An AC/DC receiver design is a style of power supply of vacuum tube radio or television receivers that eliminated the bulky and expensive mains transformer. A side-effect of the design was that the receiver could in principle operate from a DC supply as well as an AC supply. Consequently, they were known as "AC/DC receivers".
In the early days of radio, mains electricity was supplied at different voltages in different places, and either direct current (DC) or alternating current (AC) was supplied. There are three ways of powering electronic equipment. AC-only equipment would rely on a transformer to provide the voltages for heater and plate circuits. AC/DC equipment would connect all the tube heaters in series to match the supply voltage; a rectifier would convert AC to the direct current required for operation. When connected to a DC supply, the rectifier stage of the power supply performed no active function. DC-only equipment would only run from a DC supply and included no rectifier stage. DC is almost never used in mains power distribution anymore.
Different radio set models were required for AC, DC mains, and battery operation. For example, a 1933 Murphy radio with essentially the same circuit had different models for AC supply, DC supply, and battery operation. [1] The introduction of AC/DC circuitry allowed a single model to be used on either AC or DC mains as a selling point, [2] and some such models added "Universal" to their name [3] (such sets usually had user-settable voltage tapping arrangements to cater for the wide range of voltages). [4]
The first ever AC/DC design of radio was the All American Five. The sole aim of the design was to eliminate the mains transformer. [5] [6] The lower cost of transformerless designs remained popular with manufacturers long after DC power distribution had disappeared. Several models were produced which dispensed with the power transformer, but had circuit features which only allowed operation from AC. [7] [8] Some early models were available in both AC-only and AC/DC versions, with the AC/DC versions sometimes slightly more expensive. [9]
Television receivers were first commercially sold in England in 1936 for the new 'Television Service' broadcast by the British Broadcasting Corporation. All pre World War II sets used mains transformers and consequently were AC only. In 1948 Pye released the first television receiver, the B18T, to employ the AC/DC design [10] to eliminate the mains transformer when operated off 240 V mains. [11] While sufficient for radio, the voltage was not high enough to power some television circuits, so energy was recovered during the flyback period from the primary of the line output transformer to provide a boosted HT supply; [12] this was not possible with a lower mains supply voltage—even 220 V was insufficient. Pye's marketing material did not mention the set's ability to operate from a DC supply, possibly because there were no DC supplies within the reception range of Alexandra Palace television station, then Britain's only operating transmitter. Other manufacturers adopted the design; they, and later also Pye, sold them as AC/DC sets; the technique was used for many decades.
Vacuum tube equipment used a number of tubes, each with a heater requiring a certain amount of electrical power. In AC/DC equipment, the heaters of all the tubes are connected in series. All the tubes are rated at the same current (typically 100, 150, 300, or 450 mA) but at different voltages, according to their heating power requirements. If necessary, resistance (which can be a ballast tube (barretter), a power resistor or a resistive mains lead are added so that, when the mains voltage is applied across the chain, the specified heating current flows. [13] Some types of ballast resistors were built into an envelope like a tube that was easily replaceable. [14] With mains voltages of around 220 V, the power dissipated by the additional resistance and the voltage drop across it could be quite high, and it was common to use a resistive power cable (mains cord) of defined resistance, running warm, rather than putting a hot resistor inside the case. If a resistive power cable was used, an inexperienced repairer might replace it with a standard cable, or use the wrong length, damaging the equipment and risking a fire.
AC/DC equipment did not require a transformer, and was consequently cheaper, lighter, and smaller than comparable AC equipment. This type of equipment continued to be produced long after AC became the universal standard due to its cost advantage over AC-only, and was only discontinued when vacuum tubes were replaced by low-voltage solid-state electronics.
A rectifier and a filter capacitor were connected directly to the mains. If the mains power was AC, the rectifier converted it to DC. If it was DC, the rectifier effectively acted as a conductor. When operating on DC, the voltage available was reduced by the voltage drop across the rectifier. Because an AC waveform has a voltage peak that is higher than the average value produced by the rectifier, the same set operating on the same root mean square AC supply voltage would have a higher effective voltage after the rectifier stage. In areas using 110–120 volt AC, a simple half-wave rectifier limited the maximum plate voltage that could be developed; this was adequate for relatively low-power audio equipment, but television receivers or higher-powered amplifiers required either a more complex voltage doubler rectifier or warranted the use of a power transformer with a conveniently high secondary voltage. Areas with 220–240 volt AC supplies could develop higher plate voltage with a simple rectifier. Transformerless power supplies were feasible for television receivers in 220–240 volt areas. Additionally, the use of a transformer allowed multiple independent power supplies from separate transformer windings for different stages.
