Selenium rectifier

A selenium rectifier is a rectifier of semiconductor diodes on the basis of the semi-conductive element selenium. Today selenium rectifier hardly find application and have been largely replaced by semiconductor rectifier germanium and later of silicon.

For the production of a diode which selenium is deposited as a layer on a nickel-plated iron or aluminum base plate. The top electrode consists of a low melting alloy, which in turn may be by means of contact springs on a threaded bolt to a diode stack, the selenium rectifier, connected together, for example.

Designs

Selenium rectifiers were often constructed as a bridge rectifier ( half and full bridge ), the electrical connections have been taken out as solder tails between the metal plates. With this construction, the individual diodes plates were assembled with the contact springs, and in an insulating axis on a threaded rod. In some cases, two separate top electrodes were realized on a plate so that originated on a sheet two diodes with a common cathode. This design required for mounting two parallel threaded rods.

Based on selenium and diodes for low currents were made to reverse voltages of about 20 kilovolts. They consisted of a large number consecutively housed in a tube or stacked selenium diodes small area (about 1 mm2). They served, inter alia, to produce the anode voltage for the picture tube in black / white televisions and were as " selenium rod " known.

Selenium rods for lower voltages were made correspondingly shorter and often made ​​of hard paper; for a maximum current of 10 mA, they had, for example, 10 mm in diameter - they were so comparatively huge compared to silicon diodes. Furthermore, large-area selenium - photodiodes ( about 4 cm2, transparent top electrode similar to solar cells) were manufactured for use in exposure meters.

Small selenium rectifier currents for 10 ... 100 mA were also made ​​of held together with a clamp plates, very small rectifier were potted in plastic housing. Anti-parallel interconnected selenium diodes were used as a so-called hearing protection diodes in telephones; they limited by their non-linear, but "soft" characteristic crackling noise to a tolerable level without causing severe distortion during loud conversations.

History

The rectifying effect of a selenium layer on metal was discovered in 1874 by the German physicist Karl Ferdinand Braun. Greater economic importance was this rectifier, however, until early to mid 20th century.

Selenium rectifiers were as well known since 1925 cuprous oxide rectifier, dry rectifier called, there was also an electrolytic process for rectifying earlier. 1927/28, developed at Ernst Presser TeKaDe the selenium rectifier ( DRP 501 228 ).

They were isolated from about 1930 and still widely until after the invention of the silicon rectifier 1952 used small to medium currents and voltages, among others in tube radios (anode voltage) or chargers rectify - germanium diodes also had only low reverse voltages, but could not be connected in series; Although silicon diodes had high blocking voltages, but were expensive initially. Among other things, as a high- voltage rectifier " selenium rods " at least until 1978 were in use.

Compared to the earlier to the anode voltage generation (200 ... 300 V ) used in radio electron tube ( rectifier tube ) have selenium rectifier the advantage of greater durability and the elimination of the cathode heating. The selenium rectifier but was now especially for low-voltage a much more effective rectifier available, as it was the used until around 1930 cuprous oxide rectifier. Cuprous oxide rectifier, however, were still manufactured as a measuring rectifier due to their low forward voltage up to about 1950.

Construction

Long after its invention, the exact internal structure of the selenium rectifier was unclear. First, the selenium has to be brought into the crystalline form on the carrier plate with heat and pressure. Halogen additives improve conductivity. After the melting of the contact electrode and subsequent shaping process is diffused of cadmium in selenium. Crystalline selenium is an n-type conductor, and cadmium selenide is a p-type conductor, and therefore between the two substances, a pn junction, that is, a barrier layer is formed.

Electrical properties

The plate size of a selenium rectifier determines its current-carrying capacity - it was made ​​with a size up to about 400 cm2 plates having a rated current of 20 amperes. Its field of application were therefore among other power supplies for charging accumulators.

Single selenium rectifying plates have, depending on the manufacturing process, a maximum blocking voltage of 15 V to about 30 V per element. For higher reverse voltages several plates or diodes may be connected in series. The current-voltage characteristic is in the transmission range, as opposed to silicon - rectifiers a very flat response. Therefore small diode selenium could also be used as a measuring rectifier. The differential internal resistance of the diodes is however higher than selenium that of silicon diodes.

The thermal losses of the rectifier is relatively high, therefore, the heat loss is, however, effectively released through the metal plate to the surroundings. This along an axis on one or two threaded rods deposited metal plates give this component also its typical appearance, as can be seen in the picture above.

Selenium rectifiers have relatively large reverse currents, however, seem to be relatively robust in the reverse voltage is exceeded, as long as the power dissipation limit is not exceeded. Inter alia therefore they have been used for a long time after the invention of the silicon diode as a high voltage rectifier in televisions - they survived here better that occur in a high- voltage flashover in the picture tube overvoltage transients and currents.

One advantage of the "soft" characteristic of selenium rectifiers and their high disk capacity is that they hardly cause spurious emissions and less of a extra burden on the dining transformer due to the larger current flow angle. In contrast, silicon rectifier must often be suppressed and even cause severe distortion reactive power. Depending on performance, the rectifier were manufactured in different sizes - higher current required larger boards, higher voltage longer plate stack.