Analyst, July, 1968, Vol. 93, jbjb. 433435 433 A Stable, Solid-state, High Voltage Source for Electrode Polar isa tion BY E. BISHOP (Chemistry Department, University of Exeter, Stocker Road, Exeter, Devon) A simple, cheap, versatile, solid-state, high voltage source is proposed as a replacement for high tension battery supplies. It is capable of providing up to 70 pA a t 1000 volts, with drift and noise less than 0.1 per cent., and can be used for electrode polarisation, radiation counting tube supplies and other small-current applications. THE obsolescent high tension radio battery of 120 volts has been commonly used in high voltage sources for the supply of small currents for polarisation of electrodes in techniques such as differential electrolytic p0tentiometry.l The high cost and bulk of, e g ., a 1000-volt source built up from such batteries, and the short shelf life and increasing scarcity of the tapped batteries, have initiated a search for a simple, cheap alternative. A Cockroft - Walton voltage multiplier driven by a square-wave generator2 offered a promising start. SOURCE VOLTAGE AND CURRENT STABILITY- In differential electrolytic pot entiometry and other cons tant-curren t techniques, the electrode potential is a function of current density, and if the potential difference between the electrodes is EA volt, the source voltage is Vs volt, the stabilising resistance in series with source and cell (the ballast resistance) is RB, and the cell resistance plus the internal resistance of the source is Rint, then the current is given by- .... .. .. . . (1). vS - E A I = RB + Rint In a differential electrolytic potentiometric titration, for instance, EA will vary from zero to a maximum value dependent on I and, if the electrode processes are fast, on the Q of the t i t r a t i ~ n . ~ A maximum EA of 100 mV would require a Vs of 100 volts to restrict variation of I to 0.1 per cent., while a maximum EA of 1 volt would require a 1000-volt source for the same stability. RB must be correspondingly large, not only to minimise the effect of any change in Rint, but also to give the proper value of I . Partial differentials emphasise the dependences so, plainly, the larger RB the better. Ballast load (the product Vs x RB) is a significant parameter in differential electrolytic potentiometry, and partial differentiation with respect to this product- shows that the larger Vs and the ballast load the better within the limits of Johnson noise.As EA changes rapidly in the equivalence point region and also changes greatly with current density, inadequate stabilisation leads to erratic and drifting potentials. In view of these requirements it was decided to turn attention to a variable voltage source capable of supplying up to 50pA at 1000 volts. 0 SAC and the author.434 BISHOP: A STABLE, SOLID-STATE, HIGH VOLTAGE [Arta&St, VOl. 93 CIRCUIT PRINCIPLES- The circuit, shown in Fig. 1, consists of an inverter and a voltage multiplier. The inverter is a push - pull amplifier with two driver transformers. The output is fed back via C, to the primary of T, at such a level as to overdrive the transformer, giving a clipped waveform that approaches a fast rise square wave.The output of T, carries a nominal 100-volt peak ax. signal and is isolated, so that consistent earthing of positive or negative battery terminals can be used. The multiplier is a conventional series of voltage doublers, and the number of stages can be reduced or increased if a lower or higher maximum output voltage is required. Any silicon diodes capable of handling 200 peak inverse volts can be used, and the capacitors should be generously rated. Regulation is effected by connecting miniature neons across any suitable taps. As shown in Fig. 1, three 70-volt neons in series are connected across adjacent 200-volt taps, and give a nominal 1000-volt output in steps of 100 volts.Connected over two taps (e.g., -300 to -700) the output would be 500 volts in steps of 50 volts, while a single neon in place of three as in Fig. 1 would give a 350-volt output in steps of 35 volts. The unit consumes less than 500mW and is conveniently powered by small dry batteries, of from 6 to 15 volts; the output voltage will vary with supply voltage. Fig. 1. Circuit diagram of inverter multiplier. T, and T, are Rex LT44, 20KO/lKOCT Details of the obtained from Alpha Radio Ltd. circuit are given in the text Leads: R red, B black, W white, G green. CONSTRUCTION- The circuit can be laid out as in the circuit diagram and occupies about 4 x 2 inches on a piece of drilled paxolin. The board can be mounted in a small box, together with the battery and a ceramic wafer switch connected to the various taps so that ten steps of output voltage can be selected.A small square electrical conduit joint box makes a convenient container, and reduces the effect of temperature variations. The neons serve as indicator lamps, and glow steadily when operating satisfactorily; near overload they flicker, while if too large a current is drawn the lamps are extinguished. PERFORMANCE- For a 12-volt supply a typical maximum output is 70pA at 900 volts. Stability was examined by connecting the output to a ballast resistor and a standard (+O.Ol per cent.)July, 19681 SOURCE FOR ELECTRODE POLARISATION 435 resistor in series, monitoring the voltage drop across the standard resistor by means of an E.I.L. 39A pH meter, and recording the difference from a standard voltage on a Honeywell 513 X17 strip chart recorder.The stability of the output is a function of the stability of the supply to the inverter. With a raw 12 volt d.c. laboratory supply, produced by a TABLE I OUTPUT STABILITY OF 1000-VOLT SOURCE Current, RB, RS, Duration, Drift $lus noise, P A n a minutes per cent. 1 109 IW 100 <0.1 2 5 x 108 1 06 60 0-08 5 2 x 108 106 35 0-04 10 1 08 106 100 0-06 20 5 x 107 1 0 4 120 0.09 0.5* 2 x 109 105 70 2.0 2* 5 x 108 l@ 75 1-5 * Raw 12-volt d.c. supply f 10 per cent., 200 to 400-mV noise. Remainder with battery or regulated d.c. supply. 3-phase transformer rectifier, regulated to 10 per cent. and containing 200 to 400mV of noise, an output stability of 1 to 2 per cent. was obtained. With batteries or a regulated d.c. supply (Solartron AS 1411) regulation of the output was 0.1 per cent. or better, and high frequency noise was of the same order. Typical results are given in Table I. The author thanks Mr. M. Riley who carried out the performance tests. REFERENCES 1. 2. 3. Bishop, E., Analyst, 1968, 83, 212. Bollen, D., Wireless Wodd, 1965, 71, 381. Bishop, E., in Shallis, P. W., Editor, “Proceedings of the SAC Conference, Nottingham, 1965,” Received November 7th. 1967 W. Heffer & Sons Ltd., Cambridge, 1965, p. 291.