U.S. patent number 3,870,616 [Application Number 05/320,165] was granted by the patent office on 1975-03-11 for current controlled regulation of gas evolution in a solid polymer electrolyte electrolysis unit.
This patent grant is currently assigned to General Electric Company. Invention is credited to Russell M. Dempsey, Anthony B. La Conti, Mary E. Nolan, Robert A. Torkildsen.
United States Patent |
3,870,616 |
Dempsey , et al. |
March 11, 1975 |
Current controlled regulation of gas evolution in a solid polymer
electrolyte electrolysis unit
Abstract
A gas generator utilizing an electrolysis cell having a solid
polymer electrolyte is described in which the gas output is
controlled by controlling the current to the electrolysis unit. In
a preferred embodiment a hydrogen containing compound such as water
is electrolyzed to generate hydrogen and the rate of gas evolution
is controlled by varying electrical current to the cell. The output
gas pressure is sensed and used to regulate the current flow to the
cell to control the rate of gas evolution. In one instance a form
of time-ratio current control is utilized to regulate the current
whenever the outlet gas pressure exceeds a preset level.
Alternately, a strain gage type of pressure sensor is utilized to
produce a continuous output signal proportional to the output gas
pressure and the signal is fed back to control the cell current
with pressure variations.
Inventors: |
Dempsey; Russell M. (Hamilton,
MA), Nolan; Mary E. (Marblehead, MA), La Conti; Anthony
B. (Lynnfield, MA), Torkildsen; Robert A. (Danvers,
MA) |
Assignee: |
General Electric Company
(Wilmington, MA)
|
Family
ID: |
23245166 |
Appl.
No.: |
05/320,165 |
Filed: |
January 2, 1973 |
Current U.S.
Class: |
204/228.5;
204/229.2; 204/266 |
Current CPC
Class: |
C25B
15/02 (20130101); G05D 16/2066 (20130101) |
Current International
Class: |
C25B
15/02 (20060101); C25B 15/00 (20060101); G05D
16/20 (20060101); B01k 003/00 (); B01k
003/10 () |
Field of
Search: |
;204/230,263-66,128-129 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mack; John H.
Assistant Examiner: Valentine; D. R.
Claims
What is claimed as new and desired to be secured by Letters Patent
of the United States is:
1. In a gas generator having a controllable gas output the
combination comprising:
a. an electrolysis cell having:
1. a solid polymer, ion-exchange electrolyte membrane,
2. catalytic electrodes positioned adjacent to opposite surfaces of
said membrane to dissociate a chemical compound to evolve gas,
b. a source of electrical power coupled to the electrodes of said
cell,
c. means to furnish a chemical compound to one of said electrodes
for dissociation, the ionic form of one of the elements of the
dissociated compound being transported across said
ion-exchange-membrane to evolve the gas,
d. means for sensing the output gas pressure and producing a
control signal in response thereto;
e. means responsive to the control signal responsive to the sensed
gas pressure for controlling the current flow from said source of
electrical power on a time ratio basis to vary the current level to
the cell electrodes thereby to control the rate of gas evolution
including:
1. a current source including a pair of alternately conducting
solid state switching devices for supplying the cell current,
2. a voltage responsive variable repetition frequency, pulse
generator coupled to said switching devices having a voltage
responsive timing network for controlling the repetition pulse
frequency, said pulse generator producing triggering pulses for
said switches to control the average current level to said
cell,
3. means coupling the signal from said gas pressure sensing means
to said pulse generator timing network to vary the pulse repetition
rate of the pulses from said generator in response to said signal
to vary the cell current as a function of the outlet gas
pressure.
2. In a gas generator according to claim 1 wherein said pulse
generator comprises a relaxation oscillator including a solid state
switching element, a timing network coupled to said switching
including a storage element and an electrically controlled variable
resistance element for coupling the signal from said pressure
responsive means to said variable resistance element to vary the
resistance of said resistance element and thereby the time constant
of said timing network to vary the output pulse repetition
frequency from said relaxation oscillator in response to the gas
pressure.
3. The gas generator according to claim 2 wherein said solid state
switching device is a unijunction transistor and said timing
network includes a resistance capacitance network.
4. In the gas generator according to claim 1 wherein said current
source includes a pair of Silicon Controlled Rectifiers connected
in push-pull.
Description
Ths instant invention relates to a method and apparatus for
controllably generating gases in an electrochemical electrolysis
cell, and more particularly, to a gas generator in which evolution
of the gas is controlled by selectively varying the current flow to
the electrolysis unit.
