Radio Broadcast (May 1929-Apr 1930)

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CeCo Manufacturing Company, Inc. PROVIDENCE, R. I. Licensed under patents and applications of the Radio Corporation of America, the General Electric Company, and the Westinghouse Electric and Manufacturing Company. MILLIONS IN USE i«o • No. 290 Radio Broadcast Laboratory Information Sheet July, 1929 FILTER CIRCUIT DATA Tn "Laboratory Sheets" Nos. 258 and 259 some data were given on filter circuits showing the effect on the regulation, output voltage, and tube load when using and when not using a choke at the input to the filter system. Similar data will be found in Roger Wise's article "Characteristics of Power Rectifiers" in the April, 1929, Radio Broadcast. Several readers have written us to the effect that they have not been able to duplicate these curves when using a choke in the input. Some have obtained greater and others less output voltage than was indicated by the curves in "Laboratory Sheet" No. 259. This discrepancy between the values we gave and that readers have obtained is undoubtedly dueto the use of a different size choke coil in the input than was used to obtain the curves on Sheet No. 259. For these curves a standard filter choke of some 30 henries was used. The output voltage, of course, is greatest when there is no choke coil connected in the input and will decrease when a choke is placed at the input. The greater the inductance of the choke, the greater the decrease in output voltage. Those experimenters who obtained greater voltages than the curves indicated probably used an input choke of low inductance and those that obtained smaller voltages used a choke of high inductance. It could appear from these considerations that a small choke should be used but it should be remembered that the primary reason for the use of choke input circuits is to reduce the instantaneous load on the rectifier tubes so that their life will be a maximum. With chokes of low inductance this desirable effect of reduced load on the tube is not obtained to any considerable degree. In designing such systems a compromise must, therefore, be reached between the use of a large choke giving a good tube load characteristic and a small choke giving a less desirable load characteristic but greater output voltage (assuming that the transformer input voltage is not changed.) No. 291 Radio Rroadcast Laboratory Information Sheet July, 1929 VOLTAGE-DOUBLING CIRCUITS TN cases where there is need of a plate-supply device delivering greater voltages than can ordinarily be obtained from the usual type of rectifier tube, such as the type 281, it is possible to use these tubes in "voltage-doubling" circuits. Two circuits together with their regulation characteristics are given on Laboratory Sheet No. 292. Although the 281-type rectifier in ordinary circuits can supply only about 600 volts to the filter system it should be noted that when using these special circuits it is possible to obtain an input to the filter of approximately 1600 volts. The circuit shown at A is probably the more familiar type of voltage-doubling connection. As indicated by the curves the voltage regulation is rather poor but, when only small amounts of current are to be drawn from the system, this circuit can be used and has the advantage that it requires but little apparatus. Improved results can be obtained from the circuit shown at B. With this arrangement four rectifier tubes are used in a full-wave system supplied from a center-tapped transformer having a secondary potential of from 500 to 700 voles a.c. either side of the center tap. Two separate transformers, each supplying from 500 to 700 volts might, of course, be used. The curves for this rectifier system show it to have much better regulation than that obtained from the circuit in Sketch A. With circuit B a maximum current of up to 170 milliamperes can be drawn from the filter system. The disadvantage of circuit B over circuit A is that the former requires three separate filament windings each of which must provide the full output voltage. The first filter condensers in these circuits must be capable of withstanding a potential of one half the load voltage. The second condensers must, of course, be able to withstand the full load voltage. The filaments of the tubes should be turned on before the highvoltage winding is closed. If this is not done, the initial charging current may overheat the tubes or cause them to arc over. Across the output of the filter systems resistors should be connected as indicated at R. In general a 100,000-ohm resistor may be used and it should be capable of carrying some 20 milliamperes. This resistor is especially necessary when using circuit A since, with this arrangement, the voltage tends to increase quite rapidly when the load is less than 20 milliamperes. The data for this "Laboratory Sheet" were supplied by E. T. Cunningham, Inc. No. 292 Radio Broadcast Laboratory Information Sheet VOLTAGE-DOUBLING CIRCUITS July, 1929 500 700 V. ' IMMJ '■ ■ Rectifiei ml-. VOLTAGE DOUBLING CIRCUIT ■ ■ — Two Tubes — — — Four Tubes IB.) FULL WAVE VOLTAGE DOUBLING CIRCUIT • JULY 1929 •