WO2006058369A1

WO2006058369A1 - Reversible polarity electrode systems

Info

Publication number: WO2006058369A1
Application number: PCT/AU2005/001802
Authority: WO
Inventors: William Leslie Stephen Smith
Original assignee: Poolrite Equipment Pty Ltd
Priority date: 2004-12-01
Filing date: 2005-12-01
Publication date: 2006-06-08
Also published as: EP1831431A4; EP1831431A1

Abstract

A method for reversing electrode polarity in an electrolytic cell includes the steps of isolating electrodes of the cell from electrical charges applied thereto, effecting a controlled discharge to a predetermined value of residual charge carried on said electrodes, reversing the polarity of the applied electrical charge and then reapplying said electrical charge whereby the direction of current flow between electrodes is reversed. Suitably, the controlled discharge is effected by interrupting current flow from residual charges in said electrodes before a complete discharge occurs.

Description

TITLE "REVERSIBLE POLARITY ELECTRODE SYSTEMS"

FIELD OF THE INVENTION

This invention is concerned with improvements in reversible polarity electrode systems for electrolytic cells.

The invention is concerned particularly, although not exclusively, with improvements in self-cleaning reversible polarity chlorinators for swimming pools, spas and the like.

Although the invention is exemplified herein with reference to a swimming pool chlorinator, it should be understood that the invention is applicable to other reversible polarity electrolytic cells utilizing a wide variety of electrolytes.

BACKGROUND TO THE INVENTION

Electrolytic chlorinators have evolved to overcome the problems associated with chemical dosing of swimming pools, spas and the like to prevent the accumulation of algae and bacteria therein. Hitherto the process of dissolving large quantities of expensive calcium and sodium based chloride salts in a body of water has resulted in high levels of hypochlorite ions which, apart from causing skin and eye irritations in bathers, frequently necessitated the addition of equally expensive pool chemicals such as hydrochloric acid to adjust water pH, bicarbonate of soda to act as a pH buffer and soluble calcium salts to maintain a total dissolved solids balance to reduce leaching of concrete or plaster pool wall surfaces. For the average domestic pool owner, careful and frequent maintenance of pool chemistry was seen as burdensome with the result that, for example, a low chlorine level would be responded to by overdosing with chlorine giving rise to very large variations in concentration of pool chemicals either side of an optimum value. With the advent of electrolytic chlorinators, a low level of sodium chloride dissolved in the pool or spa water permitted generation of chlorine gas in an electrolytic cell to produce ionized sodium hypochlorite in an aqueous solvent on a regular cycle thereby avoiding large variations in chlorine concentration. These electrolytic cells typically included spaced electrodes comprising at least one cathode and at least one anode fabricated from flat or expanded sheet titanium, the anode further including a catalytic coating including rare earth metals such as ruthenium platinum and iridium. Electrodes embodying a catalytic coating are exemplified in United States Patent 3711385. Prior art pool and spa chlorinators typically comprised "in-line" and "in- pool" electrolytic cells.

In line cells were usually plumbed into the return line between the filtration system and the pool and were designed to operate only when the pool filter pump was operating to circulate water through the cell. Because their duty cycle was limited to the duration of the filtration system, in-line cells are generally designed as high capacity chlorine generators and typically operate at a voltage of from 24-32 volts and a current density of from 300- 400 amps/m². Prior art "in-line" chlorinators are described in United States Patents 4472256, 4808290, 4861451 , 5221451 , 5460706 and 6059942. "In-pool" type chlorinators are described in Australian Patent 569026, United States Patent 4997540 and United States Patent 5228964.

