Metal Rectifiers

 The history of commercially produced metal rectifiers officially began in 1928 and continued until the more familiar semiconductor rectifiers ousted them sometime in the 1960s. They were used in communications equipments for detection, automatic volume control applications, noise suppression, and clipping, and in power units for radio and television receivers and battery chargers.

A Westector W6

The first Westectors appeared in 1933. At a little over an inch long and rated at 25 piv, 0.1mA and 5MHz, the W6 was used in equipments manufactured during WWII. This is one of four kindly donated by Barry Smith, G4IAT. What were they used in, was it an 18 Set I can't remember?

There were five types of Westector by 1938 as follows:-

W4 half wave, 24v peak to peak 0.25mA

W6 half wave, 36v peak to peak 0.28mA

WX6 half wave, 36v peak to peak 0.12mA

WM24 full wave, 24v each side at 0.5mA

WM26 full wave, 36v each side at 0.5mA

Metal rectifiers encountered in the immediate post war era, say around 1950, were selenium, small signal copper oxide and larger types for battery charging. Competing with metal rectifiers after this date were early semiconductor types. For example, germanium types were soon available for relatively low voltage low current rectifier applications (see below). However other semiconductor varieties had already far outstripped metal varieties and had no competition from them. Some, silicon-based types known as "crystal valves", were used for example, in waveguide systems where they were good for over 30GHz. These types had emerged, as a matter of dire need (defence radar), from the discovery of the PN junction in 1940. Metal rectifiers however held their ground in EHT applications, up to over 17kV, and in HT applications in half wave, full wave, voltage doubler and bridge configurations and by 1960 the range of these rectifiers had increased to its peak and then went into sharp decline as silicon semiconductor devices improved and became increasingly cheaper and more efficient. Although semiconductor devices were certainly feasible much earlier, their development for mass markets was marking time whilst refinement techniques of silicon and germanium to acceptable purities was being perfected.

Strangely, although the metal rectifier was available before 1930, it never really became popular enough to oust valves which were still used in most HT and detection applications to the end of the valve TV era. Although some manufacturers used the odd metal rectifier, for example Lotus, Eddystone and Grundig, their most common application remained in battery chargers and battery eliminators and in multimeters for providing AC ranges although there was a small surge in popularity of the flat, aluminium-cased, selenium rectifier in the last AC only rather than AC/DC, valve radios and tape recorders. In AC/DC sets I suppose it was better to use a rectifier valve with its heater requirement than to waste the power (if the valve was substituted by a metal rectifier) in a ballast resistor.

So what were the origins of the metal rectifier? A research engineer by the name of Grondahl in the US was said to have invented the device, whilst development in the UK was led by The Westinghouse Brake and Saxby Signal Company.

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Copper Oxide Rectifiers

 The most suitable rectification material in the early days was found to be copper, and rectifiers took the form of series-connected oxide-coated copper disks usually interspersed with radiating fins for cooling. Devices were not available just in the small sizes used in radio equipment. In the 30s one could find rectifiers rated up to 1,000 amps for electroplating or up to 100,000 volts for X-Ray machines. The maximum economical power rating in the 30s was 10 Kwatt, beyond which, the size and cost would have been excessive.

Delving back beyond the official date of the invention of the metal rectifier and its mass production what do we find?

In 1920 it was already common knowledge that rectification of AC could be obtained by passing current through a circuit made by immersing an aluminium plate and a lead plate in a solution of ammonium phosphate or sodium borate. It had been found that when this was done a layer of oxide formed on the aluminium plate which then acted as a "valve" to the passage of alternating current in the circuit. Not only simple half-wave rectification was possible but full wave rectification was also achieved through the expedient of using two aluminium plates with the lead plate in between. For these "electrolytic" rectifiers, it had been found that some 60% efficiency was obtained, the other 40% being dissipated as heat. Future development had been forecast at the time, through cooling the aluminium plates, but because mercury vapour rectifiers were then the better option, the impetus of further experimentation leading to new devices was not a priority.

If the experimenters had continued their work and had the foresight to try their devices, already oxidised and without the electrolyte, together with cooling fins to dissipate waste heat, the metal rectifier would have been around much earlier. As it was, the metal rectifier was almost invented some 25 odd years before it finally arrived in about 1928.

What about those earlier experimenters and what were their contributions?