In an AC/DC design there was no transformer to isolate the equipment from the mains. Much equipment was built on a metal chassis which was connected to one side of the mains. [15] Because no power transformer was used, "hot chassis" construction was required: one of the mains power lines became the negative side of the power supply, connected to the chassis, and all metal parts in metallic contact with it, as common "ground". With AC power, the neutral, rather than live, line should be connected to the chassis; touching it, while highly undesirable, is usually relatively safe—the neutral conductor is normally at or near earth potential. But if used with a two-pin power plug (or an incorrectly wired three-pin one), any metal that the user could touch was an electrocution hazard, connected to mains live. Consequently equipment was made with no metal connected to the chassis exposed even in predictable abnormal situations, such as when a plastic knob came off a metal shaft, or small fingers poked through ventilation holes. Service personnel working on energized equipment had to use an isolation transformer for safety, or be mindful that the chassis could be live. AC-only vacuum tube equipment used a bulky, heavy, and expensive transformer, but the chassis was not connected to the supply conductors and could be earthed, making for safe operation.
Transformerless "hot chassis" televisions continued to be commonly manufactured long after transistorisation rendered live-chassis design obsolete in radios. By the 1990s, inclusion of audio-video input jacks required elimination of the floating ground as TVs needed to be interconnectable with VCRs, game consoles and video disc players. The widespread replacement of cathode ray tubes with liquid crystal displays after the turn of the millennium resulted in televisions using primarily low voltages, obtained from switching power supplies. The potentially-hazardous "floating chassis" was no more.
In the past, 110–120 V was not high enough for higher-power tube audio and television applications, and only suitable to operate low-power radio and audio equipment such as radio receivers. Higher-powered 110–120 V audio or television equipment needed higher voltages, which were obtained using a step-up transformer based power supply, or sometimes an AC voltage doubler, therefore operating off AC only.
Some AC/DC equipment was designed to be switchable to be able to operate off either 110 V AC (possibly with a voltage doubler) or 220–240 V AC or DC. [7] Television receivers were produced which could run off 240 V AC or DC. [8] The voltage was not high enough to power some circuits, so energy was recovered during the flyback period from the primary of the line output transformer to provide a boosted HT (vacuum tube) (high tension) supply. [16] In a typical vacuum tube colour TV set, the line output stage had to boost its own HT supply to between 900 and 1200 volts (depending on screen size and design). [17] Transistor line output stages, although not requiring supply voltages above the rectified mains voltage, nevertheless still developed extra voltage over the normal supply rail to avoid complicating the power supply circuitry. A typical transistor stage would produce between 20 and 50 'extra' volts. [18] Some details of the way in which the nominally 190 volts HT supply was boosted to nearly 500 volts in the 1951 Bush TV22 are described in a technical publication. [19] AC/DC televisions were produced well into the color and semiconductor era (some sets were tube/semiconductor hybrids).
With widespread adoption of solid-state design in the 1970s, voltage and power requirements for tabletop portable radio receivers dropped significantly. One common approach was to design a battery-powered radio (typically 6 volts DC from four dry cells) but include a small built-in step down transformer and rectifier to allow mains electricity (120 V or 240 V AC, depending on region) as an alternative to battery-powered operation.
A vacuum tube, electron tube, valve, or tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation is performed by an inverter.
A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As a result, power supplies are sometimes referred to as electric power converters. Some power supplies are separate standalone pieces of equipment, while others are built into the load appliances that they power. Examples of the latter include power supplies found in desktop computers and consumer electronics devices. Other functions that power supplies may perform include limiting the current drawn by the load to safe levels, shutting off the current in the event of an electrical fault, power conditioning to prevent electronic noise or voltage surges on the input from reaching the load, power-factor correction, and storing energy so it can continue to power the load in the event of a temporary interruption in the source power.
A power inverter, inverter, or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the opposite of rectifiers which were originally large electromechanical devices converting AC to DC.
A switched-mode power supply (SMPS), also called switching-mode power supply, switch-mode power supply, switched power supply, or simply switcher, is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently.
