Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to a two wire current transmi-tter
where current through a DC source and load is controlled by
the transmitter to correspond with the magnitude o~ a value of
a parameter to be sensed by a sensing element, which may typi-
cally be a thermocouple wherein DC isolation is provided, or
a temperature sensitive resistor without isolation.
U.S. Patent No. 3,573,599 issued April 6, 1971,
discloses a transformer coupling to provide DC isolation to
a sensing circuit from a supply circuit in a two wire current
transmitter. The sensing circuit includes AC amplifier, whose
input repetitively samples and compares the signals of the DC
sensor network from the DC feedback network which is transformer
coupled to the supply circuit. A DC amplifier on the supply
side acts as the current controller for the supply current.
Multiple AC coupling means are required between a sensing element
and a supply current.
A two wire transmitter is described in U.S. Patent No.
3,764,880 issued October 9, 1973, wherein a single transformer
is provided for DC isolation between a transducer circuit and a
source circuit. The source circuit is connected to an input
side of a voltage regulator which provides a regulated voltage
t~ a DC to DC converter over the current range of the trans-
mitter which may be, for example, 4-20 milliamperes. The vol-
tage regulator of this circuit requires only a small, substan-
tially constant operating current to provide required operating
voltage regulation to the converter circuit.
The present invention comprises a two wire electrical
transmitter having a voltage regulator circuit which provides
two sources of start~up current to the voltage regulator com-
ponents upon energization of the transmitter circuitry. Theadditional current source overcomes a long standing problem
associated with two wire transmitter circuits of slow circuit
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start-up or circuit initialization. The voltage regulator
circuit may be used with an isolated sensing circuit as shown
herein, or with nonisolated sensing circuits if desired.
Reference is made to the accompanying drawing wherein:
The single figure of the drawing is a block diagram
representation of a typical transmitter circuity and a detailed
electrical schematic representation of a voltage regulator
circuitry incorporated into the two wire transmitter of the
present invention.
Referring to Figure 1, a DC power supply 10 and a
series load resistor 11, both of which may be remotely located
from the rest of the circuitry are series connected to a vol-
tage regulator 20 outlined in dotted lines, by a line 12 through
a terminal 12A and by a line 13 through a terminal 13A. Voltage
regulator 20 provides a stable output DC voltage which is im-
pressed on a DC to AC converter 40 through a voltage regulator
output line 41 and line 13. DC to AC converter 40 is a semi-
conductor oscillator which converts stable output voltage of
regulator 20 to an alternating voltage in a known manner. The
AC voltage is impressed on a primary winding of a transformer
45 which is directly connected to the output of DC to AC
converter 40. 'rransformer 45 provides for electrical isolation
of transducer circuitxy of a sensor portion on the output side
from undesirable interference and transients. The functions
and operation of the transformer are further explained in U.S.
Patent 3,764,880, for example. Changes in current flows in the
transformer secondary result in a corresponding change in the
current in the transformer primary ~inding.
Voltage regulator 20 has a diode 12B in line 12 for
circuit polarity reversal protection. Diode 12s is connected
at its anode to line 12. A resistor 18 is connected to the
cathode of diode 12B and in turn resistor 18 connects in series
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to a resistor 17, which is connected at its opposite end through
a diode l9A to the cathode of a Zener diode 19 ~hich in turn
has its anode connected to line 13. A transistor 15 is coupled
to a transistor 14 to comprise a dif~erential amplifier. The
noninverting input of this amplifier comprises the base of
transistor 15, which is connected between the resistor 17
and the anode of diode l9A. In operation the combination of
diode 19A and diode 19 provide a stable reference voltage at
the base of transistor 15. A capacitor 16 is connected across
diodes 19 and l9A to suppress transients. The inverting input
of the differential amplifier comprises the base of transistor
14. The base of the transistor 14 is connected to line 41
through a diode 23, a resistor 21 and a capacitor 22. One end
of a line 21A is connected between the cathode of diode 23
and resistor 21 and at its second end between resistors 17 and
18. The base of transistor 14 is also connected between a re-
sistor 24 and a resistor 25 which are in series and comprise
the regulated voltage sense line connected across lines 41
and 13. A capacitor 18A is connected across input terminals
12A and 13A for RFI (radio frequency interference) suppression
and a filter capacitor 41A is connected across line 41 and 13.
