as only one transmitter sends
data, all receivers receive the data.
(Only one device
can transmit at a time)
Figure 2: Simplex 20 mA Circuit
Figure 3: Comparison of signal levels in an RS-232 Circuit
and a 20-mA Current Loop Circuit
2.3 Problems with 20 mA Current
Loop
The main problem with 20 mA current loop is that there
is no mechanical or electrical standard defined for
this interface. This makes every interface somewhat
unique and the user must know some of the technical
details about the circuits used in the interface.
Figure 4: Simplified One Direction Current Loop
Figure 4 is a simplified one-way
current loop implemented with two optocouplers, a voltage
source, and a resistor. Optocoupler U1 is the transmitter
and optocoupler U2 is the receiver. The value of the
loop current in this circuit is:
I loop = (Vs - V transmitter -V
receiver)/Rs
for typical optocouplers
When turned ON:
V transmitter (U1) = 0.2 V
When input LED is conducting:
V receiver (U2) = 1.8 V
If Vs = 12 volts & Rs = 470 ohms
then
I loop = (12V - (0.2V + 1.8V))/470 ohms
I loop = 10V/470 ohms = 21.3 mA
If we changed Vs = 60 V and left
Rs = 470 then
I loop = (60V -(0.2V + 1.8V))/470 ohms = 123 mA
If we changed Vs = 5 V and left
RS = 470 ohms then
I loop = (5V - (0.2V + 1.8V)/470 ohms = 6.4 mA
The point of showing these different
calculations is to demonstrate that the loop currents
circuit can vary by considerable amounts, if Vs is varied.
Likewise, if Rs was changed the loop currents could
also vary considerably. The only way to determine that
currents are near 20 mA is to examine the circuit in
detail.
2.4 Current Regulation in
Current Loops
Several methods can be used to control the amount of
current in a current loop circuit. This section will
illustrate several common methods of regulating the
current in a current loop.
2.4.1 Constant Current Generator
Current Source
Figure 5 is a circuit that uses a linear voltage regulator
integrated circuit to serve as a constant current source.
Almost any fixed or adjustable voltage regulator can
be used. The example shown in Figure 6 uses an LM317
adjustable regulator because is provides a low amount
of voltage drop (3 volts) across the current regulator
circuit. For example, if Vs was 12 volts in this circuit,
then the maximum voltage that the constant current regulator
could drive would be 9 volts. The 62 ohm, Rg resistor
sets the regulator current because there is an internal
voltage reference in the LM317 between VO and the ADJ
pins of 1.25 volts.
Figure 5: Constant Current Generator for a 20 mA Current
Loop
In a current loop, the sum of all
the voltage drops across all the devices must be less
than the voltage source, Vs driving the loop. Each device
in the current loop whether it is a transmitter (current
switch) or receiver (current detector) has some voltage
drop across it. For instance, a typical transistor switch
can have typically 0.2 volts drop across it. For most
of B&B Electronics converters, the voltage drop across
the transmitters can be as much 2.3 volts when the switch
is turned ON. The reason for this is that the transmitter
switch must provide for the reverse bias of the internal
photo detector diode inside the optocoupler. An optocoupler
used as a current detector will have from 1.2 to 2.0
volts drop across it.
Figure 6: Current Limiter built into Transmitter
2.4.2 Transmitter Current
Limiter
Some current loop interfaces incorporate current limiting
into the transmitter (current switch) itself. Figure
6 is an example of a circuit that has built-in current
limiting so that the loop current cannot exceed 20 mA.
In this circuit Rg provides a source of bias current
for Q2 so that if the loop current tries to exceed 20
mA Q2 will shunt Q1 base bias current so that Q1 will
not conduct more than 20 mA.
Figure 7. Current Limiter built into Receiver
2.4.3 Receiver Current Limiter
The circuit shown in Figure 7 is used not to regulate
the loop current, but to regulate the maximum emitter
current in the optocoupler, U1. This is done because
some optocouplers require less than 20 mA to operate
at maximum speed. Transistor Q1 is used to shunt some
of the loop current around the emitter of optocoupler,
U1.
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