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The description AC/DC refers to equipment designed to operate on either alternating current (AC) or direct current (DC). This term typically describes certain types of vacuum tube radio or television receivers. AC/DC equipment was necessary because in the early days of vacuum tubes, some regions were supplied with AC power, others with DC. Equipment which is able by its nature to use either AC or DC, e.g., heating devices and incandescent light bulbs, is not usually described as “AC/DC”.

Alternative and direct current

Direct current flows only in one direction. This is the type of current that is supplied by batteries, and was used in early central station generators for electric lights. Alternating current regularly reverses direction at a frequency that is set by the local power system, and is used in modern distribution systems since it can readily be changed in voltage with transformers. Electronic devices such as vacuum tubes and transistors are usually operated on direct current.

One form of electric power can be converted to the other,using a motor-generator system or power electronics devices such as rectifiers or an inverter circuit.A power supply in an electronic device converts the wall-socket AC power to the DC levels required. Some types of electric motor operate only on direct current, others can only operate on alternating current. Heating elements, and incandescent lamps, can operate on either direct current or alternating current of the same root mean square value.

Applicability to early radio and television

Vacuum tube equipment used a number of tubes, each with a heater requiring a certain amount of electrical power. Tubes require relatively high voltages on some of their electrodes; these voltages can conveniently be derived directly from mains electricity. There are three ways of powering such equipment:

• AC equipment: a transformer converts mains electricity into both a low-voltage (typically 6.3V) supply connected to the parallel-connected heaters of all the tubes in the equipment, and one or more high-voltage supplies which are rectified and filtered to give high DC voltages required by the equipment. Transformers operate on AC only, so that this type of equipment is AC-only.
• 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 450mA) but at different voltages. If necessary, resistance (which can be a ballast tube (barretter), power resistor or resistive cable are added so that, when the mains voltage is applied across the chain, the required current flows. With mains voltages of around 220V, 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. A rectifier and a filter are connected directly to the mains. If the mains power is AC, the rectifier converts it to DC. If it's DC, the rectifier effectively acts as a conductor. In both cases DC at about the same voltage as the mains is available to drive the circuitry. The tube heaters do just as their name describes and heat the cathodes, whether AC or DC power is applied. There is no transformer to isolate AC/DC equipment from the mains. Much equipment was built on a metal chassis which had to be connected to one side of the mains. A typical low-cost radio would have 5 tubes, plus a ballast built into an envelope like a tube that was easily replaceable.
• DC-only from DC mains (no longer applicable: see War of Currents).

The operation of the power supply is further described in another article.

AC/DC equipment was suitable for use on either AC or DC, an important consideration when DC distribution was still used. Manufacturers were able to produce a single range of equipment for all power, and users did not have problems when moving house. Because no power transformer was used, so-called "hot chassis" construction was required and the equipment power supply was conductively connected to the input power source. Any exposed metal on the device connected to the circuit common was also connected to the power supply. For safety, no exposed metal could be connected to the circuit common. Service personnel working on energized equipment had to be mindful that the chassis could be at line potential with respect to earth ground.

If a resistive power cable was used, an inexpert repairer might replace it by a standard cable, or use the wrong length, damaging equipment and risking 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.

• Older AC-only equipment uses a bulky, heavy, and expensive 50- or 60-Hz transformer, but the chassis is never live and can be earthed, making for safer operation. Additionally, the use of a transformer allowed higher voltages to be generated (e.g., for high-powered audio amplifiers), and allowed multiple independent power supplies from separate transformer windings for different stages.
• DC-only equipment was a little cheaper than AC/DC, but became obsolete as AC power became dominant.

Regional variations

In the past, 110-120V was not high enough for high-power audio and television applications. Therefore, it was used to operate low-power audio equipment such as radio receivers. Higher-powered 110-120V tube audio or television equipment needed higher voltages which had to be stepped up by a transformer power supply, or sometimes a 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 220-240V AC or DC. Television receivers were produced to run off 240V AC or DC. The voltage was not high enough to power some of the circuits, so it was boosted with energy recovered from the deflection coils during flyback. Some details of the way the voltage was boosted are to be found in a technical description of the 1951 Bush TV22. AC/DC televisions were produced well into the color and semiconductor era (sets were tube/semiconductor hybrids).

Motorized tools and appliances

Many hand-held power tools or small kitchen appliances use universal motors. When DC distribution was still used, some motorized appliances were name-plated for operation both on DC and AC systems. The universal motor connects its field in series with the armature. The motor is small enough that the reversal of magnetic fields in both the armature and field occur at almost the same time, meaning the motor can run equally well on both types of current. Although modern tools and appliances often still use lightweight and powerful universal motors, they are no longer rated for operation on DC systems, usually because the switch is only suitable for AC. Safety standards no longer test for DC operation, switching devices in AC-only appliances are smaller and unsuitable for DC circuits of the same current, and many modern power tools and appliances incorporate AC-only speed control based on thyristors that will not work on direct current supplies.

Modern equipment

Since solid-state electronics displaced vacuum tubes, circuits required high currents at relatively low voltages, and the use of transformers has become almost universal. The decreasing cost of complex electronics, with massive functionality available in a single, cheap, integrated circuit has made it feasible to power equipment safely from either AC or DC mains without a conventional mains transformer. The supply is rectified and filtered if AC, and used to power a high-frequency oscillator whose output is connected to the primary winding of a small, cheap, high-frequency transformer (which is often part of the tuned circuit used by the oscillator) which isolates the circuitry of the equipment from the mains electricity. In principle, modern AC/DC equipment would be no more complex or costly than AC-only. In practice, DC mains electricity is no longer used, and DC operation direct from the mains is irrelevant. Although unrelated to AC/DC as used in the past, modern equipment is often powered from low-voltage DC, typically a 12V battery in a motor vehicle. Much equipment operates from 12V or less, but an inverter can be used to provide AC output or a DC to DC converter based on a switch-mode power supply (SMPS) for higher DC output. Modern SMPSes can accept a DC input without any problem, though the DC voltage does need to be around 25% higher than the rated RMS AC voltage.

See also

• All American Five
• War of Currents
• AC/DC conversion
• DC/AC conversion

Notes and references

1. All About Ballast and Resistor Tubes (1939)
2. From Resistive Line Cords To Ballast Tubes (1939)
3. A five-valve receiver for AC/DC mains with ballast (barretter), detailed description and circuit diagram
4. An eight-valve 110VAC or 220V AC/DC superheterodyne receiver with push-pull output stage, detailed description and circuit diagram
5. Description of Pye B18T true AC/DC 240V (190-220V operation needed an additional AC-only autotransformer) monochrome TV and other equipment, 1948
6. Technical description of the 1951 Bush TV22

• Electrical engineering
• Electric current