Generating gas by electrolyzing a chemical compound into its
constituent elements, one of which may be a gas, is, of course, an
old and well-known technique. One recently developed form of such
gas evolving electrolysis unit involves the use of a cell which
utilizes an electrolyte in the form of a solid polymer
ion-exchange-membrane. In a cell of this sort, an
ion-exchange-membrane such as a sulfonated perfluorocarbon membrane
has a pair of electrodes of a suitable catalyst positioned on
opposite sides thereof. Through an oxidation reaction the ionic
form of one of the constitutents elements (hydrogen ions, for
example, when H.sub.2 O is electrolyzed) is produced at one
electrode. The ion is transported across the ion-exchange-membrane
to the other electrode where a reduction reaction takes place that
the positive hydrogen ion gains electrons to produce hydrogen
molecules. The solid polymer ion-exchange-membrane electrolyte
electrolysis unit is particularly advantageous because it is
efficient, small in size, and does not require any corrosive
electrolytes. Gas generators employing such an electrolysis unit
may thus be used in many new applications which formerly required
the use of stored gas and makes possible the manufacture of a
small, compact assembly for producing gas.
It is highly desirable in gas generators of this type to be able to
control the gas flow from the generator preferably by controlling
the rate at which the gas is evolved at the cell electrodes. Such
control has obvious advantages in that the rate of gas evoluion may
be increased when gas usage is high and correspondingly lowered
when the usage is low. Furhermore, control of the gas evolution
rate may be utilized to prevent buildup of excess gas pressure in
the unit while leaving the generator in an operationally ready
state thereby enhancing the usefulness of the device. It has been
found that all of these desirable objectives may be attained by
sensing an operating parameter such as the outlet gas pressure and
varying the current flow to the electrolysis cell in response
thereto, thereby controlling evolution of gas at the
electrodes.
It is therefore a primary objective of this invention to provide a
gas generating solid polymer electrolyte electrolysis unit in which
the flow of gas from the unit may be automatically controlled by
controlling evolution of the gas at the cell electrodes.
A further objective of the invention is to provide a solid polymer
electrolyte electrolysis unit in which the flow of gas from the
unit is electrically controlled to control the rate of gas
evolution at the electrodes of the unit.
Still another objective of the invention is to provide a gas
generator having a solid polymer electrolyte electrolysis unit in
which the rate of gas evolution is continuously controlled through
an automatic feedback loop.
Other objectives and advantages of the invention will become
apparent as the description thereof proceeds.
The various advantages and objectives of the invention are achieved
in a gas generating electrolysis cell which utilizes a solid
polymer ion-exchange-membrane electrolyte to evolve the gas. The
output from the cell is controlled by sensing the output gas
pressure and varying the current flow to the cell to control the
rate of gas evolution. The output from the gas generator may be
thus controlled either continually or in an intermittent or
time-ratio manner to control both the rate of flow or to control
the maximum pressure buildup in the generator. In a preferred
embodiment, the cell current is controlled by a pair of silicon
controlled rectifiers (SCR's) which are alternately gates by a
variable pulse rate pulse generator. A comparator and error signal
circuit is provided to produce a control signal proportional to a
reference signal and a gas pressure signal to vary the repetition
frequency of the output pulses from the pulse generator to control
the firing angle of the SCR's and thus the current flowing to the
electrolysis cell. Any variation in the output pressure, or
alternately, the exceeding of a preset output pressure, results in
a control signal from the comparator and error signal network which
varies the repetition rate of the pulse generator so as to modify
the current flow in the cell and thereby control the rate of gas
evolution at the cell electrodes.
The novel features which are believed to be characteristic of the
invention are set forth with particularity in the appended claims.
The invention itself, however, both as to its organization and
method of operation, as well as additional objectives and
advantages thereof, will best be understood from the following
description when taken in connection with the accompanying drawings
in which:
FIG. 1 is a schematic diagram of a gas generator in which the rate
of gas evolution is electrically controlled in response to the
output pressure of the cell.
FIG. 2 is a schematic of a portion of the solid polymer electrode
ionexchange-membrane which is useful in understanding its mode of
operation.
FIG. 3 is a circuit diagram of the pressure responsive, current
control network forming part of the system of FIG. 1.
FIG. 4 is a partial illustration of a modified form of the control
network of FIG. 3.
