Selenium rectifier
A selenium rectifier is a rectifier made of semiconductor diodes based on the semiconducting element selenium . Selenium rectifiers are rarely used today and have largely been replaced by semiconductor rectifiers made of germanium and later made of silicon.
To manufacture a diode, the selenium is applied as a layer on a nickel-plated iron or aluminum carrier plate. The top electrode consists of a low-melting tin alloy, which in turn uses contact springs z. B. on a threaded bolt to form a diode stack, the selenium rectifier, can be interconnected.
Designs
Selenium rectifiers were often built as bridge rectifiers (half and full bridges ), the electrical connections were brought out as soldering lugs between the metal plates. In this construction, the individual diode plates were mounted with contact springs and insulating washers in an axis on a threaded rod (see picture above). In some cases, two separate cover electrodes were also implemented on one plate, so that two diodes with a common cathode were created on one sheet. This design required two parallel threaded rods for assembly.
Diodes for small currents up to blocking voltages of around 20 kilovolts were also manufactured on the basis of selenium (see picture on the right). They consisted of a large number of selenium diodes with a small area (approx. 1 mm 2 ) placed one behind the other in a tube or stacked . They served u. a. to generate the anode voltage for the picture tube in black and white television sets and were known as "selenium bar".
Selenium bars for lower voltages were correspondingly shorter and often made of hard paper ; for a maximum current of 10 mA they had z. B. 10 mm in diameter - so they were comparatively huge compared to silicon diodes. Furthermore, large-area selenium photodiodes (approx. 4 cm 2 , transparent cover electrode similar to solar cells ) were manufactured for use in exposure meters .
Small selenium rectifiers for currents around 10 ... 100 mA were also made from plates held together with a clamp (see picture on the left), very small rectifiers were encapsulated in plastic. Selenium diodes connected in anti-parallel were also used as so-called hearing protection diodes in telephones; With their non-linear but “soft” characteristic, they limited clicking noises to a tolerable level without causing strong distortions in loud conversations.
history
The rectifying effect of a selenium layer on metal was discovered in 1874 by the German physicist Karl Ferdinand Braun . However, this rectifier did not acquire greater economic importance until the beginning to the middle of the 20th century.
Selenium rectifiers, like the copper oxide rectifiers known since 1925 , were called dry rectifiers , because earlier electrolytic rectification processes were also used. In 1927/28 Ernst Presser developed the selenium rectifier at TeKaDe (DRP 501228).
They were occasionally used from around 1930 and were still widely used until after the invention of the silicon rectifier in 1952, in order to reduce small to medium currents and voltages and the like a. rectified in tube radios (anode voltage) or chargers - germanium diodes also had only low reverse voltages, but could not be connected in series; Although silicon diodes had high reverse voltages, they were initially expensive. Among other things, as high-voltage rectifiers, "selenium rods" were still in use until at least 1978.
Compared to the electron tubes ( rectifier tubes ) previously used to generate anode voltage (200 ... 300 V) in radios , selenium rectifiers have the advantage of a longer service life and the elimination of cathode heating. With the selenium rectifier, however, a significantly more effective rectifier was now available, especially for low voltage, than the copper oxide rectifiers used until around 1930 . However, due to their low forward voltage, copper oxide rectifiers continued to be manufactured as measuring rectifiers until around 1950.
construction
Long after its invention, the exact internal structure of the selenium rectifier was unclear. Schottky initially assumed that the metal cover electrode on the selenium was a Schottky barrier . At the beginning of the development, the selenium was applied to the carrier plate with heat and pressure ("smeared on"). Later the plates were steamed with selenium. Halogen additives improve conductivity. The base plate was made of nickel-plated iron, later of aluminum. The top electrode made of a cadmium-tin alloy is applied in a molten state and then the plate is thermally and electrically formed, the cadmium diffusing into the polycrystalline selenium and the barrier properties being improved. The selenium is a p-conductor and the cadmium selenide is an n-conductor, which is why a pn junction , i.e. a barrier layer, is formed between the two substances . The technical current direction in the forward direction is therefore directed towards the cover electrode, which is why the positive pole of the selenium rectifier is there.
Electrical Properties
The plate size of a selenium rectifier determines its current carrying capacity - plates with a size of up to about 400 cm 2 were manufactured, which had a nominal current of about 20 amperes. Its area of application was therefore, among other things, power supplies for charging accumulators .
Depending on the manufacturing process, individual selenium rectifier plates have a maximum reverse voltage of 15 V to approx. 30 V per element. Several plates or diodes can be connected in series for higher blocking voltages. In contrast to silicon rectifiers, the current-voltage characteristic curve shows a low lock voltage and a flatter curve in the pass band . Therefore, small selenium diodes could also be used as measuring rectifiers. The differential internal resistance (sheet resistance) of selenium diodes is, however, significantly higher per area than that of silicon diodes. While the current density in selenium plates is 0.05 to 0.1A / cm 2 , it is about 3000 times as high in silicon diodes. The thermal losses of a selenium rectifier are comparatively high, especially at high voltages, the heat loss is effectively dissipated to the environment through the metal plates, but the maximum junction temperature is only around 60 ° C compared to 120 to 170 ° C for silicon diodes.
The plates are attached to one or two threaded rods along an axis and give the rectifiers their typical appearance (: File: Selen-Gleichrichter.jpg | picture above). Depending on the output, the rectifiers were manufactured in different sizes - higher current required larger plates, higher voltage longer stacks of plates.
Selenium rectifiers have relatively large reverse currents, but behave relatively robustly when the reverse voltage is exceeded as long as the power loss limit is not exceeded. For this reason, among other things, they were used as high-voltage rectifiers in televisions for a long time after the invention of the silicon diode - they withstood the short-term overvoltages and currents that occur in the picture tube in the event of a high-voltage flashover.
One advantage of the “soft” characteristic of selenium rectifiers and their high plate capacity is that they hardly cause any interference emissions and, due to the larger current flow angle, they represent a lower additional load for the feeding transformer. On the other hand, silicon rectifiers often have to be suppressed and even then cause strong distortion reactive powers .
Trivia
Because of the strong warming and the associated odor development, the rectifier was jokingly referred to as "Selenium it smells the same". The smell is described as "like rotten eggs" or "rettig".
Individual evidence
- ^ Kai Christian Handel: Beginnings of semiconductor research and development. (pdf; 2.4 MB) June 29, 1999, accessed on February 12, 2011 .
- ↑ Eberhard Spenke: Electronic semiconductors: An introduction to the physics of rectifiers , Springer-Verlag 2013, 386 pages, page 71
