Process control instrument with loop overcurrent circuit

A process control instrument receives a DC current from a two wire loop and has overcurrent protection and reverse current protection circuits which pass a portion of the DC current as a device current to a process controlling device. The overcurrent protection circuit includes a current sensing circuit which conducts the device current and generates an output indicative of device current. The overcurrent protection circuit also includes a current diverting circuit coupled across the two wire loop and responsive to the sensor output for diverting loop current in excess of a predetermined upper limit of device current back to the loop when the sensor output indicates the upper limit is reached. The reverse current protection circuit is coupled across the loop and includes a variable impedance which conducts the device current. The variable impedance inhibits the device current from flowing through the device when a loop potential is reversed from a predetermined polarity. Thus, the device current is limited to a current range controlled by the variable impedance and the upper limit.

BACKGROUND OF INVENTION 
This invention relates to industrial process control instruments such as 
pressure transmitters, current to pressure (I/P) converters, and the like 
having overcurrent and reverse current protection circuits. 
In industrial process control systems, overcurrent and reverse current 
protection circuits operate between a two-wire DC current loop and a 
process control instrument. These protection circuits reduce the incidence 
of damage or degradation to the process control instrument from excessive 
and reverse polarity currents from the loop. Examples of such process 
control instruments include pressure, temperature, flow, pH, conductivity 
and the like transmitters, are shown for example in U.S. Pat. No. 
3,975,719, and current to pressure converters, valve actuators and the 
like, are shown for example in U.S. Pat. No. 4,481,967, both assigned to 
the assignee of the present application and incorporated herein by 
reference. 
Although a variety of operating ranges are used, two wire transmitters and 
I/P's typically operate in a loop current range of 4 to 20 mA, where loop 
current flows in one continuous loop. Energization of the loop is 
typically limited to a lower energy level incapable of igniting a 
combustible atmosphere. Therefore, since process control instruments 
operate remotely from control centers, the potential drops induced in each 
the loop wires supplying loop current and the process control instrument 
are critical in determining a maximum wire length for remote operation of 
the process control instrument. The application of process control 
instruments in the industrial process industry requires careful 
consideration of several system design parameters including lift-off 
potential of the process control instrument, which is desired to be 
reduced. Lift-off potential is a minimum potential necessary at an 
instrument to ensure that the process control instrument operates 
properly. Reducing potential drops of the overcurrent and reverse current 
protection circuits, each of which increase the lift-off potential of the 
process control instrument, allows for an increased potential drop in the 
loop wire thus increasing permissible wire length. 
Some instruments include a diode in series with the two wire loop to block 
the flow of reverse current, as taught in U.S. Pat. Nos. 3,975,719 and 
4,783,659 for two wire transmitters, both assigned to the assignee of the 
present application and hereby incorporated by reference. The diode 
produces a large potential drop in the instrument when current flows in 
the proper direction, which significantly increases the lift-off voltage 
and reduces the permissible wire length. 
U.S Pat. Nos. 3,975,719 and 4,783,659 also teach overcurrent protection 
circuits in two wire current transmitters. These Patents teach a resistor 
in series with the two wire loop and connected to the emitter of a 
transistor with one end of a Zener diode connected to a remote end of the 
resistor and the other end of the Zener diode connected to the base of the 
transistor. While this combination limits the maximum value of the loop 
current, a large potential drop is produced across the resistor during 
normal operation which further contributes to increasing the lift-off 
potential of the process control instrument. 
There is a need to provide a protection circuit that protects a process 
control instrument from large and reverse polarity currents from a loop 
where the currents can damage or degrade the instrument, while reducing 
the potential drop in the loop thus reducing a lift-off potential of the 
instrument. Further, there is a need to provide a protection circuit which 
diverts substantially no current back to the loop during normal operation 
thus improving accuracy of the instrument, in a simple, reliable and cost 
effective manner. A circuit that is substantially undamaged by either a 
decreased impedance or a short circuit of the two wire loop is desirable. 