FIG. 1 shows the gas generator of the instant invention which
includes means for controlling the rate of gas evolution at the
electrodes of a solid polymer electrolyte electrolysis unit in
response to the output gas pressure. The gas generator includes an
electrolysis cell assembly shown generally at 10 of the solid
polymer ion-exchange-membrane electrolyte type in which a hydrogen
containing compound such as water or hydrogen chloride, for
example, is dissociated electrochemically to generate the desired
gas such as hydrogen, chlorine, oxygen, etc. In the following
description, the invention will be discussed in connection with a
hydrogen generator in which H.sub.2 O is dissociated to produce
hydrogen as well as oxygen. It will be apparent, however, that the
invention is broadly applicable to all gas generators of the solid
polymer electrolysis type including those capable of dissociating
other hydrogen containing compounds such as hydrogen chloride or,
for that matter, any dissociable chemical compound. Cell assembly
10 includes a housing 11 which is separated into anode and cathode
chambers 12 and 13 by a solid polymer ion-exchange-membrane
electrolyte 14. The solid polymer electrolyte ion-exchange member
may, for example, be a thin, 10 mils or so, sulfonated
perfluorocarbon membrane of the type manufactured and sold by the
Dupont Co. under their trade designation Nafion and which is
characterized by the fact that positive ions are transported across
the membrane. Positioned on either side of the
ion-exchange-membrane are electrodes 15 and 16 which are energized
from current control network 17. Electrodes 15 and 16 include both
a platinized titanium current conducting screen pressed against a
suitable catalyst which adheres to the membrane for enhancing
dissociation of the water. One suitable catalyst may be a mixture
of Platinum (Pt) and Iridium (Ir) in the proportions of 50 percent
and 50 percent by weight for the anode and platinum for the
cathode. Anode chamber 12 communicates through conduit and water
inlet solenoid valve 18 with main water supply tank 19. The water
is dissociated at anode electrode 15 into positive hydrogen ions
plus oxygen. The positive hydrogen ions move across
ion-exchange-membrane 14 and are converted into molecular hydrogen
at cathode electrode 16. Associated with the cathode chamber and
communicating therewith, is a hydrogen/H.sub.2 O accumulator
chamber 20 which acts as a storage reservoir for the hydrogen
evolved at the cathode electrode, as well as that water which is
pumped across the ion-exchange-membrane along with the hydrogen
ions, i.e., each hydrogen ion carries several molecules of water
with it as it moves across membrane 14 to the cathode. Positioned
in chamber 20 is a water level float 21 which actuates a suitable
switch such as a reed switch, for example, when the water reaches a
predetermined level. This actuates water inlet solenoid valve 18 to
shut off the water supply to the anode chamber.
The hydrogen passes from accumulator chamber 20 over conduit 22 to
a dessicant chamber 23 where any moisture in the gas is removed.
The gas then passes through a pressure regulator 24 to the exterior
of the unit. Mounted between the pressure regulator and the outlet
valve is a pressure gage 25 to indicate the gas gage pressure. A
pressure sensing element 26 which is shown in FIG. 1 as being a
pressure switch is coupled to outlet conduit 22 between the
dessicant chamber and the pressure regulator, and senses the outlet
hydrogen pressure in conduit 22. Pressure sensing element 26 is
connected to cell current control network 17 to vary the current
supplied to the electrolysis cell and thus, the rate of hydrogen
evolution at the electrodes.
The water dissociated in anode chamber 12 to produce the hydrogen
ions also produces molecular oxygen which is retained in the
chamber. An outlet conduit 30 is connected to the chamber and
transports the liberated oxygen and any water vapor which may be
contained therein to an oxygen-water separator 31 in which the
entrained water vapor is removed from the oxygen and returned to
main water supply tank 19 whereas the oxygen is vented to the air
or stored in a suitable container.
As will be described in detail subsequently in connection with the
structure of FIG. 4, the instant invention is not limited to using
pressure switch which produces control of the cell current on an
on-and-off basis, thus effectively producing a time-ratio
modulation of the cell current. A pressure sensing element which
produces a continuously varying electrical signal responsive to the
pressure variations may be used with equal facility. One type of
such a continuously varying pressure sensing device which is
illustrated in FIG. 4 is a pressure strain gage which produces an
electrical analog output signal proportional to the pressure and
which is continuously variable over a given pressure range.
The main water supply tank 19, as pointed out previously, supplies
water to the anode electrode for dissociation. However, not all of
the water which is supplied to the anode electrode is dissociated.
In fact, the bulk of the water supplied to the anode electrode is
transported across the ion-exchange-membrane into the cathode
chamber. Part of this water returns to the anode chamber by
diffusion across the ion-exchange-membrane, however the rate of
protonic pumping by the hydrogen ions is much greater than the
diffusion rate so that eventually a buildup of water takes place
and accumulator chamber 20 is provided for this purpose. It will
also be apparent that water level float 21 which controls water
inlet solenoid valve 18 is provided to cut off the water supply
from the main supply from the main supply tank whenever the water
in the accumulator chamber rises above a predetermined level. When
this occurs, solenoid valve 18 is closed shutting off the water
from the main supply tank. The cell operates on the water diffusing
from the cathode chamber back across the ion-exchange-membrane to
the anode electrode until the water level in the accumulator
chamber has been reduced below the predetermined level. Float 21
then deenergizes solenoid valve 18 opening the valve so that the
main water supply chamber again supplies water to the anoid. Thus,
tank 19 normally supplies the water required for the generation of
hydrogen but from time to time the water collected in accumulator
chamber 20 is so utilized thereby minimizing the need for shutting
the system down and emptying the accumulator chamber.
