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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