SUMMARY OF THE INVENTION 
In the present invention, a process control instrument includes overcurrent 
and reverse current protection circuits, having reduced potential drops 
and reduced leakage currents, to reduce incidence of damage or degradation 
to a process control instrument, such as a pressure, temperature, flow, 
pH, or conductivity transmitter; or a current to pressure (I/P) converter, 
a valve actuator, or the like. 
The overcurrent protection circuit receives a DC loop current from a two 
wire loop and passes a portion of the loop current through the process 
control instrument as a device current. First current sensing means 
conducting the device current between the loop and the device provides a 
first output indicative of the device current amplitude. Current diverting 
means coupled across the two wire loop responds to the first output when 
device current flows at an upper limit by diverting a portion of the loop 
current back to the loop as a shunt current. Second current sensing means 
conducts the shunt current and provides a second output indicative of the 
shunt current amplitude. First impedance means having a first variable 
impedance conducting the device current responds to the second output by 
varying the first impedance to limit the device current to no greater than 
a predetermined upper limit. 
The reverse current protection circuit conducts the device current between 
the loop and the process control device and responds to the loop 
potential. The circuit has second impedance means having a second variable 
impedance for inhibiting the device current from flowing through the 
device when the loop potential is reversed from a predetermined polarity. 
The circuit diverts substantially no device current back to the loop when 
loop potential comprises a proper polarity and loop current is within the 
predetermined normal range.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a block diagram of a first embodiment of two-wire process 
control instrument 10 wired in series with power source 44 and a separate 
process control device 42 to form two-wire loop 11 carrying a DC loop 
current. For example, under fault conditions, loop current or voltage can 
exceed a predetermined normal range, e.g. 0-60 mA for a 4-20 mA loop where 
60 mA is an upper limit, which would damage or degrade performance of 
process control device 24, such as a pressure transmitter, in process 
control instrument 10. To reduce incidence of damage or degradation to 
process control device 24 from excessive and reverse polarity current, 
instrument 10 includes protection circuit 17 coupling current from loop 11 
to device 24. Protection circuit 17 receives DC loop current I.sub.1 
+I.sub.2 and passes only normal current or device current I.sub.1 of the 
loop current on to device 24. Circuit 17 limits device current I.sub.1 to 
a level in the normal range which prevents or reduces degradation of 
device 24. The remainder of the loop current, shunt current or overcurrent 
I.sub.2, is shunted back to loop 11 for controlling the level of device 
current I.sub.1. Shunt current I.sub.2 is substantially zero until 
amplitude of normal current I.sub.1 attains the upper limit in the normal 
range. 
Process control device 24, such as a pressure transmitter as disclosed, 
senses process variable "PV" as illustrated in FIG. 1. Process control 
device 24 controls amplitude of variable normal current I.sub.1 as a 
function of amplitude of process variable PV and process control device 42 
functions as a current sensing instrument, such as a recorder, by sensing 
amplitude of loop current I.sub.1 +I.sub.2, and correlating loop current 
amplitude to the process variable PV. 
On the other hand, process control device 24 can be an output device, such 
as a current-to-pressure(I/P) converter or a current-to-position 
converter. In this embodiment, the separate process current control device 
42, such as a controller, controls amplitude of loop current I.sub.1 
+I.sub.2, and process control device 24 controls the magnitude of its 
output, process variable PV, as a function of amplitude of variable normal 
current I.sub.1. 
The process control device 24 can also include a circuit for generating and 
receiving a time-varying-signal sent over loop 11 such as taught in U.S. 
Pat. No. 4,665,938 to Brown et al., hereby incorporated by reference. 
The present invention is suitable for use in a process control system 
utilizing multidrop process instruments, such as pressure transmitters, 
each connected in parallel across a two wire loop and processing 
information by superimposing digital data over a DC line. 