These "in-pool" type chlorinators are mounted within a swimming pool submerged under water or are directly plumbed to the pool interior independent of the filtration system. With the exception of United States Patent 5228964 which teaches an electric pump for circulation of electrolyte through the cell, each of the other prior art "in-pool" type systems relies upon convection currents created by hydrogen gas generation at the cathode(s) and chlorine gas generated at the anode(s) within the hollow cell interior. While generally effective for their intended purpose, these prior art electrolytic chlorinators suffered from a progressive loss in electrical efficiency due to the plating out on the cathode of dissolved metal salts, particularly calcium carbonate. This necessitated regular cleaning with hydrochloric acid to remove the built up scale. Of more recent times it has been proposed to provide chlorinator cell circuitry which permits a self-cleaning function by periodic reversal of the polarity between the electrodes. United States Patent 4997540 describes polarity reversal in an "in-pool" chlorinator cell and United States Publication 2503/0024809 described polarity reversal in an in-line chlorinator cell. With the advent of reverse polarity "self-cleaning" electrolytic chlorine cells, it has been noted that the service life of the electrode assembly is often less than the prior art non-self-cleaning electrode assemblies and this necessitates an expensive replacement process for the pool owner and usually some pro-rata warranty compensation by the cell manufacturer. It was initially considered that an electrical "shock" induced by instantaneous polarity reversal under normal operating voltage and current conditions induced cracking in and flaking off of the rare earth metal catalytic coating on the titanium electrodes. In order to overcome electrical "shock" effects it was proposed in

United States Patent 5034110 to provide cell control circuitry which allowed a progressive ramping down of input cell voltage to a zero value at which point polarity was reversed before progressively ramping voltage back up to normal operating levels. The polarity reversal system of United States Patent 5034110 is said to be suitable for both "in-line" and "in-pool" chlorinator cells.

United States Patent 6391167 describes the construction of an "inline" chlorinator having an electrode assembly accessible through a removable cover for maintenance. This specification describes a power supply for the cell to facilitate both current and voltage regulation to protect the cell components under abnormal operating conditions. In particular a gas trap is employed as a flow sensor such that the cell is shut down in a controlled manner whereby fora predetermined period of after-run, hydrogen is generated to remove scale from the electrodes. An additional self cleaning function may be achieved by reversing the polarity between electrodes in a 90 second polarity reversal cycle comprising a 30 second cell discharge phase, a 30 second polarity changeover phase, and a 30 second recharge phase. However it is stated that where the anodes and cathodes are of different materials, polarity reversal is unnecessary. It is an aim of the present invention to provide an improved reverse polarity electrode system for electrolytic cells, an improved method of polarity reversal and otherwise to provide consumers with a more convenient choice.

SUMMARY OF THE INVENTION According to one aspect of the invention there is provided a method of reversing electrode polarity in an electrolytic cells, said method including the steps of:- isolating electrodes of said cell from an electrical charge applied thereto by a power source; effecting a controlled discharge to a predetermined value of residual charge carried on said electrodes; effecting reversal of polarity of said electrical charge applied by said power source; and reapplying said electrical charge whereby a direction of current flow between said electrodes is reversed.

Suitably, said residual charge is at least partially discharged by the application to said electrodes of charge pulses of opposite sign to said residual charges on respective electrodes.

If required, said charge pulses may be of a predetermined voltage. The charge pulses may be of a predetermined duration.

Suitably, said charge pulses may be applied with a predetermined frequency.

Preferably, a predetermined number of charge pulses are applied to at least partially discharge said residual charges on respective electrodes. If required, said power source may include one or more SCR switch mechanisms.

Suitably, said one or more SCR switch mechanisms are energized immediately before change of polarity in said cell whereby a driven side potential of said one or more SCR switch mechanisms and a potential of said residual charge approach an equilibrium value.

Preferably, said SCR switch mechanisms are energized for a number of times until said potential of said residual charge reaches a predetermined value. Alternatively, application to said electrodes of charges pulses is effected by energizing the gate of a transistor switching device with a pulse width modulated signal whereby a potential of said residual charge decays at a predetermined rate towards a zero value.

The residual charge on said electrodes may be at least partially discharged by effecting an electrical short circuit between said electrodes.

Suitably, said electrical short circuit discharges, at least partially, residual inductive charges in electrical circuitry coupled to said electrode conductors.

Preferably, a controlled discharge is effected by interrupting current flow from residual charges in said electrodes before a complete discharge occurs thereby allowing a controlled discharge rate.

Preferably, said pulse width modulated signals are applied periodically to discharge said electrodes to a predetermined value.

According to another aspect of the invention there is provided an electrolytic cell assembly adapted for reversible polarity operation, said cell assembly including:- at least two spaced electrodes; control circuitry, in use, to selectively reverse polarity in electrical charges applied to said electrodes at predetermined intervals, said electrolytic cell assembly characterized in that said control circuitry is adapted to at least partially discharge residual charges on said at least two spaced electrodes before reapplication of an operating charge of reversed polarity. Suitably, controlled discharge is effected by interrupting, with a switching mechanism, in said control circuitry, current flow from said residual charge before complete discharge of said residual charge occurs thereby allowing a controlled discharge rate.

