Identify It! Image originally posted on 10-29-2009
Answer: Metal Oxide Rectifiers
Specifically Selenium Rectifiers and Copper Oxide Rectifiers
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The large square green stack is about 2” on a side and was removed from an unrepairable Motorola 17” TV from the early 1950’s (similar model to 17T3). The light blue stack in the front of the picture is about 1” on a side. It is no longer functional and was removed from a 1950’s Sherwood model 36 high fidelity monaural audio amplifier. (The amplifier was repaired by replacing the selenium rectifier with a pair of high current silicon diodes). The “round” green stack is actually a copper oxide rectifier and was removed from a 1960’s era rotary converter (also sometimes known as a dynamotor, a device for converting low DC voltage to a higher AC voltage and then back into a higher DC voltage). The other rectifiers were removed from other 1950’s and 1960’s era televisions and other equipment.
So What is it?
Metal Oxide Rectifiers are electrical devices that allow electrical current to only flow in one direction in a circuit. They are analogous to one-way valves as used in an air or vacuum pump.
Metal oxide rectifiers are used in power supply circuits to convert alternating current (A.C.) into direct current (D.C.) They were invented in 1933 many years prior to the invention of the semiconductor diode. Selenium rectifiers were used up until about 1975 in place of vacuum tube diodes in electronic devices such as television sets and photocopiers because they were more efficient (about 85% vs. 60% for vacuum tubes), could handle a larger amount of current, and are “instantly-on” since they don’t have to warm up like a vacuum tube does before it can operate. Although they are mostly obsolete today they are still manufactured and used in applications that require rectification of a large amount of electrical current such as in battery chargers and welding equipment.
Most Selenium or Copper Oxide rectifiers were put together as a stack of individual cells. Each cell could withstand about 20 volts. By adding multiple cells in series the voltage rating could be increased almost indefinitely. Selenium rectifiers could handle a larger amount of current (around 50 milliamperes per square centimeter, 50ma/cm2) than Copper Oxide rectifiers and replaced them in most applications. The large metal plates (either square or round) function as a heat sink, providing a large surface area to transfer heat to the surrounding air.
Each cell in a Selenium rectifier is made up of a pressure plate made of either aluminum or steel. This plate is then covered with a very thin coating of another metal such as lead, bismuth, or nickel. A much thicker layer of selenium (usually doped with a halogen) is first heated in an annealing process to form tiny gray hexagonal crystals and is then deposited as the next layer. Another steel or aluminum plate is added next in the stack, as is a steel heat sink plate.
Exploded Diagram of a 2-cell Selenium Rectifier
How does it work?
First some quick definitions:
Voltage is a measure of electrical potential energy (also sometimes called electromotive force or EMF). Since voltage is potential energy it represents the ability to do work with electricity. Voltage is measured in units of “volts” named after the experimenter Alessandro Volta.
Current is a measure of electricity in motion. Current is measured in units called “Amperes” (Amps for short) which is named after the experimenter Andre’-Marie Ampere. Current is a measure of the FLOW of electricity, i.e. how many electrons pass a given point in one second. The movement of electrons through a circuit is what causes useful work.
Electrical Engineers say that when electricity flows from a negative terminal of a power supply through a circuit and then back into a positive terminal the electrical current is called D.C. or Direct Current. D.C. has two characteristics:
- It flows in one direction only
- Once a D.C. power supply is turned on, the voltage rapidly rises to 100% and remains there while the circuit is in operation. In other words the voltage does not usually fluctuate.
The electricity we use in our homes is called Alternating Current
A.C. (Generator Position vs. Voltage Output) over 1 rotation
A moving magnetic field causes an electrical current to flow through a conductor. By rotating a coil of wire inside a magnetic field a changing voltage can be generated. When the coil is perpendicular (at right angles) to the magnetic field zero voltage is generated. As the coil rotates into a position parallel to the magnetic field a maximum voltage is generated. As the coil continues to rotate to 180 degrees (again perpendicular to the magnetic field) the voltage drops to zero again. Then as the coil moves to 270 degrees it is again parallel to the magnetic field, but facing in the opposite direction. At this point a maximum voltage is again produced only this time it is negative since it is flowing in the opposite direction. As the coil returns to zero degrees rotation the voltage again drops back to zero and the cycle starts over again. Generators spin fast enough that the electricity we use in our homes is changing like this sixty times in one second.
We use A.C. like this because D.C. is susceptible to losses over distances due to the resistance of the transmission lines. Devices known as transformers can be used with A.C. to step the voltage and current up and down. This makes for a much more efficient system of power transmission.
