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1945 Theatre Catalog, 4th Edition, Page 316 (292)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 316
Page 316

1945 Theatre Catalog, 4th Edition, Page 316

ward resistance, is relatively low. In production, the ratio of resistance will be in order of 10,000 to 1. Ratios up to 60,000 have been obtained in laboratory tests. Any material which possesses such asymmetric resistance characteristics can be used as a rectifier. The commercial application depends upon satisfactory efiiciency, regulation, costs, size, and life. That copper oxide rectifiers meet these standards to a very high degree is evidenced by their wide use in industrial applications.

Rectifier Circuits

The simplest rectifier circuit is the single-phase, half-wave type. Its use is restricted to low-current battery chargers and a few special applications requiring half wave rectified current.

A full-wave, single-phase rectifier may be either the center tap or the bridge type. The wave form is the same for both circuits, the instantaneous current value dropping to zero twice every cycle. Such circuits are Widely used for battery charging and the majority of magnetic coil leads.

For motion-picture equipment, a conventional nlter is ordinarily required to reduce the ripple to an acceptable value. Filtering is economical for capacities up to several hundred watts, but for larger sizes it is customary to use polyphase rectification. Copper oxide rectifiers for projection arc supply use their connection for maximum efficiency and to insure a light free from fiicker.

One type of circuit occasionally used for power rectifiers is the Scott connection for transforming three-phase to two-phase before rectification. It is ordinarily used for tube-type rectifiers to save the cost of two tubes, but for copper oxide rectifiers there is no advantage in its use. The wave form indicates that it may be responsible for screen fiicker, particularly when used with the Suprex arc.

Rectifier Ratings

The output of the copper-oxide rectifier, like most other electrical equipment, is limited by temperature. The earlier designs of 3 to 4 watts rating were made without cooling fins: but, as the ratings increased, cooling fins were added to increase the output of each washer. The addition of cooling fins increased the output of a 11/2-inch diameter washer from .08 to .55 watts.

For ratings of 500 Watts or over, the size of naturally cooled rectifiers becomes rather excessive. The addition of a cooling fan increases the fiow of air and consequently the rating of each washer. Corrugated copper cooling fins to provide maximum heat dissipation were designed and first used for motionpicture projector-arc supply. Continued investigation indicated further economies by the use of plate type rectifiers, in which the rectifying surface also serves as the cooling surface. This development was made possible by the development of the metal coating over the oxide, which eliminated the necessity for high-contact pressure. I

By a long series of life tests in the laboratory, and verified by field opera

tion, the limits of voltage and current have become well defined.

For example, four ll/z-inch diameter discs without cooling fins, connected in a bridge-type circuit, are rated at 3 volts, 0.11 amperes. By addition of cooling

fins and spacing washers, the rating is .

increased to 4.5 volts, 4.15 amperes. The use of a suitable fan increases the rating to 5 volts, 1.0 ampere.

Four fan cooled plates, 4%" x 12", connected in a single-phase, bridge circuit, are rated at 5 volts, 30 amperes. When connected in a three-phase, bridge circuit and used for intermittent service, such as projector arc supply, ratings up to 7 volts and 65 amperes a plate are used.


Commercial installations and laboratory life tests of copper-oxide rectifiers over a period of many years have definitely established their exceptionally long life. The rectifier resistance, however, increases with time, resulting in a drop in output. This aging is most pronounced in the first six to twelve months of operation, after which the characteristics stabilize.

Compensation for aging is usually provided by transformer taps, for increasing the voltage applied to the rectifier. It is recommended that output be checked after six months of operation and at oneor two-year intervals thereafter.


The naturally cooled pressure-type assembly has been used for motion-picture equipment for approximately ten years. They have been supplied for loudspeaker field excitation, for amplifier-tube filament supply, and for exciter-lamp supply.

With the introduction of the Suprex arc came the development of a fan cooled copper-oxide rectifier for are supply. The earlier models used the washer type of assembly but present designs use the more recent plate type construction.

The use of copper oxide rectifiers for are supply has a number of decided advantages. These advantages briefiy are (1) no moving parts, with the exception of a small cooling fan; (2) quiet operation; (3) dry metallic rectifying elements, containing no glass or fragile parts; .4) high efficiency; (5) low maintenance; (6) . instantaneous operationno time lag in startingewith the full load being carried at once; (7) voltage adjustment made by changing transformer taps, eliminating losses in ballast rheostats; and (8) unit assembly, with all parts contained in one cabinet and no separate motor starters or ballast resistors required.


The rectifier should be located in or near the projection room to eliminate voltage drop in long cables.

Circulation of air through the ventilating holes must not be impeded, and provision should be made for ventilating the room in warm weather.

Humidity also affects the disc-type of rectifier, if unprotected, by acting on the


Aquadag co'ating and increasing the forward resistance of the rectifier. These effects can be guarded against by the use of suitable high-grade varnish coatings. Varnish coatings also protect against mildly corrosive conditions.

The fan and contactor coil are connected to a 110-volt supply through a sWitch on the projector. The three-phase power supply connections are made to terminals either on the contactor or terminal board. The output is connected to the arc lamp terminals. The knife switch for controlling the arc should be removed from the circuit or wired in closed position to prevent operation of the rectifier on open circuit.

Proper line voltage taps must be selected by checking line voltage with a meter. Proper secondary taps must be selected by test.

Operation To start the operation of this rectifier, turn on the 110-volt switch at the projector lamp, which applies power to the cooling fan and also to the small contactor, which in turn energizes the prime winding of the transformer. As soon as this contactor closes, power is available and the arc may be struck. The operation of the arc is the same'as with any other sort of power supply. To shut down the arc, turn the switch at the projector lamp, which will stop the fan and de-energize the transformer primary.

The transformer primaries are provided with taps for line voltages from 190 to 250 volts a.c. Secondary taps are provided for current adjustments and also to take care of rectifier aging. After the rectifier is installed and properly adjusted, it will be unnecessary to change taps more than about once a year. The cooling fan is equipped with ball bearings which are permanently lubricated; and which will require no further attention.

It will be noted that by proper design of the transformer, it is possible to limit the current at the moment the arc is struck and at the same time obtain good voltage regulation in the current range through which the Suprex carbon should operate.

The high efficiency obtained from copper oxide rectifiers indicates an appreciable saving in the annual power bill for projection. The efiiciency of the copper-oxide rectifier is approximately 68 per cent; for a typical bulb-type rectifier of the same rating, 58 per cent; and for the copper-sulphide-type about 40 per cent. The efficiency of a typical 60-volt generator for this service is approximately 65 per cent, but since a ballast resistor is required to bring the voltage down to approximately 35, the over-all efiiciency of the combination from ac. line to the arc is only 36 to 37 per cent. (The smart theatreman, of course, would install a less capacious generator and not use the ballast, thus effecting a greater economy in operation.) Tests have shown that in some locations, depending upon power schedules, the bill for projection power may be reduced as much as $30 a month by changing from motor-generators to copper-oxide rectifiers.

1945 Theatre Catalog, 4th Edition, Page 316