AN INEXPENSIVE POWER UNIT FOR HIGH-VOLTAGE DIRECT CURRENT GRAEME WELCH AND F. E. BLACET E
FIOURE 1.-WIRING DIAGRAM OF PIER
THE
University
of California, Los Angeles, California
THREE-PHASE RECTI-
ALTHOUGH i t is convenient to have in the laboratory direct cnrrent of two or three hundred volts, few schools are thus equipped, presumably because of the common belief that an expensive motor generator is required. For some time the authors have been operating a high-pressure mercury arc requiring this voltage range with the aid of direct current furnished by a three-phase rectifier constructed, for the most part, from standard and relatively inexpensive electrical supplies. This power unit has given excellent service and a description of it is presented herewith, in the belief that it will be of use to teachers and experimenters in many fields who do not have ready access to high-voltage direct current but do have three-phase alternating current. Figure 1 is an electrical diagram of the equipment. T is a transformer designed for 240 volts primary and having three 5-volt-3-ampere and one 5-volt-9-ampere
center tap secondaries with 1000 volts insulation between windings and above ground. The rectifier tubes are standard type 83. The plates of each of these are connected to 10-ohm-10-watt center tapped resistors, R, as indicated. The filaments of the upper three tubes in the diagram operate on the 5-volt-9-
ampere secondary, while the lower three have their filaments heated from individual 5-volt-3-ampere secondaries. Chis a choke of 1500 turns of No. 16 cotton enamel copper wire on a laminated iron core of approximately six square inches in cross sectioq and with an air gap of about 2 mm. F is a 3-ampere fuse. Figure 2 is a photograph of the complet$ apparatus showing transformer, rectifier tubes, fuse, choke, and leads for direct current and for three-phase alternating current. The three-ply wood base is 10 inches wide and 15 inches in length. The resistors and connecting wires are all beneath this board and are readily accessible by turning the complete unit on its side. Operating on 240 volts, three-phase A.c., this setup gives approximately 270 volts D.C. with a load of 2.5 amperes. The no-load voltage is slightly over 300. In our laboratory this apparatus has been used for over 900 hours to carry a 2.5-ampere mercury arc load and the tubes are still performing perfectly. The tubes are the only parts which will deteriorate appreciably with time and our experience indicates that the upkeep on these will not exceed one-third of a cent per hour and may become considerably less than this amount. This is true in spite of the fact that 2.5 amperes are being used, and consequently each tube is continuously passing slightly more than its rated peak load of 0.8 ampere.
In order to determine the amount of ripple in the output, cathode ray oscillograms of current were taken' and compared with known alternating current as shown in Figure 3. Number 1is a reference oscillogram of 0.061 ampere operating on 60 cycles A.C. In comparison to this, No. 2 shows thefluctuations in 2.50 amperes D.C. from the rectifier, operating a mereury arc load. From the heights of the waves and the currents involved, it can be shown by simple computations that the maximum deviation from 2.50 amperes is plus and minus 0.015 ampere. The 3-ampere fuse is a necessary protection for the tubes against accidental overloading. In our mercury arc circuit, where variable resistance is eliminated as the arc strikes and increases in length, we have a simple magnetic switch in the D.C. circuit to prevent heavy overload of the rectifier should the arc go out. In any use of this power unit i t is important tbat some type of switch be present in the circuit so tbat the load is not applied to the rectifier until the tube filaments have had the opportunity to become hot. The purpose of the choke is to smooth the D.C. current and to reduce the peak currents through the tubes; thus the dimensions of the choke described here are not critical and considerable variation is allowable without materially affecting operation. D.C.
FIGURE ~.-CATHODERAY OSCILLOGRAMS OF CURRENT No. 1 is a comparison standard produced by 0.061 amperes of 60 cycle A.C. No. 2 is the ripple component of the D.c.-whendelivering 2.50 amperes to the mercury arc load
It should be pointed out that the voltage of a rectifier of this general type may be changed by placing suitable transformers in the three-phase circuit, and the current output may be approximately doubled by using twice the number of tubes, etc. In our laboratory this apparatus has been used only to run a mercury arc, but there appears to be no good reason why it should not be employed to supply direct current for electromagnets, lecture experiments, and perhaps many other purposes.
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The authors are grateful to Mr. Leo Delsasso of the Physics Department of the University of California for assistance in taking the oscillograrns.
