A Stable, Multipurpose DC Power Supply for Electroanalytical Work M. Jaffar Department of Chemistry, Ouaid-i-Azam University, Islamabad, Pakistan Qazi Zahid University Grants Commission, Islamabad, Pakistan
With the introduction of modern electronic integrated components, electroanalytical measurements of a varied nature are now possible with at a high degree of precision and accuracy. This is specifically true of potentiometric work involving the use of standard and reference electrodes, including ion-specific electrodes, whereby relative potential differences are determined in various analytical situations such as millivolt measurements, redox titrations, precipitation reactions, and amperometric estimations. A prerequisite to these experimental situations is a stable power supply capable of delivering dc power to the relevant circuits. In addition, most CMOS chips available commercially these
days also require noise-free dc power for their smooth performance. Normal voltage requirement in such applications is f5 V, f9 V, f12 V, and *24 V. Only a versatile supply could possibly afford to cover this diverse range of operating voltage. I t has become possible recently to evolve such multipurpose power supplies. We have introduced a convenient and cost-effective solution to the problem by fabricating a dc power supply using easily available components. This work describes the necessary details of the supply. The circuit diagram is given in the figure and includes details on component specifications.
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Circuit diagram for the dc power supply.
Volume 65
Number 8 August 1988
743
Typlcal Parameters of the Regulators Used In the Power Supply
Parameter
7805 7905
7812 7912
7815 7915
7824 7924
Input voltage range Load regulation Ripple rejection Output resistance Line regulation output noise'
7-25 V 0.2% 70 dB 30 m 0.2 % 0.05 m v
15-30 V 0.4% 80dB 75 m 0.2% 0.08 mV
18-30 V 0.5% 60dB 95 m 0.3% 0.09 mV
27-38 V 0.8% 56dB 150 m 0.3% 0.2 mV
"up to
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Basically, the power supply operates on the diode-based ac to dc conversion. The step-down transformer provides appropriate voltages rectified in later stages through bridge rectification and filtered for effective removal of ripples. An
744
Journal of Chemical Education
IC regulator, series 78/79, of the fixed voltage type (RS, England) is used for voltage regulation in individual stages. This regulator has internal fold-hack, thermal, and shortcircuit protection. The supply has the flexibility of operation on 220 V ac or 110 V ac, 50-60 Hz, power lines. The output current load is rated at 1 A. Typical output voltage characteristics are i5 V (f0.2 V); i 1 2 V ( f 0.5 V); f15 V (f0.7 V); i 2 4 V ( f 0 . 9 V). Performance parameters of the regulators are summarized in the table. The supply has the flexibility of increasing output voltage, say from 5 V to 6 V or 9 V. Flexibility of increase in output voltage is true for other ranges. This can he achieved easily by incorporating auxilary circuitry at the appropriate output level. The total cost of the finished supply is US $20. We are grateful to the University Grants Commission for financial assistance under the low-cost project scheme.
