Wilbur 1. Baker
and Charles E. Boozer1 Emory University Atlanta, Georgia
Direct Current Power Supply Suitable for Freshman Laboratory
Telephone type dry cells used in student determination of the FaradayZ are troublesome and expensive when used only once a year; therefore we attempted to find a suitable replacement. Many power supplies have been described, but none suited this application. A full-wave silicon rectifier was designed for this use and 15 units were assembled a t a cost of983.83 each. These devices have been very satisfactory. Twentyfive freshmen averaged 92.4% ," of the theoretical value using dry cells and stock meters, and 25 high school students averaged 98.3% of the theoretical value using the power supplies and calibrated meters. Figure 1 shows an assembled unit. The table lists typical items which are readily available. All parts are "bargains" and "special sales" of electronics suppliers. The circuit diagram (Fig. 2) is ~ t a n d a r d . ~For some uses a filter choke and a bleeding resistor are required. Because the choke would have doubled the cost, an inexpensive capacitor was substituted.' The resistor is not needed for this use. A rheostat is included in the circuit. Polarity is according to electron flow, i.e., the (-) terminal reduces. The output has some "ripple," but this has no effect because the meter is a currentaveraging device. The transformefl is mounted a t one end of the base using a metal strap. The rectifiers (diodes) are mounted on a strap of metal which also serves as a heat sink to keep them cool. As shown in Figure 1, the strap for the rectifier has a 'lrinch offset at one end for a mounting lug and has space underneath for the studs. Multi-section capacitors should be connected in parallel for maximum capacitance. A short capacitor can be extended by soldering the (+) terminal(s) to a short piece of machine screw and mounting on the heat sink the same way as the rectifiers. A longer capacitor can be fastened to the base using straps.
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1 Present address: Copolymer Rubber and Chemical Corp., P.O. Box 2591, Baton Rouge, La. 2 SIENKO, M. J., AND PLANE,R. A,, "Experimental Chemistry," 2nd ed.. NcGraw-Hill Book Co., Inc., New York, 1961, pp. 12&8. 3 The Headqumters Staff, "The Radio Amateur's Handbook," 41st ed., The American Radio Relay League, Inc., Newington, Conn., Flg. 7-I(B), p. 222. [See also Fig. 7-I(C).] 4 Recommended ratings are (approx.): (a) twice the transformer secondary voltage, and (b) capacitance should be as large as possible, but not less than 500 mfd/amp. 6 This circuit requires a. transformerwitha center-tappedsecondmy. The two secondaries are set out-of-phaae to one another and connected to the rectifiers. Another circuitS for a. cheaper transformer (one secondary) has the same output for the same volt-ampere rating, but uses four diodes. The costs of the components and labor should be evaluated before a choice is made if a sizable number of instruments is needed.
The cover is made of pegboard for ventilation. Corners are braced by wood strips. The cover is securely attached to the base using the simplest method possible. One suggestion is to drop the sides of the cover the thickness of the base and put one screw in each edge. The rheostat is mounted in the cover. Components are connected as shown in the circuit diagram, and all connections are soldered. If some terminals or wires of the transformer are not used, they should be taped or otherwise protected against shorts and shocks. Although this circuit is burn-out proof for short-circuits of several minutes duration, overload -
Figure 1. (left) Uni! orrembied. c~nstr~ction.
Figure 2.
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Iright) Cover removed to show detoilsof
Circuit diagram.
Item and Specifications
Supplier"
Tran~former,135 vdc at 118 ma. Transformer, p, 117 v, 50-60 cyc; s, 12.6 vet, 2 a Capacitor, electrolytic, 1200 mfd, 25 vdc Silicon rectifier. stud mte. - 2 a. . 50 piv Rheostat, variable, 25 w, 15 ohms
B-A A
Zener diode, 4.7 v, 10 w, teat cnrrent 400 ma Circuit breaker, Sylvania type MB-316, 2 a. Fuse, Buss No. MDL, 2 a. 125 v Fuse monnt. Littelflise No. 357001
Cost 0.69 2.49
$
RS
0.39
P
0.09
H $,nd
',
10/9 .OO
0
2/1.39
A
0.25
B-A B-A
5/0.85 0.14
B-A, Burstein-Applehee Co.. 1012 McGee St., Kansas City 6, Mo.; A, Allied Radio, 100 N. Western Ave., Chicago, Ill.; RS, Radio Shack, 730 Commonwealth Ave., Boston 17, Mass.; P. Poly-Pakka Co., P.O. Box 942, South Lynnfidd, Mass.; H a n d R, Herback and Rademan, Inc., 1204 Arch St., Philrtdelphis. 7, Pa.; 0 , Olson Electronics, Inc., 260 South Forge St., Akron, Ohio. Volume 42, Number 12, December 1965
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657
protection should be provided at either the input or output; the latter is easier to hook up and is illustrated (parts for both are listed in the table). The cover as painted white to prevent any accusations that we "give the students black boxes to work with."
658
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Journal o f Chemical Education
This instrument has other uses; addition of a zener diode easily and cheaply changes it to one of constant potential. With 5-15 ohms on the rheostat, the zener listed should have a potential of 4.7 v under fairly wide load variations.