FIG. 2 is a partial schematic of a section of the
ion-exchange-member electrode of cell 10 and is useful in
understanding the action taking place at the electrodes and across
the ion-exchange-membrane by means of which the hydrogen is
evolved. Membrane 14 which is a sulfonated perfluorocarbon has an
anode electrode 15 attached to its surface. The electrode consists
of a current conducting element 40 which is shown to be of a mesh
or screen like construction which is pressed against a catalyst 41.
Similarly, on the cathode side of the membrane, the electrode 16
consists of a current conducting mesh 42 pressed against a catalyst
43. The current conducting electrodes 40 and 43 are connected
through suitable leads, not shown, to a source of D-C voltage which
estabilshes a suitable potential at the electrodes to dissociate
the hydrogen containing compound and results in the evolution of
the gases at the respective electrodes. Thus, as shown in FIG. 2,
an oxidation reaction takes place at the anode electrode whereby
the water is dissociated to form positive hydrogen ions and oxygen
with an accompanying loss of electrons. The reaction taking place
at the anode is shown as follows:
2H.sub.2 O.fwdarw.4H.sup.++ O.sub.2 +4e (Oxidation-Loss of
electrons)
The hydrogen ions are transported across the ion-exchange-membrane
to the cathode electrode as shown by the upper arrows, where a
reduction reaction takes place in which the hydrogen ions gain
electrons to produce molecular hydrogen. Thus, the reaction taking
place at the cathode is as follows:
4H.sub.]+ 4e.fwdarw.2H.sub.2 (Reduction-gain of electrons)
In summary, water is dissociated at the anode to produce molecular
oxygen and hydrogen ions. The hydrogen ions are transported across
the ion-exchange-membrane to the cathode where a reduction reaction
takes place and the hydrogen is converted into molecular
hydrogen.
In addition to the transport of hydrogen ions across the
ion-exchange-membrane, each hydrogen ion in moving across the
membrane transports seven molecules of water from the anode to the
cathode. It is this transport of water by the hydrogen ions, i.e.,
protonic pumping of the water, which results in the accumulation of
water at the cathode and dictates the need for an accumulator
chamber to hold the water. While water is pumped protonically from
the anode to the cathode by the hydrogen ions, the accumulation of
the water in the cathode chamber results in a pressure gradient
which produces diffusion of water back across the
ion-exchange-membrane from the cathode to the anode, as indicated
by the arrow D shown in the lower portion of the
ion-exchange-membrane. However, the rate of protonic pumping to the
cathode side is substantially greater than the rate of diffusion to
the anode side so that overall there is an accumulation of water on
the cathode side of the cell. Consequently, an accumulator must be
provided as well as a means for intermittently stopping the supply
of water to the anode when the water accumulation at the cathode
side exceeds a predetermined level.
As pointed out previously, the dissociation of water ions involves
the loss of electrons on the anode side, the gain of electrons on
the cathode side and the transport of positive hydrogen ions across
the membrane. As a result, current flow in the external circuit and
the magnitude of this current flow controls the evolution of
hydrogen at the cathode with the rate of evolution being
proportional to the amount of current flowing in the external
circuit. By controlling the current flow to the electrochemical
cell as a function of the output hydrogen pressure, the rate of
evolution of the hydrogen gas may be controlled thereby providing a
ready means for controlling both the flow rate of the hydrogen from
the generator, as well as controlling the pressure levels in the
system.
In the arrangement of FIG. 1, the output gas pressure from the
generator is sensed by a pressure switch 26 which is actuated to
produce an electrical control signal whenever the output pressure
exceeds a given level. The electrical signal from the pressure
switch is then applied to cell current control network 17 to
control the current to the electrolysis cell assembly to vary the
current level by a time ratio mode thereby varying the rate of gas
evolution at the electrodes. FIG. 2 illustrates a preferred
embodiment of the circuitry for controlling the current to the
electrolysis cell in response to the gas outlet pressure. The
network of FIG. 3 contains 4 major elements including a cell
current source 50 which includes a pair of Silicon Controlled
Rectifiers (SCR's) connected in push-pull for supplying current to
the electrolysis cell, a variable PRF trigger pulse generator 51
for controlling the firing angle of the SCR's, a comparator and
error signal generating network 52 for producing a control signal
which controls the repetition frequency of the triggering pulses
from pulse generator 51, and a power supply circuit 53 for
supplying an A-C supply voltage to the anodes of the SCR's and
positive and negative unidirectional supply voltages to the
remaining circuit components.
