Economical Electrophoresis with a Controlled-Current Power Supply David R. Hatiman' and Willlam H. Courtney Western Kentucky University, Bowling Green. KY 42101
Electrophoresis is a powerful tool used in biochemistry and molecular biology for the purification and identification of proteins and nucleic acid fragments. Commercially available apparatus is expensive ($600+). The two parts of the apparatus, chamber and power supply, can be constructed from available plans (1-3) at considerable savings. Circuit diagrams for economical dc power supplies for electrophoresis are available in the literature (5, 7,8). The power supply diagrams proposed by Davis (7)and Madeira and Pires (5) are quite simple with limited ability to vary the dc current. Meyer (8) utilizes a more sophisticated circuitry but relies on an external source of dc power. The electrophoresis power supply illustrated (see figure and table) features a controllable current (0-90 mA, dc), the use of common 120 V, ac power, and a materiala cost of annroximatelv 550. The pilot light, PLI, and switch, SI are for convenience of ooeration. The isolation transformer. T I .allows the neeative ohput terminal to be connected tb the building ground making the unit safer to operate. Another safety feature is the fuse, F1.The dc voltage at "1" is approximately 180 V; the positive lead of CI must be connected at this point. The voltage at "2" is approximately 375 V with reference to the negative bus, "3" (not the negative terminal). Resister, RI, limits the current in diode, DQ.Diode, D3, holds the base of transister, Ql, at 7.5 V with respect to the negative bus, "3". Resistor, R3, limit9 the current to 150-mA maximum with a short circuit across the load terminals. Potentiometer, Rz, permits the adjustment of the current to the load (an electrophoresis chamber). The maximum current available is limited by the load resistance; the highest load resistance which will allow 100 mA is approximately 2300 0.A typical electrophoresis apparatus has a resistance of 1,000-15,MH) fl and requires 1&30 mA of dc current. This Dower s u ~ ~isl capable v of maintaining constant current for electropho~e& chambers as long as thecurrent control is set below maximum. An ammeter or voltmeter could be inserted
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in the load circuit:. ammeters are ouite ex~ensive.but voltmeters are widely available at a low cost. ' Caution: This power supply is capable of delivering high voltages at lethal currents. When using the power supply, do not connect it to the 120 V outlet until all dc connections have been made and the electrophoresis experiment is to begin. The circuitry for the power supply should be housed in a box, grounded as shown in the figure. Female output jacks should be installed to prevent accidental contact with the output potential. ~
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1. Brewer. J. M.:Ashworth,R.B. J. Chem.Edur. 1969,46,4146. 2. Racu%m.D.:White. L.J. Chern.Educ. 1912.49.439440.
3. Tibbs. J. J.Cham.Educ.1973,60,862863. 4. Brsboan,G. D.; Waugh,D.S. J. ChemEduc. 198463,54b542. 5. Madeira, V. M.C.:Pires, E. M . V.J. ChemEduc. 19S6.63.11W-1I11. 6. Hopkin%.T.R.:Sreekrishna, K. J.Cham.Educ. 1987.64.279-280. 7. Davis. B. J. N.Y. Aeod. A n ~ l s1964.121.404-427. 8. J. J.J. Chem. Edur. 1983.60.143.
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Conbolledcunem power supply.
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Presented to the Kentucky Academy of Science, November 1986. 72nd Annual Meeting. ' lndlvidual to whom requests for reprints should be addressed.
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20 kR. 10 W 1800 R. 50 W pol. 50 0,10 W 40 pF. 250 WVdc 40 pF. 450 WVdo ECG 163A(700V. 10A)
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PL,
ECG 125(1000V. 25 A) 7.5 V. 112 W Zener 1201120V. 35 VA 0.1 A, fast acting SPST 120 V. 3 A Neon pllot lighl, 120 V
Volume 67 Number 6 August 1990
703