Well in 1910 there was an established device called an "Electrolytic Detector". This was an intriguing instrument for indicating the presence and direction of an EMF between two points. Two electrodes immersed in a solution of phenolphthalein or other chemical indicator showed, by colouration at one of the electrodes, which was positive and which was negative. But more interesting is the fact that the "Electrolytic Rectifier" was around then. It went by lots of names but it was most commonly called the "Nodon Valve" and was linked also to the "Gratz" method of rectifying. Read on....

When experimenting with Electrolytic Rectifiers it had been observed that metals of low atomic weight exhibited the "valve" effect at high potential differences and metals of high atomic weight at low PDs. Nodon's "Electrolytic Valve" consisted of an aluminium rod in a lead vessel filled with ammonium phosphate. In a Nodon Valve, aluminium hydroxide forms on the surface of the aluminium and this exhibits a very high resistance in one direction and a low resistance in the other; hence when AC is passed through the device one obtains a measure of DC. With two such Nodon Valves connected in opposition, full wave rectification is obtained. It had been noted that efficiency wasn't too high and had the effect of producing an accompanying electrolyte temperature rise, so that in order to prolong its use, external fan cooling was employed. Full wave connection was known as the "Gratz" method of connection and best efficiencies of between 65% and 75% were obtained at 140 volts at frequencies of 25 to 200 Hertz. A typical rectifier was a "5-amp valve" and can be traced back to before 1905.

A variation of the Nodon Valve was the Buttner Valve which used a cathode of magnesium-aluminium alloy and iron or lead as its anode. Buttner's electrolyte was borate and his device was also developed before 1905.

Another type was the Churcher Valve which used two cathodes within one cell and could therefore handle full wave rectification with a single device. The circuit diagram of this looked essentially like that of a full wave thermionic valve and was good up to 65-0-65 volts AC. This device was around in 1903.

The De Faria Valve used a hollow aluminium cylinder within a larger cylinder of lead in an electrolyte of sodium phosphate, all contained in an ebonite vessel. The lead cylinder was perforated to allow cooling and in 1907 was rated at 8 amps.

The Grisson Valve used an aluminium plate, a sheet of lead and an electrolyte of sodium carbonate and was cooled by circulating water through tubes immersed in the vessel. This was around in 1906.

The Pawlowski Valve of 1905 is perhaps closest to the "modern" metal rectifier in that it used a copper plate coated with a layer of copper hemisulphide in direct physical contact with an aluminium sheet. When first constructed, on connection to an AC circuit, sparking occurs as the device "forms", after which the device works well as a rectifier.

All these various rectifiers fell by the wayside, maybe due to a lack of demand, or maybe because other avenues of experimentation were more rewarding?

Shown above: (top) a meter rectifier coded 280LU1670 from a wartime radio equipment and (bottom) one of the last copper oxide bridge rectifiers from inside a moving coil meter designed for reading AC.

Now back to the future. Copper oxide rectifiers are made up of copper disks, which having been heated to 1,020 degrees centigrade for several minutes, followed by subsequent annealing by heating to 600 degrees centigrade, are then rapidly quenched. Black cupric oxide is then removed by cyanide solution, after which the disk is left coated with cuprous oxide which is then removed from one side. The remaining oxide layer is coated with graphite to make a sound electrical connection and rectifier "piles" are then made by fitting successive sets of prepared washers on a threaded rod. Each copper washer is accompanied by a lead disk in contact with its graphite surface, together with a copper-plated spacer and a copper-plated steel cooling fin. For a three-quarter inch diameter disk the average forward current is a third of an amp and the maximum safe reverse potential is 9 volts. A full-wave bridge rectifier for a 12 volt battery charger might have 12 such assemblies.

Selenium Rectifiers

 Selenium rectifiers, developed in Germany, use steel disks on which are deposited a thin layer of selenium, a hard grey coloured element. Selenium rectifiers can be constructed in much the same way as their copper oxide equivalent but are more efficient and have a smaller physical size than an equivalent copper oxide type. They have an unusual odour, especially when they have suffered from an overload, which can fill a room with a smell like un-neutered tom cats. The last high voltage selenium rectifier, used say for powering a tape recorder had an aluminium case about two inches by three inches and was less then a quarter of an inch thick. The rectifier would be bolted to the chassis of the equipment thus ensuring adequate cooling.

An example (full wave bridge) rated at 30 volts 1 amp, is shown here.



A selection of early semiconductor germanium diodes which were in competition with the last of the metal rectifiers. Top to bottom and left to right:-

OA70, CV7047 (OA5), OA10, CV448 (OA71) and a GEX541 (80v 6A Ge type).
See some metal rectifiers

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