The cell current source 50 includes a pair of alternately
conducting silicon controlled rectifiers 54 and 55 connected in a
push-pull configuration to supply current to the electrolysis cell
shown schematically at 10. Anode voltage for SCR's 54 and 55 is
supplied via leads 56 from the power supply module 53. The anode
voltage for SCR's 54 and 55 is an A-C voltage so that the SCR
anode-cathode voltages are positive during opposite alternations of
the A-C supply voltage. Consequently, SCR's 54 and 55 conduct
alternately during alternate half-cycles of the supply voltage. The
cathodes of SCR's 54 and 55 are connected together and to the
positive terminal of electrolysis cell 10 so that conduction of
each of the SCR's produces current flow through the cell, the
magnitude of which is controlled by the firing angle of the SCR's.
The gate electrodes 57 of each of the SCR's are transformer coupled
to pulse generator 51 with the gates connected respectively to
secondary windings 58 and 59 of a pulse transformer 60, the primary
of which is connected to the output of trigger pulse generator 51.
Secondary windings 58 and 59 are so wound, as indicated by the dots
adjacent their junction, that the pulses from pulse generator 51
are applied in phase to the gate electrodes so that the positive
pulses are applied to both gate electrodes simultaneously. However,
since only one of the SCR's has a positive voltage at its anode
during any given half-cycle of the supply voltage only one of the
SCR's can be triggered at a time.
The output of the trigger pulse generator 51 controls the firing
and thus the phase angle of conduction of the SCR's and thereby the
current flow through the cell. For example, during the half-cycle
when the anode of SCR 54 is positive and that of 55 is negative,
the application of a trigger pulse to the gate electrode of SCR 54
causes that SCR to conduct whereas the application of the same
pulse to the gate electrode of SCR 55 will have no effect since the
anode-cathode path is reverse biased. The average current flowing
during each SCR conducting period depends on the point in time
during the positive anode voltage cycle that the SCR is triggered
into conduction. That is, the conduction phase angle, and hence,
the average current depends on the firing point. If the SCR is
triggered early in the 180.degree. positive anode voltage cycle, as
close to 0.degree. as possible, the average current is high. If the
SCR is triggered late in the positive half-cycle of the cycle,
i.e., closer to the 180.degree. point when anode voltage goes
negative again and the SCR stops conducting, the average current is
low. The value of the average current thus may be varied from a
very high value to zero current by varying the conduction phase
angle between 0 and 180.degree.. The conduction phase angle in
turn, depends on how early or late in the anode voltage cycle the
SCR is triggered. It will also be obvious that the greater the
repetition frequency of the triggering pulse, the earlier in the
cycle the SCR's will fire and hence, the greater the average
current. Conversely, as the pulse repetition frequency is reduced,
the SCR's fire later and later in the cycle thereby reducing the
average current flowing through the cell. Thus, by controlling the
repetition frequency of the pulses from pulse generator 51, the
firing angle of the SCR's may be varied and the current level
correspondingly controlled.
Trigger pulse generator 51 is a simple relaxation oscillator
including a programmable unijunction transistor and a R-C timing
network which includes a variable resistance controlled by an error
signal from comparator 52 to vary the pulse repetition frequency.
Thus, trigger pulse generator 51 includes a programmable
unijunction transistor 63 having a gate electrode 64, a cathode 65
and an anode 66. Cathode 65 is connected to the A- bus by resistor
67 and gate electrode 64 is connected through voltage divider
resistors 68 and 69 to the A- and A+ busses of the power supply to
establish the firing voltage for unijunction 63.
An R-C timing network controls the voltage level at anode 66 and
hence, the rate at which the unijunction transistor is driven into
conduction. The timing network includes a capacitor 71 connected
between the anode and the negative A- supply bus. A fixed resistor
72 is connected in series with the emitter-collector path of
transistor 73 which functions as a variable resistance between
capacitor 71 and the positive A+ supply bus. Storage capacitor 71
charges through resistor 72 and the emitter collector path of
transistor 73 toward the positive voltage at the regulated A+ bus.