Loop 11 provides loop current I.sub.1 +I.sub.2 to terminal 12 in instrument 
10, which returns loop current to loop 11 from terminal 40 in instrument 
10. In instrument 10, overcurrent protection circuit 16 and reverse 
current protection circuit 20 isolate process control device 24 from 
excessive currents from the loop 11. Loop current I.sub.1 +I.sub.2 floss 
from terminal 12 to node A in overcurrent protection circuit 16. At node 
A, any overcurrent I.sub.2 flows to current sensing means 13, while normal 
current I.sub.1 flows to node B in reverse current protection circuit 20, 
as will be explained later. From node B, normal current I.sub.1 flows on 
to terminal 22 of process control device 24. Normal current I.sub.1 flows 
through process control device 24 back through terminal 26 of process 
control device 24 to conductor 31. Normal current I.sub.1 flows from 
conductor 31 through reverse current protection circuit 20 to conductor 33 
and on through overcurrent protection circuit 16 to node C of overcurrent 
protection circuit 16. At node C, overcurrent I.sub.2 from current 
diverting means 38 is summed with normal current I.sub.1 such that the 
entire loop current I.sub.1 +I.sub.2 flows from node C out through 
terminal 40 back to loop 11. 
Normal current I.sub.1 flowing through process control device 24 induces 
potential V.sub.2 across terminals 22 and 26. Potential V.sub.2 is 
representative of polarity of loop current I.sub.1 +I.sub.2. Potential 
sense means 18 coupled across a portion of loop 11 senses potential 
V.sub.2 and generates output 3 indicative of potential V.sub.2. Potential 
sense means 18 diverts substantially no device current I.sub.1 back to 
loop 11 when loop potential V.sub.2 is a proper polarity. When potential 
sense means 18 senses loop potential V.sub.2 to be the proper polarity, 
impedance of variable impedance 30 is reduced in response to output 3 and 
normal current I.sub.1 flows freely through process control device 24 and 
through variable impedance means 30 to overcurrent protection circuit 16. 
When, instead, potential sense means 18 senses potential V.sub.2 as 
reversed from the proper polarity or insufficient in magnitude, then 
impedance of variable impedance 30 is increased in response to output 3 
thus reducing normal current I.sub.1 flowing through process control 
device 24 and variable impedance means 30. Thus, reverse current 
protection circuit 20, having variable impedance 30 conducting device 
current I.sub.1, is responsive to output 3 which is a function of a loop 
potential V.sub.2, the protection circuit 20 diverting substantially no 
device current I.sub.1 back to loop 11 when loop potential V.sub.2 is the 
proper polarity. 
Amplitude of normal current I.sub.1 flowing through current sense means 34 
controls the level of output 1. The variable impedance of current 
diverting means 38 is responsive to the level of output 1. When normal 
current I.sub.1 flows at the predetermined upper level, output 1 reaches a 
level such that the impedance of current diverting means 38 is reduced so 
that shunt current I.sub.2 now flows through current diverting means 38 
and also through current sense means 13, which is in series with current 
diverting means 38. The amplitude of shunt current I.sub.2 flowing through 
current sense means 13 controls the level of output 2. The level of output 
2, which is indicative of shunt current I.sub.2, controls the impedance of 
variable impedance 32. The impedance of variable impedance 32 controls the 
amplitude of normal current I.sub.1 flowing from loop in series through 
process control device 24 and variable impedance 32 such that normal 
current I.sub.1 does not exceed the predetermined normal range. 
Hence, overcurrent protection circuit 16, which senses the magnitude of 
device current I.sub.1, prevents device current I.sub.1 from exceeding a 
predetermined upper limit which can damage or degrade the performance of 
process control device 24 even when loop current exceeds the predetermined 
upper limit. Further, the circuit induces a reduced potential drop in the 
two wire loop. Moreover, the circuit is relatively insensitive to the 
impedance of the additional device 42. 
FIG. 2 shows a schematic diagram of a second embodiment of two-wire process 
control instrument 46 wired in series with power source 44 and process 
control device 42 to form two-wire loop 43 carrying a DC loop current. 
Instrument 46 comprises overcurrent protection circuit 52, reverse current 
protection circuit 54 and process control device 24 where the two circuits 
are interposed between the loop 43 and the process control device 24. 
