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October 1947 QST
 Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
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The advent of selenium rectifiers in
the 1940s was a very welcome new option to circuit designers, consumers, and servicemen.
Before that, vacuum tubes did the job (with some use of copper oxide rectifiers).
Selenium rectifiers have the advantage of ruggedness and reliability over tubes
(~85% vs. ~60%, respectively). Not requiring a heater voltage eliminates needing
to create heat in excess of that dissipated due to the innate inefficiency. Voltage
and power handling is adjusted by stacking appropriate layers and adjusting the
physical size, respectively. A failed selenium rectifier reportedly often emitted
a very foul odor, which although offensive to the user, proved to be a nice bonus
for the serviceman since it immediately gave a clue as to what went wrong.
Electronics magazines of the era published many articles about selenium
rectifiers, including
After Class: Working with Selenium Rectifiers,
The Semiconductor Diode,
New Selenium Rectifiers for Home Receivers,
Selenium Rectifiers,
Applications of Small High-Voltage Selenium Rectifiers, and
Using
Selenium Rectifiers.
Using Selenium Rectifiers - A Symposium
Fig. 1 - A simple half-wave circuit.
S.R. Circuits and Their Applications
By Ralph Berkman, * W7HWY
To many, the new selenium rectifiers are just a substitute for the rectifier
tube of the more common a.c.-d.c. sets, but to those who have investigated their
possibilities, they represent the heart of an ideal power supply for many of the
pieces of gear that the ham, for one reason or another, is continually acquiring.
Using a rectifier of this type, the power supply will require very little space
as compared to the more conventional type, and components may be placed wherever
convenient. If operated within their limits, rectifiers of this type will give off
very little heat. No warm-up period is required, as in the case of the filaments
of tube rectifiers. In VFOs, converters and receivers, heat has always been a problem
and in these applications the dry rectifier with its low heat radiation merits special
consideration.
Construction of the selenium cell varies with the various manufacturers, but
all are basically the same in that the selenium is placed on one electrode and then
crystallized. The plate is then formed and the barrier film forms on the surface
of the selenium. Against this the other electrode is pressed, making up the cell.
Cells are stacked to produce the desired current and voltage rating, making up what
is known as the selenium rectifier. Rectification action takes place in much the
same manner as in any other rectifier. In the forward direction there is good conductivity
but in the back direction there is not perfect cut-off, and, as a result, there
will be some a.c. ripple content.
Fig. 2 - Voltage-doubling circuits.
These units come in the 100-, 150- and 200-ma. sizes and the additional cost
of the larger sizes is very little. The efficiency of rectifiers of this type run
high. The voltage drop across a unit averages about 5 volts. The critical temperature
of these units is very close to 155° F. Therefore, protective resistors should
be installed in series with them to limit the condenser charging current to a safe
value. Temperatures may be kept down by not trying to push the unit to its full
capacity, and then some. Give it a little safety margin and you will get longer
life and have a trouble-free power supply.
All selenium rectifiers mount with a No. 6 screw and the unit is insulated from
the mounting hole. This permits mounting directly to any metal surface, which will
help to dissipate some of the heat. It will be found that mounting the unit in a
vertical position will provide better air circulation. If these few precautions
are observed, up to 450 volts at 200 ma. is available with standard component parts
and you won't have a supply that is several times heavier than the rig it powers.
Transformerless Circuits
Although selenium rectifiers may, of course, be used in any of the standard transformer-rectifier-filter
systems, it is natural to associate their advantage of compactness and light weight
with transformerless supplies. Fig. 1 is a straightforward half-wave rectifier
circuit which may be used in applications where 115 to 130 volts d.c. is desired.
It makes an ideal bias supply, for instance. In this, as well as other circuits,
it will be observed that the negative side of the output is common with one side
of the a.c. line and it is suggested that this side be fused with a 1/2-ampere fuse.
Fig. 2 shows several voltage-doubler circuits. Of the three, the one shown
at B is the most desirable since there is no series condenser. It is a full-wave
circuit and there will be very little ripple voltage appearing at the output. On
the other hand, the circuit of C has one very desirable feature in that point X
is common to both condensers in the rectifier and also to the first condenser in
the filter. This means that a single-unit three-section condenser may be used, saving
space. If less than 100 ma. is being used, this, in the author's opinion, is the
best circuit. The ripple content under these conditions, and the leakage between
sections, will not be excessive. These three circuits will find ready application
in communications receivers, converters, VFOs, test equipment, etc., and especially
in cases where heat has been a problem.
Fig. 3 - Voltage-tripling and quadrupling circuits.
Fig. 3-A and -B shows voltage-tripler and quadrupler circuits respectively,
for use where higher voltages are desired. They are ideal for powering the small
portable or fixed-station rig and the compactness and light weight will be appreciated.
The writer uses the tripler circuit for powering a small 'phone-c.w. rig where the
weight of a comparable conventional power supply would make portable operation prohibitive.
All components are standard. C1 in all circuits is for" hash" filtering
and its value is not critical. A 0.05-μfd. 600-volt-working condenser should
serve. All other condensers should be 40-μfd. 200-volt units, except those in
the tripler and quadrupler circuits. Those in the circuit of Fig. 3 should
have a rating of 450 volts working. In the voltage multipliers and in other circuits
where a condenser is passing the full current, good condensers should be used because
the a.c. ripple mentioned above appears across the condenser and increases as the
load increases. If the current is allowed to become too high, it will cause heating
and deterioration of the condenser. This can be kept to a minimum by using a capacitor
of high value and making sure it is of good make. Aside from this, no particular
difficulties should be encountered even when using voltage-tripler and quadrupler
circuits. R1 should be 25 ohms, but if it is found that the rectifier
units are running a little too warm, this value may be increased to as high as 100
ohms, with a corresponding drop in output voltage, of course.
A single-section filter, as shown in Fig. 2-C, will provide sufficient smoothing
for most applications.
At W7HWY the circuit of Fig. 3-A is being used for powering a BC-474 and
the circuit in Fig. 1 for the receiver portion. The transmitter is running
15 watts on 75 'phone and 18 watts on 80 c.w., while on the road, and in my estimation
it's tops.
Summing it all up, for the guy who is looking for a lot out of little things,
this is economical power in a mighty small package.
Posted November 16, 2022
(updated from original post on 7/21/2016)
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