Radio mirror (Jan-Oct 1923)

October o, 1922 R A 1 ) 1 O D f G E S T— Illustrated Determination of Resistance Values for Rheostats Relations of Resistance, Voltage and Amperage for Various Tubes By II. J. Marx THEHF lifts been created a decided confusion in the Blinds of the Radiophans in regartr-to the proper resistance In rheostat* to be OMd for the various tubes now on the market. :mmcdiate result has been the flooding of the market with a series of rheostats of a resistance ran^e rOBlrilH from one ohm for a poivpr rheostat icontrollins two or more tulusi to 60 ohms for the UV-199 ami C-j:>:> tubes. Pans are using dry cells connected in and some in parallel, or Storage batteries with single, double and throe cells. Each chanse in current source, and also in tubes used necessitates a Gonsidi of the proper rheostat required, Apparently this has been exirem. ly pttZZUng to fans but can easily be remedied by an application of Ohms I. aw. Direct Current Circuit This part of (he vacuum tube circuit. which pertains to the filament lighting, is a simple direct current circuit. As such, it follows Ohm's law: £ I = — . R The proper application of this formula will solve all problems covering the proper selection of rheostats. Function of the Rheostat For example, the familiar UV-2M and C-300 operate at 3 volts' potential and draws approximately 1 ampere. The average source of current supply is the 6 volt storage battery. By introducing a variable resistance, such as a rheostat: the voltage across the filament can be varied from about 3 volts to 6 volts. Then if the tube operates even, better at 4.5 volts than at five, this can be taken care of bymeans of the rheostat. Obviously, by increasing the rheostat resistance, the range of variation also can be increased, from even lower values to the maximum 6 volts of the battery. Since the tube does not begin functioning until about 4 volts are applied to the filament, there is no gain in the surplus control range. In selecting the rheostat for any combination of tube and battery, the resistance should be such as to furnish a voltage range covering that of the operating range of the tube. Filament Resistance Going back to the tube mentioned, if the operating voltage is five and the current consumption at that potential 1 ampere, the filament resistance can be computed by applying Ohm's law. E 5 R = — = — =5 ohms. I 1 The tube begins operating at about 3 volts on the filament. In order toxfind the consumption at this voltage, Ohm's law is again applied. E 3 volts I = — = = .6 amperes. R 5 ohms The filament resistance varies slightly as the heat of the filament is changed, but this can be disregarded. The source of current, however, has a potential of 6 volts. If only .6 amperes is wanted, the required total of resistance, including filament and rheostat will be: E 6 volts R = — = = 10 ohms. I .6 amperes The filament resistance is 5 ohms so only 5 ohms more are required in the rheostat. The average storage battery, when fully charged, is likely to have a slightly higher voltage, so the standard rheostat used was the 6 ohm type. Power Rheostats When rheostats are used to operate two or more tubes connected in parallel, conditions are a little less simple. The effective filament resistance is the result of the resistance of one divided by the number of tubes connected in parallel. The current consumption is that of one tube multiplied by the number of tubes. For example, where two tubes are operated by one rheostat, the required resistance becomes: E 6 volts R = — = = 5 ohms. I .6 amperes X 2 The effective filament resistance is only half of one, or 2.5 ohms. The required resistance in the rheostat is therefore 2.5 ohms. The standard two tube power rheostat has a resistance of 3 ohms. The factor to be considered in power rheostats is whether the resistance wire has the carrying capacity for the current required; if not, the resistance wire will fuse. If an ordinary 6 ohm rheostat is used for more than one tube, this usually happens. The safe limit for the standard 6 ohm rheostat is 1.5 amperes. Dry Cell Tubes As previously explained, it was the development of the dry cell tube that started most of the trouble in respect to rheostats. In order to make this as clear as possible, it will be advisable to analyze the problem for the two popular types, under different battery conditions. The UV-201A and C-301A tubes operate on a filnmen. voltage of five, but only consume .25 nmpo'-es of current. The filament resistance i lien will be 20 ohms. Assuming that the tube MgiM fuii'-tiuninu at :', volts, the current consumption will be .1 5 amperes. If a t! volt storage battery is used, the total resistance required is: slightly, but can be used. If three dry cells in series are used, the voltage of 4.5 is not sufficient, whereas four cells give 6 volts, making the condition parallel to that of the storage battery, UV-199 and C-299 These tubes operate at 3 volts and draw .06 amperes, tints giving a filament re i rt~C Be f. **> : 0> c ■Kg 9 ■_ < Is Bag S V. ~ — :£■ s 3 4-1 /. 0 3« i s 3 DfU> u k s 13455 u be ■z.r-~ 0(S» ■/. S 3 ... c ■ ■-■ '.T-rt Otl Cm OS rv C-300 6 5 1 5 10 5 to 6 22.5 .0005 C-301A CV-261A 6 D .25 20 40 20 to 30 22.5 45toS0 1 to 5 .00025 YVD-11 YV'D-12 1% 1.1 .25 4.4 10 5 to 6 22.5 45 60 80 100 0 1.5 3 4 2 .00025 UV-199 C-299 4%-6 3 .06 50 90tol30 30 to 75 22.5 -ill 60 SO 100 0 3 4.5 6 1 to 5 .00025 UV-201 C-301 6 5 1 5 10 5 to 6 22.5 40 60 80 100 0 4.5 6 1 .0005 E 6 R= — = — = 40 ohms. I .15 The resistance of the filament is 20 ohms, so the rheostat resistance should be 20 ohms. Naturally, a 25 or even 30 ohm rheostat simply increases the range sistance of 50 ohms. They do not start functioning until a filament voltage of about 2.3 is reached. At this voltage, the current draw is: E 2.3 I = — = — = .046 amperes. R 50 Now if three dry cells, connected In series (two cells leave no margin) are used, in' potential at the source is 4.5 volts. The required total resistance will then be: E 4.5 R = — = = 100 ohms. I .046 The rheostat resistance required will then be 50 ohms. Sixty ohm rheostats are sometimes used. If a two cell storage battery (4 volts; is used, the resistance of the rheostat should be about 30 ohms or more. Suppose, however, that a 6 volt storage battery is used, the required total resistance will be: E 6 ■V R = — = =130 ohms. I .046 This indicates that the rheostat resistance must be SO ohms. Under these circumstances it fs of particular importance that the voltage should not exceed 4.5 or even 4 volts. It is suggested, in this case, that a permanent resistance of 25 or :>n ohms be inserted in series, the rheostat requiring only 50 ohms. If, then, the rheostat is accidentally turned on full, there will always be the fixed resistance in series with less possibility of burning the tubes. The use of a fixed resistance is recommended only where the voltage at the source greatly exceeds that required for operation. A small margin over the operating voltage is always necessary in order to compensate for deterioration of battery and variations in tubes. WD-ll and 12 Tubes When using TTD-11 or 12 tubes with a single dry cell, the required rheostat resistance will be found to equal 6 ohms. SECOND ANNUAL wimmiomy ffTEKNATIONAL ANNUAL COLISEUM NOVEMBER 20th to 25th TUESDAY to SUNDAY INCLUSIVE The World's Greatest Exposition of RADIO and Associated Industries and Conclave of Manufactures, Distributors, Jobbers — Dealers and Radiophans. The Center of the Midwest Market HAVE YOU CONSIDERED IT ? WIRE OR WRITE OFFICE: 127 NORTH DEARBORN ST. TELEPHONE — STATE 4161 H. J. HERRMANN JAMES F. KERR Managing Director Manger