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The New G-10 Germanium Dot Rectifier
June 1952 Radio & Television News Article

June 1952 Radio & Television News
June 1952 Radio & Television News Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919-1959. All copyrights hereby acknowledged.

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The New G-10 Germanium Dot Rectifier

Engineering data on a new unit for radio, television, and allied power applications which uses no critically-short materials.

By T. J. Ferguson

Crystal Eng. Sec., Comm. & Gov't. Equip. Dept.

Electronics Div., General Electric Company

Fig. 1. Over-all view of the General Electric germanium power rectifier which has been designated the G-10.

General Electric Company has long been aware of the important characteristics that make germanium rectifiers superior to other types of rectifiers. These characteristics, are their lower forward resistance, higher back resistance, longer life expectancy, and reliability. The advantageous characteristics of the germanium diode plus the techniques for increasing the area of the rectifying surface, developed by the General Electric Research Laboratories, have been responsible for a germanium rectifier that could be used in television, radio, and allied power supply applications. The G-E Germanium Dot Rectifier, designated G-10, having these characteristics, grew out of this investigation. In addition, it was found commercially feasible to manufacture the Germanium Dot Rectifier without the use of critical materials.

The G-10 is composed of two "button" rectifiers, each consisting of a 1/8 square inch pellet of spectroscopically pure germanium placed in the center of metal cups sealed with butyl rubber, as shown in Fig. 2. These rectifiers are series mounted on two inch diameter, 1/32 inch thick metal dissipating fins. A high coefficient of thermal conductivity is the governing qualification for the choice of metal used in the dissipating fins. Copper and aluminum have been successfully used. Production units will probably use aluminum dissipating fins.

Rectifiers are stabilized electrically by 24 hours of operation at full load. This stabilization serves to remove the remaining gaseous impurities from the rectifying surface and to relieve any mechanical strains within the button due to changes of temperature with operation.

In order to insure an equal division of the peak back voltages across series-connected rectifiers, it was found necessary to use units with the same dynamic back currents. Any increase in temperature results in an increased back current. There are two factors that vary from rectifier to rectifier that must be considered when selecting rectifiers for their inverse currents. The first is the heat generated by the currents in the forward and back resistance. The second is determined by the thermal characteristics of the contact between the rectifier and the dissipating fin.

Both of these factors can be evaluated simultaneously by applying full forward current during the selection test. Therefore, the rectifiers are sorted with respect to their dynamic back currents at the full rated load. This sorting and selection is done at the factory, hence the finished products are electrically stable and only a very slight change occurs in the inverse current during the life of the rectifier.

Fig. 2. Cross-section view of one of the "button" rectifiers used in the G-10 assembly.

To further insure that mismatched, damaged, or otherwise defective units are not shipped to customers, the completed rectifiers are checked under maximum operating conditions for forty-eight hours at 130 volts a.c., 350 ma., and 55�C. This results in an extremely efficient rectifier that does not change its electrical characteristics significantly with time.

Fig. 3 shows the static characteristics of an average G-10 taken at different ambient temperatures. The rather rapid increase in inverse current at the higher temperature tends to place an upper limit on the useful temperature range of the Germanium Dot Rectifier. This upper limit is believed to be inherent in most semiconductor devices, as it is thought that inverse currents are largely generated by thermal agitation at the rectifying surface.

Fig. 4 shows the static resistance, at room temperature, for an average G-10. It is of interest to note that the rated peak-to-peak voltage of the G-10 (400 volts) falls on or near the point of maximum resistance of the Germanium Dot Rectifier at room temperature.

In many circuits the efficiency of this unit is at least 98%. For example, with a 50-watt resistive load, the power dissipated within the rectifier as heat is usually less than 1 watt. Operation at 50°C does not materially affect this efficiency. Other types of rectifiers may lose as much as 5% of their room temperature efficiency at 50°C.

Another advantage of this rectifier is the low effective capacity, usually about 20 μμfd.; making it possible to operate dry disc type power supplies from 25 cycles to about 50 kilocycles. This characteristic is of particular advantage in lightweight installations, such as aircraft and mobile power units. Here a high frequency alternator with a G-10 rectifier and a small filter will provide low noise and ripple content d.c. power over wide extremes of temperature, vibration, and altitude.

The life expectancy of the Germanium Dot Rectifier is believed to be well in excess of 10,000 hours at full load and 40°C. At the present time, several units have been on test for more than 4000 hours at full load, at 40°C with no significant changes in their electrical characteristics. This figure has been limited only by a lack of time in which to take the test to completion.

Some typical characteristics for the Germanium Dot Rectifier with various filter condensers at room temperature are shown in Figs. 5 and 6 for two types of rectifier circuits. An average selenium rectifier is shown for comparison. It will be noted that the slope of the curve is mainly a characteristic of the size of the condensers used, but that the efficiency of the rectifier determines the position of the curve.

A doubler circuit has been successfully used at temperatures as high as 90° C ambient with the characteristics as shown in Table 1.

Excessive overload in the half-wave circuit, or the voltage doubler circuit, generally causes the rectifier to short-circuit. This usually opens a series fuse or surge resistor and damages the rectifier irreparably. If this occurs, there is no disagreeable odor from the G-10 as is often the case with other type rectifiers.

The recommended surge or current limiting resistance has been tentatively established at the same values used with selenium rectifiers, i.e., approximately 5 ohms. There is some indication, however, that this may be reduced to approximately half of this value. Even at 5 ohms, the series condenser ripple-current ratings must be increased to prevent condenser damage, due to the higher surge and ripple currents that result from the lower forward resistance of the Germanium Dot Rectifier.

The Germanium Dot Rectifier is not seriously deteriorated by humidity due to the butyl rubber sealed metal case; several test units have successfully completed 50 or more cycles of operation under maximum load conditions at 90% to 95% relative humidity. Each cycle consists of two periods of four hours of rectifier operation separated by an eight hour period of inactivity.

Additional research is underway to improve the accuracy of tests and to provide a faster and more complete stabilization, together with operation at higher temperature ambients and current ratings than are presently possible.

Fig. 3. Static characteristics .of an average G-10 taken at different ambient temperatures ranging from 25 to 75 degrees C.

Fig. 4. Static resistance at room temperature.

Table 1. G-10 characteristics in a doubler circuit at ambient temperatures to 900 C.

Table 2. Tentative electrical characteristics for the G-10 Germanium Dot Rectifier.

Fig. 5. Some typical characteristics for the G-10 in condenser-input power circuit,

Fig. 6. Performance graph of the G-10 when used in a conventional doubler circuit.

 

 

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