When the voltage at anode 66 becomes sufficiently positive to
forward bias the anode-gate junction, unijunction transistor 63
conducts, rapidly discharging capacitor 71. This rapid discharge
produces current flow through cathode resistor 67 and a short
positive pulse is generated each time the unijunction transistor
conducts. This pulse is coupled to the base of NPN transistor
amplifier 74. Transistor 74 has its collector connected through a
collector resistor 75 and a diode 76 to the A+ bus and its emitter
directly to the A- bus. The output from the transistor 74 is
coupled to primary winding 61 of pulse transformer 60 and thence,
through secondary windings 58 and 59 to the gating electrodes of
SCR's 54 and 55. A capacitor is connected between the junction of
resistor 75 and diode 76 and charges to the positive A+ supply. The
voltage on capacitor 77 maintains the voltage across transistor 74
even when the A+ supply voltage which is a clipped. full rectified
sine wave goes to zero twice during each cycle of the rectified A-C
supply. Diode 76 is provided to prevent discharge of capacitor 77
when the voltage on the A+ bus goes to zero since it is poled to
block current flow when the A+ bus is a zero volt and capacitor 77
is charged to a positive voltage.
When capacitor 71 discharges the voltage drops and the anode-gate
junction is again reverse biased terminating conduction so that
capacitor 71 again begins to charge towards the voltage at the A+
bus. The rate at which capacitor 71 charges, i.e., the R-C time
constant of the R-C network, and hence, the repetition rate of the
output pulses from this relaxation oscillator is controlled by the
resistance of the emitter-collector path of the PNP transistors 73.
Transistor 73 thus functions as a variable resistor which controls
the time constant of the network and hence, the repetition rate of
the output pulses from the oscillator. The resistance of transistor
73 may in turn be selectively varied and the pulse rate controlled
by means of a control signal from comparator 52 which is applied to
the base of the transistor. The base electrode 78 of the transistor
is connected through resistor 79 to the A+ bus and through current
limiting resistor 80 to comparator 52. Resistor 79 normally keeps
the base positive resulting in a high resistance for the emitter
collector path and hence a low pulse rate. The signal from
comparator 52 changes the voltage at the base electrode to control
the resistance of the emitter and collector path of transistor 73.
Thus, the output or error signal from comparator 52 controls the
pulse rate of the pulses from trigger pulse generator 51 and hence,
the firing angle of the SCR's and the current level supplied to
electrolysis cell 10.
Comparator and error signal generator 52 includes a summing
amplifier 81 which integrates the various input signals to the
amplifier and includes a pair of input terminals 82 and 83.
Terminal 82 is connected to a current setting potentiometer 84
consisting of resistors 85 and 86 shunted by zener diode 87 which
maintains the voltage across the potentiometer constant. A movable
slider 88 is positioned along the lower resistor 86 and couples a
reference signal which establishes the reference current level to
the summing amplifier. In the absence of any other input signals,
the output from the summing amplifier as established by the setting
of slider 88 controls the resistance of the emitter collector path
of transistor 73 to establish a pulse rate from pulse generator 51
which results in a given current level from the SCR's. Input
terminal 83 of summing amplifier 81 receives a feedback signal from
the cell which is proportional to the actual current flowing
through the cell. To this end, a current shunt resistor 90 is
connected between one terminal of cell 10 and ground. The voltage
drop across resistor 90 which is proportional to the current
flowing in the cell is applied over lead 91 to input terminal 83
and is thus compared with the reference signal from potentiometer
slider 88. The feedback signal from cell 10 corrects for current
variations due to variations in line voltage, temperature, cell
conditions, etc. That is, if due to line voltage variations or
conditions in the cell itself, the actual current flowing through
the cell differs from the predetermined current level established
by the position of slider 88 on the current setting potentiometer
84, a signal is fed back of magnitude and polarity to produce an
output from the summing amplifier which varies the pulse repetition
frequency rate sufficiently to bring the current to the desired
level.
Also connected to input terminal 82 is an electrical signal
responsive to the outlet gas pressure for modifying the current
flow through the cell. To this end, single pole double throw switch
element 91 which is controlled by the pressure sensor is connected
between the B- supply bus and input terminal 82. Switch 91 has a
movable armature 92 which is normally maintained in the open
position so that switch 91 has no effect on summing amplifier 81
and the flow of current through cell 10. In the particular
arrangement illustrated in FIG. 1, when the outlet gas pressure
reaches a predetermined level, pressure switch 26 is actuated
thereby moving armature 92 of switch 91 to connect the B- voltage
bus to input terminal 82 through a current limiting resistor. As a
result, the output from summing amplifier 81 and from comparator 52
goes heavily positive. With the signal from summing amplifier 81
going more positive, PNP transistor 73 becomes less conductive and
the resistance of the emitter-collector path rises thereby
increasing the time constant of the R-C timing network associated
with unijunctions transistor 63. The repetition rate of the pulses
from generator 51 is correspondingly reduced to a very low value
and, in fact, the pulse rate drops below the SCR supply voltage
frequency. Consequently, SCR's 54 and 55 are not triggered at all
during each half-cycle of the supply voltage and the current
supplied to cell 10 goes to zero. When the current goes to zero,
gas evolution at the cell electrodes ceases and the outlet pressure
begins to drop. When the outlet pressure again drops below the
critical value, pressure switch 26 is deactivated moving armature
92 of the switch 91 into the open position and removing the B-
voltage from input 82. The output from the comparator circuit
becomes less positive which decreases the resistance of the emitter
collector path of transistor 73. The time constant of the timing
network is reduced and the repetition rate of the pulse generator
is increased sufficiently so that SCR's 54 and 55 are again
triggered and conduct current. This sequence, with the SCR's being
disabled for the entire cycle of the supply voltage, continues
until the outlet gas pressure of the generator reaches and
stabilizes at a voltage below the critical pressure level. Thus, in
effect, a time ratio control of the cell current is produced by
enabling and disabling the SCR's until the average current level is
reduced sufficiently to reduce the rate of gas evolution to
maintain the output pressure below the critical level.