A thyratron is a type of gas-filled tube used as a high-power electrical switch and controlled rectifier. Thyratrons can handle much greater currents than similar hard-vacuum tubes. Electron multiplication occurs when the gas becomes ionized, producing a phenomenon known as Townsend discharge. Gases used include mercury vapor, xenon, neon, and hydrogen. Unlike a vacuum tube (valve), a thyratron cannot be used to amplify signals linearly.
A DC-to-DC converter is an electronic circuit or electromechanical device that converts a source of direct current (DC) from one voltage level to another. It is a type of electric power converter. Power levels range from very low to very high.
In electronics, a center tap (CT) is a contact made to a point halfway along a winding of a transformer or inductor, or along the element of a resistor or a potentiometer.
A flyback transformer (FBT), also called a line output transformer (LOPT), is a special type of electrical transformer. It was initially designed to generate high-voltage sawtooth signals at a relatively high frequency. In modern applications, it is used extensively in switched-mode power supplies for both low (3 V) and high voltage supplies.
The term All American Five is a colloquial name for mass-produced, superheterodyne radio receivers that used five vacuum tubes in their design. These radio sets were designed to receive amplitude modulation (AM) broadcasts in the medium wave band, and were manufactured in the United States from the mid-1930s until the early 1960s. By eliminating a power transformer, cost of the units was kept low; the same principle was later applied to television receivers. Variations in the design for lower cost, shortwave bands, better performance or special power supplies existed, although many sets used an identical set of vacuum tubes.
A mercury-arc valve or mercury-vapor rectifier or (UK) mercury-arc rectifier is a type of electrical rectifier used for converting high-voltage or high-current alternating current (AC) into direct current (DC). It is a type of cold cathode gas-filled tube, but is unusual in that the cathode, instead of being solid, is made from a pool of liquid mercury and is therefore self-restoring. As a result mercury-arc valves, when used as intended, are far more robust and durable and can carry much higher currents than most other types of gas discharge tube. Some examples have been in continuous service, rectifying 50-ampere currents, for decades.
In electronics, a bleeder resistor, bleeder load, leakage resistor, capacitor discharge resistor or safety discharge resistor is a resistor connected in parallel with the output of a high-voltage power supply circuit for the purpose of discharging the electric charge stored in the power supply's filter capacitors when the equipment is turned off, for safety reasons. It eliminates the possibility of a leftover charge causing electric shock if people handle or service the equipment in the off state, believing it is safe. A bleeder resistor is usually a standard resistor rather than a specialized component.
In electronics, motorboating is a type of low frequency parasitic oscillation that sometimes occurs in audio and radio equipment and often manifests itself as a sound similar to an idling motorboat engine, a "put-put-put", in audio output from speakers or earphones. It is a problem encountered particularly in radio transceivers and older vacuum tube audio systems, guitar amplifiers, PA systems and is caused by some type of unwanted feedback in the circuit. The amplifying devices in audio and radio equipment are vulnerable to a variety of feedback problems, which can cause distinctive noise in the output. The term motorboating is applied to oscillations whose frequency is below the range of hearing, from 1 to 10 hertz, so the individual oscillations are heard as pulses. Sometimes the oscillations can even be seen visually as the woofer cones in speakers slowly moving in and out.
A double diode triode is a type of electronic vacuum tube once widely used in radio receivers. The tube has a triode for amplification, along with two diodes, one typically for use as a detector and the other as a rectifier for automatic gain control, in one envelope. In practice the two diodes usually share a common cathode. Multiple tube sections in one envelope minimized the number of tubes required in a radio or other apparatus.
An antique radio is a radio receiving set that is collectible because of its age and rarity.
An induction heater is a key piece of equipment used in all forms of induction heating. Typically an induction heater operates at either medium frequency (MF) or radio frequency (RF) ranges.
A vibrator is an electromechanical device that takes a DC electrical supply and converts it into pulses that can be fed into a transformer. It is similar in purpose to the solid-state power inverter.
In vacuum tube technology, HT or high tension describes the main power supply to the circuit, which produces the current between anode and cathode. It is also known as the plate supply or voltage, B battery supply, or simply labeled →B on circuit diagrams, from the days of battery powered circuitry.
A capacitive power supply or capacitive dropper is a type of power supply that uses the capacitive reactance of a capacitor to reduce higher AC mains voltage to a lower DC voltage.
This glossary of electrical and electronics engineering is a list of definitions of terms and concepts related specifically to electrical engineering and electronics engineering. For terms related to engineering in general, see Glossary of engineering.