Overcurrent protection circuit 52 passes only normal current or device 
current I.sub.1 of loop Current through reverse current protection circuit 
54 to process control device 24. The remainder of the loop current, shunt 
current or overcurrent I.sub.2, is shunted by overcurrent protection 
circuit 52 back to loop 43. Overcurrent I.sub.2, as described later, 
generates an output which controls the impedance between nodes D and E to 
maintain the level of normal current I.sub.1, which flows between nodes D 
and E, in the normal range. Overcurrent I.sub.2 is substantially zero 
until device current I.sub.1 attains the predetermined upper limit. 
Two-wire loop 43 provides varying DC loop current I.sub.1 +I.sub.2 to 
terminal 12 of overcurrent protection circuit 52. Loop current I.sub.1 
+I.sub.2 flows from terminal 12 to node A. At node A, shunt current 
I.sub.2 flows to resistor 50, while normal current I.sub.1 flows to node B 
in reverse current protection circuit 54. At node B, normal current 
I.sub.1 is passed on to terminal 22 of process control device 24. Normal 
current I.sub.1 flows through process control device 24 to terminal 26 of 
process controlling device 24 and on to node D. Normal current I.sub.1 
flows from node D through node E to node C in overcurrent protection 
circuit 52. At node C, shunt current I.sub.2 from transistor 68 is summed 
with normal current I.sub.1 such that the entire loop current I.sub.1 
+I.sub.2 flows from node C out through terminal 40 of process control 
instrument 10 back to loop 43. 
Normal current I.sub.1 flowing through process control device 24 induces 
potential V.sub.2 across terminals 22 and 26. Potential V.sub.2 is 
representative of polarity of loop current I.sub.1 +I.sub.2. Enhancement 
type field-effect transistor (FET) 56 coupled across terminals 22 and 26 
senses polarity of potential V.sub.2 via resistor 94 across gate 80 and 
source 82. Substantially no device current is diverted back to loop 43 
from gate 80 to source 82, unlike the leakage from a base to an emitter if 
a bipolar transistor were substituted for FET 56. Leakage current reduces 
the accuracy of process control instruments and is undesirable. When loop 
current I.sub.1 +I.sub.2 is of correct polarity, FET 56 is enhanced by 
potential V.sub.2 and is turned on, where impedance across source 82 to 
drain 84 is substantially reduced to zero such that normal current I.sub.1 
flows freely through process control device 24 and FET 56 from source 82 
to drain 84. Substantially no potential drop is induced in loop 43 by 
reverse current protection 54 when FET 56 conducts device current I.sub.1 
since impedance from source 82 to drain 84 is substantially zero in 
comparison with other loop impedances. When loop current I.sub.1 +I.sub.2 
is reversed from the correct polarity such that damage to process control 
device 24 could occur, FET 56 operates in a depletion mode and is turned 
off, such that impedance from source 82 to drain 84 is increased thus 
substantially reducing normal current I.sub.1 flowing through process 
control device 24. 
Reverse current protection circuit 54 operates independently from 
overcurrent protection circuit 52 and can be interposed between loop 43 
and overcurrent protection circuit 52 and conducting the loop current 
I.sub.1 +I.sub.2. It can also be interposed between loop 43 and process 
control device 24 when overcurrent protection circuit 52 is omitted thus 
conducting device current I.sub.1 which here is the same as the loop 
current. 
Normal current I.sub.1 flowing from node E to node C through current 
sensing resistor 60 induces potential V.sub.4 which is indicative of the 
amplitude of normal current I.sub.1. When normal current I.sub.1 attains 
the upper limit of the normal operating range, potential V.sub.4 across 
base 68 to emitter 66 biases current diverting means bipolar transistor 62 
on, which can also be a FET or the like. Biasing transistor 62 on allows 
shunt current I.sub.2 to flow from node A through current sensing resistor 
50, transistor 62 via collector 64 to emitter 66, and resistor 90 to node 
C. Shunt current I.sub.2 induces potential V.sub.5 across resistor 50 
which is indicative of the amplitude of shunt current I.sub.2. Resistor 50 
can also comprise other current sensing means that generates a potential 
corresponding a current, such as a transistor. Potential V.sub.5 generates 
potential V.sub.6 =V.sub.1 -V.sub.5 -V.sub.4 across gate 74 and source 76 
such that enhancement type FET 70, serving as a variable impedance, 
operates in a depletion mode and is turned off. 
