A Constant Current Regulator.
H. K. Patton, Vanderbilt
Pniversity, SLti.hvilie,Tenn. R E need for mhintnining a constant direct current of about Tampere in a resistance heater has led to the development of a
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regulating device, which, when used in conjunction with lead storage cells, gives satisfactory compensation for small variations in current due t o changes in circuit resiatance or battery voltage. Except for the galvarionieter and potentiometer, which are ordiiidiily available 111 a clirmistry l:ihoiatory, the regulator was conjtructed frcirn ~ r : ~ i i d : ~ ttii:iteii:iI~, d t l i c s total cost of which \vas less tils11 $10.
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Itrater and ICegirIator Circuit Uiagram Il-\olt lead batter, 100-mfd.. 150-1olt capacitor (.alvanometer
Heater Lens Milliammetrr P . ( ; . Photocell P o t . Potentiometer H,. btandard resislor. 1 ohm Hz. Rheostat, 30 ohms H;. Control resistor, about 1 o h m n4. 2-megohm, 0.5-w.att nL. 1200-ohm. 2-watt 12,000-ohm. ?-watt Ha. .S. Light 3ourc.e 1'. I17L7-GT radio tube H: 2 - v o l t lracl rrll I..
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After the initial setting is made, regulation of the desired curient strength is completely automatic. If the current through the heater does decrease, owing to an increase in resistance of the heater circuit or decrease in battery voltage, the voltage across control resistor Rs becomes smaller than that of the potentiometer, and the galvanometer is deflected in such a direction that more light is reflected from S onto P.C. The resulting increase in current supplied by the radio tube almost completely compensates for the initial decrease in heater current. A heater current larger than the desired amount causes a similar but opposite reaction of the various control elements, resulting in decreased current output from the radio tube. Hence in either case the total heater current remains essentially constant. These compensations are definite and almost instantaneous.
The range of variation in current which can he controlled by the device is limited by the maximum output of the radio tube. The tube used, Type lliLi-GT, has a mauimum output of about 50 ma. Hence the control potentiometer could he adjusted so t h a t the initial output of the radio tube is about 25 ma., and the device lvould successfully compensate for any changes in the heatcr circuit which caused the current supplied by the batteries either to increase or decrease by about 25 ma. The range of control could undoubtedly be extended by the use of a tube with a grc:iter n i a h i u n i current output, or by the use of tn-o or more t u l m in parallel. Thc sensitivity of the device is affected by the nature of sc~vernlof its coniponents. Increased sensitivity can be acconipiislied by any one, or any combination of the following: use of H nior(~intense light source, a more sensitive galvanometer, or a more sensitive photocell.
In the present setup, the light source consists of a single-element 6-volt incandescent lamp manufactured for use in automobile headlamps and rated a t 30 candle power. The galvanometer is a Leeds and Sorthrup, Type R(2500-e). The photocell is manufactured by the General Electric Co. and bears the designation C12-868/PJ-23. .4n important part of the regulator is the lens, which is used to focus light from the source onto the galvanometer mirror in such a manner that the cross section of the beam as it waches the photocell is circular and about 2 mm. in diameter. This allows a very slight movement of the galvanometer coil to vary the intensity of light falling on the photocell from practically zero to the maximum available. I t is possible that greater sensitivity might be obtained by the use of a needle-type galvanometer fitted with an opaque vane similar to that used by Longsworth and MacInnes [ J . Optical SOC.Am., 19, 50 (1929)l in their regulator. That part of the heater current supplied by the radio tube is, in the setup shoxn, rectified and filtered 60-cycle alternating current. By means of a cathode ray oscilloscope, the "ripple" voltage was estimated t o be less than 0.5% of the total plate voltage. Because the control current supplied by the tube is only n small fraction of the heater current, the error introduced due to the alternating current component is negligible in the present work. The use of a better filtered power supply, or a battery source. for plate voltage might be desirable for certain applications. %-lien the regulator is connected directly to line current as shown, the heater circuit wiring is a possible source of electrical shock, and should be handled with care. This feature can be removed by the use of an isolation transformer. llaintenance of a constant current by this device is dependent on the constancy of both the voltage of the Potentiometer working battery, W , and the resistance of resistor RS. In the regulator described, the working battery is a 100-ampere-hour lead storage cell, which is kept connected to the potentiometer a t all times. For operation over long periods of time, the change in xvorking battery vo1t:igr niight neccwitate periodic rcndjustment of the potentiom-
The currei11 fiu\virig 1 Iii~ough thc lientrr, H , consists of contt,iljutions from tww . ( J i i t ' ( ' r S , Almost all this current is furnished 1)y tlir 12-wtIt buttriirs. 13, hut a small amount, which is autoiii:iti(,:illy v u r i d. ! . I the o t l i r r elements of the regulator in such a ii?xinrr that the tut:ll lirater current remains constant, is furiiiilietl by tlir clrc,tiuri tube, 1'. The amount of this additional w r i , r n t drp,riidr on tlir iiitensit?. of light falling on the photocell, l'.('. Tlii.: light intensity in turn is controlled by the position of t h r n i i i w r on galvanonirter G , which reflects niore or less light i'voni the suuwr. i;,oilto the photocell, depending on the deflect i o n of the g u l v a r ~ u ~ n r t r The ~ ~ . deflection of the galvanometer is tisrcl initially by adjustment of the potentionietrr, Pot., which I1:ilniices f l i p p u t m t i a l U ' I ' O ~ S the control resistor against a part 01' 0i:it ~jro(Iii(,fdt)y t l i t . potrntiomr'tev ivorking battery, V.
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