If required, said switching mechanism may include one or more SCR switching devices.

Alternatively, said switching mechanism may include one or more transistor switching devices.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be more clearly understood and put into practical effect, reference will now be made to preferred embodiments illustrated in the accompanying drawings in which:-

FIG. 1 shows schematically the application of a half wave rectified power signal to a prior art chlorinator;

FIG. 2 shows a schematically control circuit and cell according to the invention;

FIG. 3 shows schematically a power signal produced by the circuitry of FIG. 2;

FIG. 4 shows schematically a modified signal produced by the circuit of FIG. 2;

FIG. 5 shows an alternative embodiment of a cell discharge device; FIG. 6 shows a further embodiment of the control circuit of FIG. 2; FIG. 7 shows graphically, the operation of the bleed circuit shown in FIG. 6; FIG. 8 shows graphically, the controlled discharge of residual electrode charges and induced circuit charges for the circuit arrangement of FIG. 7; and

FIG. 9 shows graphically, the controlled discharge of residual electrode charges and induced circuit charges for the circuit arrangement of FIG. 2.

For the sake of simplicity, like reference numerals are employed for like features described in the specification and illustrated in the drawings.

DETAILED DESCRIPTION OF THE DRAWINGS Reverse polarity chlorinators typically incorporate an electrolytic cell and control circuitry. The control circuitry usually incorporates a current rectifier to rectify mains AC power to half or full wave rectified. The rectified DC power is supplied to the cell with a voltage between 5 volts and 30 volts and a current density of typically 300 amp m².

The electrolytic cell incorporates titanium cathodes and anodes that are coated with a conductive coating material including rare earth metals such as ruthenium, platinum and iridium. The coating is slightly porous, which allows electrolyte to penetrate the coating and contact an electrode.

When an electrical charge is supplied to the cell, a scale, usually comprising predominantly insoluble calcium salts, plates out on a surface of the cathode. Over a period of time the scale substantially reduces the conductivity of the cathode and hence, reduces the ability of the cell to produce chlorine at the anode.

To overcome the accumulation of scale, prior art chlorinators typically reverse the polarity between the anode and cathode at regular intervals. Hence, after a further period of time it will again become necessary to reverse the polarity of the cell to remove scale build upon the cathode.

It has been found however, that damage is caused to the electrodes when polarity is reversed. It is known that immediately after power to the cell is switched off, a residual potential difference of up to 6 volts can be measured between the anode and cathode. This potential difference will decay fairly quickly to around 2 volts and decay very slowly thereafter over many hours. A slight residual potential difference can be detected often up to a week later. This residual charge is believed to arise from a concentration of anions within and on the coated surface of the cathode and, conversely a concentration of cations within and on the surface of the anode. When the polarity of the electrical charge is changed, it has been noted by the inventor that initially, the electrode previously acting as a cathode exhibits a very low resistance to current, possibly due to the high concentration of anions with and on the surface of the electrode coating. The low resistance permits an initial large current spike followed by a rapid increase in resistance in the electrode with a resultant voltage spike of up to say 30 volts on a cell normally operating with a 15 volt charge.

While not wishing to be bound by any particular theory it is considered that the current and voltage spikes impose an electrical shock to the fairly brittle electrode coating. Moreover, it is possible that with these current and voltage spikes, an explosive rate of formation of hydrogen gas from small quantities of electrolyte within the porous coating matrix cause cracking and/or flaking of the coating with a progressive loss in efficiency when the electrode is functioning as an anode to produce chlorine gas.

FIG. 1 illustrates schematically the initial voltage spike 14 on the first half wave of a rectified signal compared with a normal signal value 11. A positive signal 150 is communicated to the cell 30 by triggering the first SCR 70 when the first wave 130 has a phase value equal to about 90 degrees and hence, a positive voltage value. It should be appreciated that any phase value between 0 and 179 degrees may be suitable.

