Dwight F. Mowery, Jr.
Southeastern Massachusetts Technological Institute North Dartmouth, 02747
I
1
A Dual Range Direct Current Power
S I J ~for~ Student I ~ Use
In an effort to capture student interest and also provide first hand experience wit,h important electrochemical processes such as electrolysis, electrodeposition, ion mobility, electrophoresis, electrodialysis, etc., a direct current power supply was desired for general use in undergraduate chemistry laboratories (see Fig. 1). The experiments in which this supply would he used dictated a 6 amp capacity at a voltage from 0 to 120 v and a 1 amp capacity to 250 v. A small ripple could be tolerated so a bridge rectifier without further smoothing was found sufficient. An accurate ammeter, A , was required but a voltmeter, V, of 5% accuracy would satisfy the requirements of most of the contemplated uses. Proper fusing and safeguards against accidental fuse burnout and shocks when used by students was also a requiremcnt. In addition the construction had to he rugged enough to withstand hard usage arid because of the large number needcd the cost, including student labor for const,ruction, had to be kept under $100 each. The basis of operation finally chosen was a 4 amp, 240 v autotransformer with a 280 v coil tapped a t 40, 120, 200, and 240 v. With 120 v ac applied across of the coil (240 v tap) a rectified dc current a t a voltage from 0 to 120 v could be produced while 120 v ac across 3/7 of the coil (120 v tap) generated a rectified current of voltage from 0 to 255 v. A multiple pole two-position switch was used to change the power input to the autotransformer and at, the same time place a shunt across the ammeter for the lower voltage higher current range. Since this switch was required no matter what rectifier arrangement was used, an appreciable saving was effected by adding more poles to the switch so that the 8 rectifiers were changed from series to parallel in the bridge, thus allowing a larger current at lower voltages. The cost of a switch which could make or break a 6 amp, 120 v dc current was prohibitive but one which could carry this current was economically feasible. Several safety devices were incorporated to prevent damage to
the supply if used carelessly. First, the autotransformer coil was fused for 4 amp on the high voltage rangc and 8 amp on the low range. Second, a microswitch was installed which was activated by a long protruding screw replacing a short one in the autotransformer brush-holding disc in back of thc coil. This switch activates a relay which closes the main circuit to the transformer coil when the dial is set to zero. As the voltage is turned up from zero, current through the normally open relay contacts, now closed, powers the relay coil and keeps the contacts closed until the main switch is finally thrown off. This safety feature prevents the power supply from being switched on at a high voltage and small external resistance with consequent fuse burnout. A third safety feature is a mechanical lock of the rotary voltage range changing switch by means of a small solenoid activated from the normally closed contact of the rclay. A spring keeps the solenoid core pushed into a hole in a metal disc fastened to the rotating shaft of the switch. When the solenoid is activated after the main switch is throlvn on and before the relay is activated by turning the autotransformer dial to zero, the range change switch may he operated. After the voltage is turned up, the range change switch is locked and can only be changed by switching the supply off. I n addition to these safety features for protection of the power supply the operator is protected from shock by grounding the casc to the elcctrical outlet using a standard grounded plug on a three conductor supply wire. A polarized female type dc outlet with concealed contacts is used and all experimental equipment used with the power supply has been made as safe from shock hazard as possible. The student is carefully cautioned never to plug equipment into the power supply until it has been completely assembled. Figures 1, 2, and 3 show the wiring diagram, the physical layout, and the external appearance of the power supply. The switches and components are given the same letters in all diagrams for easy identification. The wiring diagram shows the switch positions when utilizing the low voltage high current range. Depending upon the relative increased cost of silicon rectifiers capable of handling a 6-8 amp load at all voltages and of the cxtra poles on the rotary switch including the labor of wiring them, it may in some circum-
Figure 1.
Figure 2.
Wiring diagram.
Physical layout.
Figure 3.
External oppearonce.