When FET 70 operates in the depletion mode, impedance from drain 72 to 
source 76 is substantially increased and substantially all normal current 
I.sub.1 flows from node D to node E through current limiting means 
resistor 58 instead of through FET 70 via the drain 72 to source 76, which 
is in parallel with resistor 58. Resistor 58, or equivalent high impedance 
current limiting means, has a large power rating for dissipating a large 
amount of power when conducting the majority of device current I.sub.1. 
Resistor 58 has a value such that the sum of potentials V.sub.2, V.sub.7 
and V.sub.4 induced by normal current I.sub.1 flowing at the upper limit 
is equal to terminal potential V.sub.1, thus limiting normal current 
I.sub.1 to the upper limit. FET 70 can be substituted by a bipolar 
transistor; however, leakage current across the base to emitter of the 
bipolar transistor degrades accuracy of instrument 46. 
When amplitude of normal current I.sub.1 is below the upper limit of the 
normal range induced potential V.sub.4 is insufficient to bias transistor 
62 on, substantially no shunt current I.sub.2 conducts through resistor 
50, potential V.sub.5 is substantially zero and FET 70 is enhanced and 
turned on. Enhanced FET 70 has a reduced impedance of substantially zero 
from drain 72 to source 76 and substantially all normal current I.sub.1 
flows from node D to node E through FET 70 via drain 72 to source 76, 
instead of through resistor 58, and potential V.sub.7 is reduced to 
substantially zero. Thus, the parallel combination of FET 70 and resistor 
58 functions as a variable impedance inducing a reduced potential drop for 
limiting device current I.sub.1 to the normal range, and is responsive to 
potential V.sub.5, which is indicative of shunt current I.sub.2. During 
normal operation, the only substantial potential drop induced in loop 43 
by overcurrent protection circuit 52 is V.sub.4, which is substantially 
less than a potential drop of a typical forward biased diode. All other 
components require only minimal wattage ratings thus reducing cost, size 
requirements and increasing reliability. Further, overcurrent protection 
circuit 52 operates independently of the impedance of power source 44, and 
does not highly stress any components while conducting normal current 
I.sub.1 flowing at the upper limit. 
Amplitude of normal current I.sub.1 below the upper limit generates a 
substantially reduced lift-off potential V.sub.1 =V.sub.2 +V.sub.4 
+V.sub.7 compared to conventional protection circuits. Process control 
instruments having reduced lift-off potentials, such as the invention 
disclosed, are very useful for the industrial process control industry. 
First of all, the likelihood of generating sparks across terminals 12 and 
40, which may ignite a combustible atmosphere, is reduced. Secondly, 
instrumentation with lower lift-off potentials increase the distance that 
a remote process control instrument can be located while operating from a 
safe power source level, such as that produced by power source 44. 
Protection circuits in process control instruments, such as the invention 
disclosed, that divert substantially no device current back to the loop 
during normal operation improve the accuracy of process control 
instruments. 
Resistor 92, connected from node E to base 68 of transistor 62, along with 
resistors 90 and 94, provide intrinsic safety protection for overcurrent 
protection circuit 52 and reverse current protection circuit 54. The 
circuits, as disclosed, have no components capable of substantial energy 
storage, such as a capacitor or an inductor, that are capable of 
discharging with sufficient energy to provide a spark which can ignite an 
explosive atmosphere. 
While the invention has been described with reference to preferred 
embodiments, workers skilled in the art will recognize that changes may be 
made in form and detail without departing from the spirit and scope of the 
invention.