The power supply module 53, which supplies both the A- Csupply
voltage for SCR's 54 and 55 as well as the positive and negative
unidirectional supply voltages for the remaining circuitry,
includes an iron core transformer 95 having a primary winding 96
connected to an A-C supply and a center tapped secondary winding
97. The center tap of secondary winding 97 is grounded so that two
secondaries produce alternating output voltages which are
180.degree. out of phase. The anodes of SCR's 54 and 55 are
connected via the leads 56 to intermediate taps on opposite sides
of the grounded center so that positive anode voltages are supplied
to SCR's 54 and 55 on alternate half-cycles of the supply voltage.
Positive and negative unidirectional supply voltages for the B+,
B-, A+ and A- busses are provided by full wave rectifying circuits
connected to winding 97. To this end, a pair of diodes 98 and 99
are respectively connected to the opposite ends of winding 97 and
are so poled as to conduct during positive alternations of the
supply voltage to produce a positive rectified voltage on lead 103.
Diodes 100 and 101, on the other hand, are so poled as to conduct
during the negative alternations. Consequently, these rectifier
pairs supply positive and negative voltages for the B+ and B-
supply busses over leads 103 and 104. Connected between the supply
busses and ground are the filter capacitors 105 and 106 which
conduct any alternating or ripple current components in the
rectified voltage to ground. A pair of zener diodes 107 and 108 are
connected respectively to the B+ and B- supply bus to regulate the
rectified D-C voltage which is then filtered by capacitors 105 and
106. The zener diodes function, in a manner well known to those
skilled in the art, to regulate the voltage and maintain it at a
given level. Thus, as the rectified positive voltage on the B+ bus
exceed the level established by the particular zener, the zeners
begin to conduct clipping the rectified voltage. Similarly, zener
108 is so poled as to conduct if the negative voltage on the B- bus
exceeds a predetermined level to clip the negative rectified
voltage. The A+ and A- busses of the trigger pulse generator are
connected to the B+ and B- busses respectively by resistors 70 and
109 and the voltage at the A- and A+ busses is maintained at a
different and lower level by means of zener diodes 110 and 111. The
voltages at the A+ and A- busses have the wave form illustrated by
Curve 112 since there are no filter capacitors provided at these
busses. The A+ and A- voltages therefore, go to zero twice during
each supply voltage cycle in order, as will be explained in detail
later, to reset the pulse generator each time the supply voltage to
the SCR's goes to zero.
Thus, in the arrangement shown schematically in FIG. 1 and the
current control circuitry shown in detail in FIG. 3, the current
level in the cell is set by means of a potentiometer from zero to a
maximum current with the signal from the potentiometer being summed
with a feedback signal from the cell and an external signal
responsive to outlet gas pressure. All these signals are integrated
in a summing amplifier to produce an increasing or decreasing
current output in response to an error input, thus resulting in a
very closely controlled output current. Current for the cell is
derived from a transformer secondary and controlled by two phase
controlled SCR's which produce a variable full wave rectified
current output. Control of this current and triggering of the SCR's
is accomplished by a unijunction oscillator which is transformer
coupled to the SCR's. The phase variation of the firing of the
rectifiers is accomplished by varying the pulse repetition
frequency according to the error signal form the summing amplifier
which error signal is in turn, controlled in response to a pressure
sensitive element from the gas outlet section of the generator and
in response to the actual current flow in the generator. Thus, a
time ratio servo control system is provided which closely controls
the current flow through the cell to control the rate of gas
evolution at the cell and in turn, the outlet pressure and gas flow
from the generator.