Volume 45, Number 1 1 , November 1968
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739
stances be more economical to eliminate the seriesparallel switching feature for the rectifiers. The electromagnctic lock on the range changing rotary switch is believed to be a very desirable feature as a sudden switching to the higher voltage range with the power supply loaded would almost certainly burn out a fuse. Thercfore, there would be no reason to make this switch heavy enough to make and break an 8 amp current. An additional outlet for alternating current could easily be provided if necessary but an ac ammeter and possibly voltmeter would he required although the no-load ac voltage is essentially as marked on the Variac dial for the hieh voltaee range and anaroxim~telvhalf the marked reading on the'low voltagc range. an additional cost of about twenty dollars a rhoke coil, two condensers, and a more accurate mult'range voltmeter could be included in the supply to smooth out the ripple but since this would be required a few experiments it was decided to include this equipment in a small package which could be plugged into the dc outlet of the power supply when needed. gives the dc voltage produced at ~b~ and at fu&load and also the ac amneraee at full load for both the low and high voltage ranges. I t can be seen that the S amp fuse will allow a full scale ammeter reading of 7.5 amp dc up to a Variac setting of about 230 hut slightly less above this setting while the 3 amp fuse will allow a full scale ammeter reading of 1.5 amp dc up to a Variac setting of about 210. The component list gives the parts required with specifications and prices both individually and in lots of 50 or more (in paren-
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Variao setting
6 15 24 43 55 60 70 80 85 95 105 110 120
20 60 40 100 120 140 160
200 220 240 260 280
voltage ~ m g e - ~ ~ i voltage g h RangA C ~ ~ A C ampa ~ 7.5 D C Volts a t 1.5 s s m p s ~ ~c o - h d 1.5 amps a m ~ DC
D C Volta N O - I O * ~7.samps 5 10 15 25 35 40 50 60 70 80 90 9s 105
1.0 1.4 2.1 3.5 4.2 4.8 5.5 6.2 6.8 7.5 8.2 8.9 9.6
13 31 50 90 105 125 140
10 25 45 80 100 120 135
0.7 0.8 1.1 1.5 1.8 2.1 2.3
165 175 195 210 23s 255
150 165 180 195 216 230
2.6 2.9 3.1 3.3 3.6 3.9
..
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740
Performance of Power Supply ---LOW
Journal of Chemiml Educofion
thesis). ~h~ total of components comes to $83.12 for a single unit or $73.01 each for a lot of 50 or more. Component List 2--Silicon reotifiers (r), full wavebridge. 3 a m ~ I$s v ao/l% v do. Leotronic lleseareh L S ~ Sho., . 715 A ~ st., C ~ ~h i l a + e b h ~ pa. a , $6.47 each or General Electric DnlAD117.55 each (S6.20 ehoh 1x1 lot of 50). I-TYP~ X ~ O variac H adiustable autotransformer (TI, General Radio Co., 135 (31.50). I-Ammeter (A) s 1 . 5 "do. Simpson 41/nin. WidcVue. 615 (121 1-Voltmeter (vj. &so0 v do 3 in. rectangular. Alco Electronic Produets Inc.. Lawrenoe Mass. Model S-35 $3.50 (3.501 I - F . p E r y . % e ~ h ( 4 , 12 pols. 2 ~ o s k i o n .Mdlory 1332L. 1 amp. 300 v so. irJ.14 ,'.I-,,
1-Snap snvitoh (c). lever t y ~ eCCA series 760-250. 60.76 (0.541 I-Toecle switoh (b) SPST b u t l e r - ~ a m m e r7501k12 15 amp. 10.52 (0.431 I - R ~ I ~ Y(R) DPDT ousrhisn series 900 115 v ae.,io amp. $1.75 (1.401 l - - ~ i o ~ i ~ ~ - pea&et & ~ l Bud G I 8 9 2 8 X 1 3 X 81/r 1". $5.95 (4.76) I-rureholder. D u ~ ~ T ~ & H I (for P ,1)' X I'/l in. fuseJ0.32 (0.32) I-l'oseholder. Duss Type H J M . for I/. X 1 in. fuse50.32 (0.321 I-Fuse (F3). 8 AG. 3 a m p $0.06 (0.06) 1-Fuse (118) 3 AD 8 arnD 10.09 (0.081, I-Polarwed k o k e i ( d ) , ohhasis mountmg, two oonthot, 10 s m p , Cinoh-JonS-302-AD, $0.27 (0.18) 1-l'arallel ground plug, Allied Eleotronioa 522717. $0.45 (0.40) eft. Power cord, 10 amp, 3 oonductor, Allied Electronioa, 482384. $0.75 ,a