The first SCR 70 prevents further communication of the first wave 130 to the cell 30 when the phase of the first wave is at about 180 degrees. Subsequently, the third SCR 90 is triggered and allows communication of the phase shifted wave 140 to the cell 30 when the phase shifted wave 140 has a phase value of about 270 degrees, and therefore, a positive voltage value. It should again be appreciated that any phase value between about 180 and 359 degrees may be suitable. The second SCR 90 prevents further communication of the phase shifted wave 140 to the cell 30 when the phase shifted wave 140 has a phase difference of about 360 degrees. This process is repeated for the duration of the positive signal 150. The first cycle is driven by stored charge and transformer potential.

The stored charge feeds back along the rectifier circuit and causes a momentary overloading on the electrode plates 40 and 50. The stored charge will normally equalize when a thyristor is excited in the first half cycle. The thyristor will remain excited while a stored charge is available to drive it in prior art devices. As shown in FIG. 1 , there is a residual negative potential 12 of one or more volts which causes the momentary overload. As the value of the negative charge changes to positive a residual positive potential 13 is generated on the electrodes.

Referring to FIG. 2, there is provided a schematic diagram of a reverse polarity chlorinator 10, incorporating a power supply 20, a microprocessor controller 25, a transformer 21 , a cell 30, having at least two electrodes 40 and 50, and a rectifier circuit 60. The rectifier circuit 60 incorporates a first SCR 70, a second SCR 80, a third SCR 90 and a fourth SCR 100. A first SCR cathode 71 of the first SCR 70, a second SCR anode 81 of the second SCR 80, a third SCR cathode 91 of the third SCR 90 and a fourth SCR anode 101 of the fourth SCR 100 are electrically connected to the cell 30. A first SCR anode 72 of the first SCR 70 and a second SCR cathode 82 of the second SCR 80 are electrically connected to a first transformer end tap 22. A third SCR anode 92 of the third SCR 90 and a fourth SCR cathode 102 of the fourth SCR 100 are electrically connected to a second transformer end tap 23 that is phase shifted 180 degrees from the first end tap.

Microprocessor controller 25 includes a wave signal generator device to produce a first wave signal and a phase shifted signal. The signal generator device may be powered by a switch mode supply or a transformer supply as illustrated.

A first SCR gate 73, a second SCR gate 83, a third SCR gate 93 and a fourth SCR gate 103 are electrically connected to the controller 25 enabling the controller 25 to trigger the first SCR 70, second SCR 80, third SCR 90 and the fourth SCR 100. The cell 30 is electrically coupled to the rectifier circuit 60 by cables 65. Controller 25 controls rectifier circuit 60.

Referring to FIG. 3, the reverse polarity chlorinator 10 of FIG. 2 utilises the first SCR 70, the second SCR 80, the third SCR 90 and the fourth SCR 100 to control the timing of a first wave 130 and a phase shifted wave 140 which are generated by the wave signal generator function of controller 25. The control circuitry as shown FIG. 2, permits control of the timing of the first wave 130 and the phase shifted wave 140 in order to restrict current flow to the cathode when the polarity of the first wave 130 and the phase shifted wave 140 is reversed. Controller 25 incorporated into the chlorinator 10 controls the timing. As shown in FIG. 3, the graph represents voltage against time, showing the first wave 130 from a first end tap 22, having a phase difference of 180 degrees in comparison to the phase shifted wave 140 from second end tap 23. The wave signal generator function of controller 25 then creates the first wave 130 and the phase shifted wave 140 with the phase difference of 180 degrees. The first wave 130 and the phase shifted wave 140 both have amplitudes of a maximum of about 30 volts and are both at a frequency of about 50-60 Hz when generated by a transformer connected to a mains supply. The frequency can be much higher when powered by a switch mode supply.

In use, a positive charge signal is applied across the cell 30, electrode 50 acts as a cathode, which results in a build up of scale on the electrode 50. When the positive signal has been communicated to the cell 30 for about 4 hours, a sufficient amount of scale is deposited on the electrode 50 to reduce the volume of chlorine that is produced at the anode 40. After about 4 hours of operation, as measured by the timing function of controller 25 shuts off the power to cell 30. Initially, the potential difference between the two electrodes 40 and 50 is equal to about 6 volts. The potential difference decreases to 2 volts over a few minutes after which the potential difference continues to gradually decrease at a much slower rate. After cell 30 is electrically isolated from its power signal supply, controller 25 repeatedly measures the potential difference between the two electrodes 40 and 50 to determine if the potential difference is below an acceptable level of 1 volt. Testing can be done at a frequency of from about 1 Hz to 16 MHz.

