[Table of Contents]People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights (if any) are hereby acknowledged.
, "A selenium rectifier is
a type of metal rectifier, invented in 1933. They were used to replace vacuum tube rectifiers in power supplies
for electronic equipment, and in high current battery charger applications. The photoelectric and rectifying
properties of selenium were observed by C. E. Fitts around 1886 but practical rectifier devices were not
manufactured routinely until the 1930s. Compared with the earlier copper oxide rectifier, the selenium cell could
withstand higher voltage but at a lower current capacity per unit area."Safety note
from RF Cafe visitor Joe B.: Selenium dust is toxic.An
MSDS sheet for selenium on a WV government website lists it as life threatening. Selenium is another one of
those elements/compounds that are essential for life, but can kill you in too large of a quantity (like water).
See all articles from
Special Information on Radio, TV, Radar and Nucleonics Working with Selenium
Fig 1. Constructional features of a typical selenium cell.
This photo shows a large 5-amp. selenium rectifier compared in size with a 500-ma. unit (left), a 50-ma. unit
(center), and a common matchbox cover (right).
Fig. 2. Selenium rectifier compared to a vacuum tube. The forward direction in the selenium is from alloy to
baseplate. Thus, the alloy is comparable to the cathode and the baseplate to the plate of a vacuum-tube
Resting on top of a modern selenium of the power variety is an old-type copper oxide rectifier. Although almost
three times the size of the copper oxide unit, the selenium weighs less. Both can handle 5 amperes, but the
selenium is rated at a five-times higher voltage, hence the extra length.
Fig. 3. Electron charge density is always greatest at the points of greatest curvature. Here, density is highest
at corners of pyramid.
Fig. 4. Forward or conductive direction (A) is from point to base of end-to-end crystals. High resistance
direction (B) is opposite.
Fig. 5. Selenium rectifiers in series (A) and in parallel (B) for increasing voltage and current handling
Selenium rectifiers have long been accepted as an efficient means of converting a.c. to d.c. in industrial
applications with relatively large power requirements. Their use has recently been expanded to include radio and
TV receivers as well as all types of electronic control gear and mobile equipment. But although many millions of
these units are now being employed, most experimenters are dangerously ignorant of their characteristics and
limitations. "To know it well is to use it wisely" may be a highly questionable aphorism in many ways but it fits
the selenium rectifier like the proverbial glove.
Figure 1 is a cross section of a finished selenium cell.
The etched-aluminum baseplate serves as the negative electrode and the low-temperature alloy as the positive
electrode. In operation, electrons flow readily from the alloy to the baseplate but encounter high resistance in
the opposite direction. The alloy plating behaves like the cathode of a vacuum tube and the aluminum baseplate
serves as the anode or plate, with the selenium crystal layer actually performing the rectifying action (Fig. 2).
. During the manufacturing process, much care must be taken in the deposition of the
metallic selenium on the baseplate, because performance of the finished rectifier depends upon the orientation of
the individual crystals in the "barrier layer," as the selenium coating is called. Although the rectifying action
is still imperfectly understood, the need for correct orientation suggests the following explanation.
Consider a single crystal having a shape like that of a pyramid (Fig. 3). All metals contain many free electrons
or carriers which distribute themselves according to certain well-known laws of electrostatics. One of these laws
states that electric charges will concentrate on surfaces with the sharpest curvature; this is called the effect
of points. As the sharpest curvatures on the surface of a pyramid are found at the corners, we should expect to
find free electron density highest at these points.
A potential applied to a line of crystals of this
shape oriented end-to-end mayor may not cause a current to flow, depending upon the polarity of the voltage. When
the direction of the e.m.f. is such as to move the electrons from a corner to a face (Fig. 4A), the carriers
readily cross the interfacial boundary at B, moving into face AC, and thence distributing themselves at the
corners. This is the forward or conduction direction. When the polarity of the voltage is reversed (Fig. 4B), the
deficiency of electrons on the flat faces limits the number of carriers, conduction does not occur readily, and
the resistance is substantially higher. If we now replace the batteries with an a.c. source, the conduction is
essentially unidirectional and rectification takes place. Voltage and Current Ratings
Seleniums are available in an almost unlimited range of voltages and currents. Those popular for radio and TV
applications may be roughly limited to a maximum r.m.s. voltage input of 130 volts and a current range from 50 ma.
to 600 ma. d.c. Such seleniums are designed to operate at a.c. line voltage, i.e., about 120 volts r.m.s. Note,
however, that even a 500-ma. rectifier is capable of carrying more than twice the current of the largest receiving
type rectifier, the 5U4G. In the real power sizes -5 ampere capacity and more-selenium rectifiers are no less
bulky than other types but they weigh considerably less.
The selenium ratings given above carry two
distinct warnings: do not exceed the rated current for any extended period of time and do not use a radio-type
selenium rectifier with step-up power transformers, unless you take steps to extend the working-range.
Series or Parallel Systems
. Units may be connected either in series to increase their voltage range (Fig.
5A) or in parallel to improve their current handling ability (Fig. 5B). Polarity must be observed at all times.
An additional point worth keeping in mind is that there is approximately 5 volts of drop across each selenium unit
in the series connection; these add up and may play havoc with the voltage regulation of such a power supply if
too many elements are included in the series circuit. In our example of the parallel connection. two 500-ma.
seleniums provide a total load current up to one ampere. The author has used ten of these 500-ma. units in a
circuit where the current demand was a continuous 5-amp. drain-with no ill effects. Surge Resistor
A resistor of low value - 5 to 22 ohms, depending upon. the particular rectifier - is always encountered in series
with a selenium stack (Fig. 6), This resistor must never be omitted.
When the equipment is first turned
on, the uncharged filter capacitor a behaves like a hungry rhinoceros with its maw wide open to gobble up its
fodder - coulombs in this case. If this large surge current were allowed to flow into the capacitor with only the
resistance of the selenium rectifier in the way. it would reach enormous values instantaneously. The heat
generated might be more than enough to destroy the barrier layer. Resistor R limits the surge current to a safe
figure. After the first few cycles, the capacitor takes on full charge and the current through the selenium then
becomes a steady value equal to the load current of the device being operated by the rectifier.
The surge resistor is also an inexpensive fuse that protects the costlier selenium rectifier if a short
Duplex Power Supply
. In Fig. 7, a low voltage isolation transformer (Stancor PA-8421)
provides protection against the ever-present shock hazard and at the same time makes available a 6.3-volt
secondary for heaters of tubes. Two selenium rectifiers are so connected that they supply either +135 volts or
-135 volts with respect to the zero level terminal.
These voltages are ideal for circuits in which positive plate and screen voltages are needed in addition to
negative bias voltages. Two wire-wound potentiometers provide control of voltage level for testing many types of
small devices, such as photo-relays, timers, bridge circuits, transistorized apparatus, etc. And the output
voltage may be doubled by taking it from the -135 and +135 volt terminals, with the former acting as the zero
level point. Threshold Voltage
. A minimum voltage is required to make a selenium rectifier
conduct in the forward direction. Most authorities agree that a good average is one volt. Under most temperature
conditions, a selenium rectifier will pass no current at all until the applied e.m.f. exceeds this value.
A low-value surge resistor must always be used to protect the selenium rectifier.
Duplex power supply with variable output voltages. Both negative and positive voltages, or voltage doubling, may