Using A.C. to transmit electrical power works well BUT many electrical devices and circuits need D.C. in order to operate properly. The process of changing an alternating current into a direct current is called rectification and is accomplished with circuits made up of devices like Selenium Rectifiers, Vacuum Tube Diodes, or Semiconductor Diodes. Since a rectifier only allows an electrical current to flow through them in one direction they may be arranged like one-way valves in order to change the A.C. into a pulsating D.C. The addition of filter capacitors and inductors can smooth out the pulsating D.C. into a usable source of D.C. electricity.
Half and Full Wave Rectifier Circuits
The physical interface between the selenium oxide and the other metal forms a kind of semiconductor junction due to the different electron affinity of the two dissimilar metals. This exchange of electrons across the junction means that electrons can easily be made to flow in one direction but are largely blocked from flowing in the opposite direction. This electron exchange process at the junction is what allows a metal oxide rectifier to function as a rectifier.
Selenium rectifiers were capable of handling a larger amount of current and voltage than early semiconductor diodes due to the large surface area of their junctions and the accompanying heat sink plates that carried destructive heat away from the junction. However over time the junctions in Selenium rectifiers do break down due to chemical changes caused by the remaining heat and the electrons traveling through the junction.
Metal oxide rectifier’s failure rate turned out to be higher than acceptable for consumer electronics and so they were phased out as soon as more reliable semiconductor diodes and rectifiers became available in the mid 1970’s. When a Selenium rectifier fails electricity can flow in both directions across the junction. This means that it acts as a conductor in its normal forward bias direction, but then starts to act as a resistor in its reverse bias direction. The resistance starts off as a very high value but as the junction fails it reduces until it becomes a complete short circuit. Once this happens the metals in the junction are heated to the point that they emit a small cloud of toxic smoke that usually has a tell tale “rotten eggs” smell. Repair technicians could often identify the failure of a Selenium rectifier simply by the smell inside a piece of equipment where one had failed.
Vocabulary Words used in this Article
- Alternating Current (A.C.): Electrical Current produced by a generator that has a voltage that changes magnitude and direction periodically usually as a sine wave function.
- Anneal: A process in metal working in which a desired texture, consistency, or harness is produced by gradually heating and cooling. Similar to tempering.
- Capacitor: An electrical device that stores charge and therefore opposes changes in voltage. Capacitors have two metal plates that are separated by a non-conductive dielectric material. Since capacitors oppose changes in voltage they are used as filters in A.C. to D.C. power supplies to help smooth a pulsating D.C. into a more flat D.C.
- Copper Oxide Rectifier: A largely obsolete electrical device made up of a junction between copper and copper oxide that functions as a kind of surface junction diode to rectify A.C. to D.C.
- Diode: The word “Diode” originally referred to a vacuum tube that had two electrodes, a cathode and anode. These diode tubes functioned as rectifiers because they only allowed electrical current to flow in one direction (from cathode to anode). Today diodes refer generically to any two terminal electrical device that has the property of allowing current to flow in only one direction. Diodes are now mostly made of semi-conductor materials.
- Direct Current (D.C.): Electrical current that flows in only one direction in a circuit. Batteries produce Direct Current.
- Doped: A material is doped if it was originally a pure material but has had an impurity intentionally added for the purpose of creating a semiconductor (effecting how electrons may flow through the material).
- Inductor: An electrical device (usually a coil of wire) that has the effect of opposing changes in electrical current. Inductors are used along with capacitors in power supply circuits to help smooth pulsating D.C. into a more pure D.C.
- Peak to Peak: The total positive difference in magnitude between the highest positive and negative peaks of a periodic function such as a sine wave. One measure of A.C. electrical power is its peak to peak value, though in practice the RMS value is more often used.
- Pulsating D.C.: An electrical current that while flowing in only one direction does vary periodically between a minimum value (usually zero) and a maximum value. Filters made up of capacitors and inductors are used to smooth pulsating D.C.
- Rectifier: An electrical device that allows electrical current to easily flow in one direction but that offers a very high resistance in the opposite direction.
- RMS: Root Mean Square is the mathematical process of taking the square root of the average of squared values of a periodic function such as a sine wave. For a sine wave this is roughly 70.7% of its peak to peak value. RMS Voltage is sometimes called effective voltage because it is the equivalent to the D.C. voltage capable of doing the same electrical work.
- Selenium: Element number 34, with symbol Se and an atomic mass of 78.96. It is related to Sulfur and Tellurium.
- Selenium Rectifier: A now largely obsolete rectifier made of Selenium Oxide that forms a kind of junction diode.
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