After reversing polarity of the cell power supply, a series of low voltage discharge pulses are applied to the cell to reduce the residual potential difference between the electrodes. As shown in FIG. 4, the discharge pulses 160 supply electrons to the cathode, similar to the rectified signal of the prior art, however, the discharge pulses 60 have relatively small amplitudes and a very short pulse duration and serve to prevent an initial massive voltage and current spike and the cathode thereby preventing damage occurring to the catalyst coating on the electrode 50 previously acting as a cathode. The first half wave 165 exhibits the same spike phenomenon but as the magnitude and duration of the pulse is relatively small, little or no shock is applied to the electrode coating. The discharge pulses 160 are transmitted after the controller 25 has determined that the potential difference is below an acceptable level, say 1 volt. It should, however, be appreciated that the discharge pulses 160 may be communicated without the controller 25 measuring the potential difference. The discharge pulses 160 are produced by triggering the first SCR 70 when the first wave 130 has a phase value of about 178 degrees. Communication of the first wave signal 130 is prevented by the first SCR 70 when the phase of the first wave 130 is about 180 degrees. Subsequently, the third SCR 90 is triggered when the phase shifted wave 140 is at about 178 degrees phase, and communication of the phase shifted wave 140 is prevented by the third SCR 90 when the phase of the phase shifted wave 140 is about 180 degrees. This process is repeated for 1 second, allowing the communication of about 100 discharge pulses 160 to the cell 30.

The residual charge is believed to be generated by charged ions that are trapped around the electrode surface. It is also caused by the addition of the inductive energy that is stored in the circuitry of the prior art devices and in the cables that connect the cells of prior art devices to the circuitry of prior art devices.

In prior art devices, the large initial negative voltage spike and the large initial positive voltage spike are commonly in excess of 30 volts and over the course of operation of the prior art devices, will do considerable damage to the surfaces of the electrodes of the prior art devices. The damage done is cumulative and reduces the ability of the prior art devices to produce chlorine and ultimately requires the replacement of the electrodes.

The reverse polarity chlorinator 10 of FIG. 1 prevents the creation of the large initial positive voltage spike and the large initial negative voltage spike by communicating discharge pulses 160, shown in FIG. 4, to the cell 30. The discharge pulses 160 are communicated to the cell 30 to produce a discharge spike 165 that has a sufficiently low voltage to prevent damaging the coating of rare earth metals on the two terminals 40 and 50. Normally, the SCR would remain excited until the bulk of the stored charge has passed in a single cycle. However, the discharge pulse duration is short enough that the change in polarity of the end tap reaches a sufficient negative polarity to stop excitation of the SCR before the bulk of the discharge occurs. When the residual electrode charge has been completely depleted, the output to the electrode may ramp up to full operational parameters.

Referring to FIG. 5, there is provided a schematic diagram of a second embodiment of the reverse polarity chlorinator 10, which incorporates a relay circuit 180. The relay circuit 180 incorporates a switch 190 and a resistor 250. When the switch 190 is closed a short circuit is created between the two electrodes 40 and 50, which discharges the potential difference between the two electrodes 40 and 50 as well as any induced charge in the control circuitry, thereby achieving the same effect as the discharge pulses 160. FIG. 6 represents an alternative embodiment to that of FIG. 2.

In FIG. 6, the rectifier circuit 60 includes transistor switch devices such as MOSFETS 250 instead of the SCR switch devices shown in FIG. 2.

Also shown in FIG. 6 is a bleed circuit 255 represented by a transistor such as a MOSFET, electronic relay device or suitable switching mechanism 256 and a regulating resistor 257. This bleed circuit 255 can also be incorporated into the circuit arrangement of FIG. 2. The gate 258 of

MOSFET 256 is electrically coupled to controller 25 by conduit 259.

FIG. 7 shows graphically, the operation of bleed circuit 255 in an electrolytic cell assembly of the invention. The circuit is activated when the electrodes are isolated by a pulse width modulated signal from controller 25 to switch on bleed circuit 255 which allows the residual electrode charge 210 and any induced circuitry charge to decay to a zero value.