A transistor 32 and a transistor 34 comprise a com-
plementary series regulator transistor pair. The collector
of transistor 15 is connected to the base of transistor 32
throuyh a line 16A. The emitter of transistor 32 is connected
to the cathode of diode 12B and the collector of transistor
32 is connected to the base of transistor 34. The collector
of transistor 34 is connected to the cathode of diode 12B
and the emitter of transistor 34 is connected to line 41.
The collector of transistor lA is connected to line
16A between the collector of transistor 15 and the base of
transistor 32 through capacitor 26. The collector of transis-
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tor 14 is also connected between capacitor 22 and resistor 21.
The emitter of transistor 14 is connected to the emitter of
transistor 15. The emitters o~ transistors 14 and 15 are
connected through a resistor 28 to line 13.
Voltage regulator 20 begins operation in the
following manner. ~s DC power supply 10 starts increasing
in voltage af~er being initially connected in the circuit,
or switched on, diode 23 is re~erse biased and diode 19, the
reference voltage diode, ~s not conducting. Transistor 15
of the differential amplifier begins to forward bias and a
further increasing of voltage from DC power supply 10 causes
transistor 15 to begin conducting, which turns on transistor
32 through the collector of transistor 15 and connection to
the base of transistor 32. Transistor 32 in turn then pro-
vides base current to transistor 34, hence, transistor 34
starts conducting. T~hen the regulator output voltage on
line 41 is greater than t~e voltage on line 21A, diode 23
starts to forward bias. This condition provides a second
start-up current for the regulator circuit through diode 23,
20 line 21A and resistor 17 to the base of transistor 15 which,
secon~ start-up current is thus summed with the first start-
up current and causes more conduction of transistor 15.
Further increase in voltage of ~C power supply 10 on line 12
starts reference diode 19 conducting, establishing the desired
reference level signal at the base of transistor 15 and
consequently regulating the voltage across lines 41 and 13.
During operation the base of transistor 14 is
biased hy the output voltage of regulator 20 across lines 41
and 13. If the output voltage increases slightly, transistor
30 14 becomes more conductive which causes transistor 15 to
decrease conduction. Reduced conduction in transistor 15
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provides less current to the complementary pair of transistor
31 and transistor 34, thus decreasin~ output volta~e. The
operation of voltage regulator 20 is s~milar but opposite
for a decreasing voltage~ Regulator 20 uses a very constant
small current which is substantially constant with changes in
temperature. The sum of the first and second start-up
currents also remain substant~ally constant during operation
(after start-up when the output voltage is regulated) in
that the voltAge drops across the diode 23, the resistor
17, the diode l9A and the diode 19 are constant. This is
important in an isolated two wire transmitter since the
current in the regulator does not pass through the delta
feedback resistor network which will be explained.
On the sensor side of transformer 45 the trans-
former secondary is connected to a rectifier 46, which
converts AC voltage from transformer 45 to a stable DC
voltage in a normal manner for powering the transducer
circuitry shown in block diagram form~ The transducer control
circuitry is shown by way of example only and may take any
desired form. As shown, rectifier 46 is connected to the
subsequent circuitry by lines 47 and 48. An oscillator 110
receives power from line 47 and is connected to line 49 and
isalso connected to the control inputs of a modulator 100 and
a demodulator 70. Oscillator 110 provides through the control
inputs to modulator 100 and demodulator 70 a series of pulses
such that a first and a second signal sensed at the first and
second inputs to modulator 100, are alternately modulated and
demodulated, respectively, An adjustable current source 130
is connected to line 47 and is connected by line 143 and
terminal 154A to a sensor network 150. Sensor network 150 on
one side has a zeroing variable resistor 154 which is connected
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to terminal 154A. Resistor 154 is also connected to a sensor
200 which may be a thermocouple as sho~l. Sensor 200 is
connected at a second end to a first input of modulator 100.