It will also be apparent that since the supply voltages on the A+,
A- busses go to zero twice during each cycle, as shown by Curve
112, the voltage at gate electrode 64 of unijunction 63 also goes
to zero. With the bias voltage at zero, the rectifying junction at
emitter 66 is no longer reverse biased and the resistance at that
junction is very low. This permits capacitor 71 to discharge very
rapidly through this junction and its voltage goes to zero. In this
fashion, the unijunction relaxation oscillator is reset each time
the supply voltage goes to zero. Since A-C supply voltage also goes
to zero at this time, it can be seen that the oscillator is reset
each time current conduction is switched from one SCR to the
other.
In the control circuitry of FIG. 3, an arrangement is illustrated
in which the rate of gas evolution at the cell is varied by
controlling the current through the cell in response to a pressure
sensing element which actuates the control circuitry whenever the
gas pressure exceeds a predetermined level and then cycles the
system until the outlet pressure stabilizes at a lower pressure
level. The invention, however, is not limited to a time-ratio or an
on-an-off system which is actuated only if the pressure exceeds a
predetermined value. The system may also function so that the
outlet pressure is continually sensed and compared with the
reference value established by the current setting potentiometer to
control the current through the cell continuously in response to
pressure variations. FIG. 4 illustrates such an arrangement in
which a pressure transducer is utilized to sense the outlet gas
pressure to produce a control signal which varies continually with
pressure. To this end, a pressure transducer having a plurality of
strain gages connected as the four active arms of a Wheatstone
Bridge may be utilized as the pressure sensing element. In FIG. 4 a
pressure transducer [which may for example, be a pressure
transducer of the kind sold by the CEC/Transducer Div. of the Bell
& Howell Co. located at Munrovia, Calif. under its designation
Series 4-236] is connected to the outlet conduit to sense the
outlet pressure. The outlet pressure varies the resistance of
four-strain gages, 113 connected as the arms of a Wheatstone Bridge
as a function of pressure. A source of D-C potential is connected
across one diagonal of the bridge and a voltage proportional to gas
pressure is produced across the other diagonal of the bridge. The
output voltage from the Wheatstone Bridge is connected by a pair of
cables to an amplifier 114 where it is amplified and applied as one
of the inputs to an input terminal 115 of a summing amplifier 181.
The other input to the amplifier is a reference voltage from a
current setting potentiometer similar to that described in
connection with FIG. 3. That is, the current setting potentiometer
includes a pair of series connected resistors 116 and 117 which are
connected between the B+ terminal and ground potential. A zener
diode 118 is connected across the two resistors to maintain the
voltage across the potentiometer constant. A movable slider 119 is
connected along the lower resistor 115 and establishes the
reference voltage from the current setting potentiometer. This
reference voltage is applied to input 118 of the summing amplifier.
The output of summing amplifier 81 thus produces an error signal
which controls the repetition rate of the trigger pulse generator
which controls the firing angle of the current supplying SCR's
connected to the electrolysis cell.
In the arrangement illustrated in FIG. 4, the current level
established initially by the position of the slider 119 establishes
the current level in the cell and the rate of gas evolution at the
electrodes. The evolution of gas is established by reference signal
from the current setting potentiometer will under normal conditions
establish an output pressure level which is sensed by the strain
gage transducer and fed back to the input of the summing amplifier.
As the gas pressure varies above or below the value desired, due to
usage or other variables the feedback signal from the strain gage
pressure transducer varies thereby varying the output from the
summing amplifier and controlling the pulse repetition rate of the
pulse generator which controls the firing angle or phase angle of
the current supplying SCR's. By controlling the firing angle of the
SCR's, the amount of current through the cell is controlled as is
the rate of evolution of the gas and hence, the outlet pressure
from the generator. The strain gage pressure transducer thus
provides a continuous signal which varies with outlet pressure to
maintain the current through the cell at a level such as to
maintain the rate of evolution of the gas at a level adequate to
maintain the desired gas pressure as conditions such as use,
temperature, etc., vary to cause changes in fluctuations in gas
pressure.
It will be appreciated that a very simple and effective arrangement
for controlling the outlet pressure and the flow rates of a gas
generator have been described in which all these desirable end
results are achieved by sensing the output pressure and controlling
the current flow to the electrolysis cell as a function of the
pressure. Control of the current in turn, controls the rate of
evolution of the gas at the cell electrodes thereby regulating both
the pressure and the rate of gas flow from the cell in a simple and
effective manner.
Although a particular embodiment of this invention has been shown,
it will, of course, be understood that the invention is not limited
thereto since many modifications both as to the arrangement and the
circuitry utilized therein may be made. It is contemplated by the
appended claims to cover any such modifications as may fall within
the true spirit and scope of this invention.
* * * * *