FIG. 8 shows graphically, the controlled discharge of residual electrode charge in the circuit arrangement of FIG. 6. The MOSFET switching devices of FIG.6 are initiated at position 301 on the power cycle with a series of pulse width modulated signals when the transformer potential is lower than the residual charge on the electrodes. Power flows back through rectifier circuit 60 from the stored charge potential on the electrodes until the pulse width modulation on the MOSFET is deactivated at point 302 on the power cycle. This function is repeated for several cycles until the residual charge decays to zero or an acceptable value.

FIG. 9 illustrates graphically, the operation of the circuit of FIG. 2. Prior to activation of the controlled electrode discharge step, a residual charge potential 401 exists on the electrodes. The SCR switching device is actuated at position 402 immediately before the transformer polarity reverses. At position 403, the transformer potential is zero but the SCR switching device remains excited by the residual charge potential on the electrodes. At position 404, the electrode charge potential has been reduced at the point where it intersects with the transformer potential 405. At this position, the excitation of the SCR switching device ceases and no further current flows in the circuitry. This series of steps is repeated for a predetermined number of times by decharge pulses of predetermined amplitude and frequency until the residual charge on the electrodes approaches zero or some other predetermined acceptable value.

It readily will be apparent to persons skilled in the art that many variations and modifications may be made to the various aspects of the invention without departing from the spirit and scope thereof.

Claims

CLAIMS:

1. A method of reversing electrode polarity in an electrolytic cell, said method including the steps of> isolating electrodes of said cell from an electrical charge applied thereof by a power source; effecting a controlled discharge to a predetermined value of residual charge carried on said electrodes; effecting reversal of polarity of said electrical charge applied by said power source; and reapplying said electrical charge whereby a direction of current flow between said electrodes is reversed.

2. A method as claimed in claim 1 wherein said residual charge is at least partially discharged by the application to said electrodes of charge pulses of opposite sign to said residual charges on respective electrodes.

3. A method as claimed in claim 2 wherein said charge pulses are of a predetermined voltage.

4. A method as claimed in claim 2 wherein said charge pulses are of a predetermined duration.

5. A method as claimed in claim 2 wherein said charge pulses are applied with a predetermined frequency.

6. A method as claimed in claim 2 wherein a predetermined number of charge pulses are applied to at least partially discharge said residual charges on respective electrodes.

7. A method as claimed in claim 2 wherein said power source includes one or more SCR switch mechanisms.

8. A method as claimed in claim 7 wherein said one or more SCR switch mechanisms are energized immediately before change of polarity in said cell whereby a driven side potential of said one or more SCR switch mechanisms and a potential of said residual charge approach an equilibrium value.

9. A method as claimed in claim 8 wherein said SCR switch mechanisms are energized for a number of times until said potential of said residual charge reaches a predetermined value.

10. A method as claimed in claim 2 wherein application to said electrodes of charges pulses is effected by energizing the gate of a transistor switching device with a pulse width modulated signal whereby a potential of said residual charge decays at a predetermined rate towards a zero value.

11. A method as claimed in claim 10 wherein said pulses width modulated signals are applied periodically to discharge said electrodes to a predetermined value.

12. A method as claimed in claim 1 wherein said residual charge on said electrodes is at least partially discharged by effecting an electrical short circuit between said electrodes.

13. A method as claimed in claim 1 wherein said electrical short circuit discharges, at least partially residual inductive charges in electrical circuitry coupled to said electrode conductors.

14. A method as claimed in claim 12 wherein controlled discharge is effected by interrupting current flow from residual charges in said electrodes before a complete discharge occurs thereby allowing a controlled discharge rate.

15. An electrolytic cell assembly adapted for reversible polarity operation, said cell assembly including:- at least two spaced electrodes; and control circuitry, in use, to selectively reverse polarity in electrical charges applied to said electrodes at predetermined intervals, said electrolytic cell assembly characterized in that said control circuitry is adapted to at least partially discharge residual charges on said at least two spaced electrodes before reapplication of an operating charge of reversed polarity.

16. An electrolytic cell assembly according to claim 15 wherein, in use, controlled discharge is effected by interrupting with a switching mechanism, in said control circuitry, current flow from said residual charge before complete discharge of said residual charge occurs thereby allowing a controlled discharge rate.

17. An electrolytic cell assembly as claimed in claim 16 wherein said switching mechanism includes one or more SCR switching devices.

18. An electrolytic cell assembly as claimed in claim 16 wherein said switching mechanism includes one or more transistor switching devices.