This input to modulator 100 carries a signal representative
of the parameter to be sensed in th~s instance, temperature.
Also connected to a second end of zeroing resistor 154 and a
first end of sensor 200 is a resistor 201 which may provide
cold junction compensation. The combination of an adjustable
resistor 203, resistor 202, and a resistor 15~ is a "delta
entrant bridge" feedbac~ network. The feedback network is
connected at one junction 202A to a second end of resistor
2Ql, at a second junction 152A to line 49 and at a third
junction 153A to line 48.
The second side of sensor network 150 is connected
from terminal 154A through a resistor 151 and through a
resistor 152 to line 49 and to the second junction 152A of
the feedback network between resistors 202 and 153. A re-
ference signal is provided at a junction between resistors 151
and 152 to a second input to a modulator 100. Hence, at a
first input of modulator 100 a signal representative of the
measured, variable parameter is sensed, and at the second
input, the reference signal is sensed. At alternate cycles
of oscillator 110, moclulator 100 outputs the variable signal
and the reference signal, respecti~ely, to an AC amplifier
80 which amplifies the signal received. AC amplifier 80
outputs the amplified signal to demodulator 70, which on
alternate cycles of oscillator 110 outputs the ampli~ied
varying parameter based signal to one input terminal of a
differential operational amplifier 60 and the amplified re-
ference signal to the other input terminal of amplifier 60.Amplifier 60 provides an output representative of the
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differential signal presented at its in~ut terminals and
outputs a signal to a current control circuit 50 which will
either increase or decrease the current through the current
control 50 to again balance sensor network 150 and equalize
the signals at the input of the modulator 100. The current
through current control 50 rebalances sensor network 150 by
sending its current through line 49 and through the delta
re-entrant feedback resistors, 203, 202, and 153 thus
producIng a ske~ing voltage to rebalance sensor network
150. Modulator 100, AC amplifier 80, DC modulator 70, DC
amplifier 60, and current control 50 are all connected to
and powered from l~nes 47 and 49 which are connected to the
output of rectifier 46. ~irtually all the current from those
components flows through the delta re-entrant feedback net-
work and through line 48, which is represented by I t. Since
current control 50 is adjusted based on the differential or
unbalance in sensor network 150, IoUt is representative of
the condition to be sensed. Iout flows through line 48
through rectifier 46 where it affects the AC current through
transformer 45 and is thus reflected into DC to AC converter
40. The current flow through lines 41 and 13 changes to
achieve a balance of currents in transformer 45. ~he changed
current flows through DC power supply 10, load resistor
11, terminal 12A, li.ne 12, regulator 20, and line 41, then
back through DC to AC converter 40, transformer 45, rec-
tifier 46, line 47 through the current control 50, and through
the delta re-entrant feed~ack resistors 202, 203 and 153 to
line 48 thus completing the circuit. Since the current re-
quired internally by the voltage regulator 20 and DC to AC
converter 40 is not passed through the feedback resistance
network of the transducer, current required to operate these
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devices must be substantially constant with temperature and
changed in IoUt ~ Since the current from these de~ices
does not pass through the feedback resistance network,
but are substantially constant, the DC component of these
currents and the current at "zero" from line 48 represented
in the current of line 13 are constant value offset currents.
This "zero" offset may be corrected if desirable by additional
circuitry external from load resistor 11.
It should be noted that the second start-up
current through diode 23 is enhanced by the gains of
transistors 15, 32 and 34 respectively.
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