DC Power Units, Physics tutorial

Power Supply Types:

Power supplies for electronic devices can be largely divided in linear and switching power supplies. Linear supply is generally a relatively simple design; it becomes gradually more bulky and heavy for high-current equipment because of need for large mains-frequency transformers and heat-sinked electronic regulation circuitry.

Battery Power Supply:

A battery is a kind of power supply which is independent of availability of mains electricity, appropriate for portable equipment and use in locations without mains power. The battery comprises of many electrochemical cells joined in series to give the voltage desired. Batteries may be primary or secondary.

Primary cell first utilized was carbon-zinc dry cell. It had voltage of 1.5 volts; afterward battery types have been manufactured, when possible, to give same voltage per cell. Carbon-zinc and related cells are still utilized, but alkaline battery delivers more energy per unit weight and is extensively used. Most usually utilized battery voltages are 1.5 (1 cell) and 9V (6 cells).

Different technologies of rechargeable battery are utilized. Types most commonly employed are NiMH, and lithium ion and variants.

Unregulated Power Supply:

AC powered unregulated power supply generally utilizes a transformer to convert voltage from wall outlet (mains) to different, nowadays generally lower, voltage. If it is utilized to generate DC, rectifier is utilized to convert alternating voltage to the pulsating direct voltage, followed by the filter, including one or more capacitors, resistors, and at times inductors, to filter out (smooth) most of the pulsation. A small remaining unwanted alternating voltage component at mains or twice mains power frequency-ripple-is inescapably superimposed on direct output voltage. For purposes like charging batteries ripple is not problem, and simplest unregulated mains-powered DC power supply circuit comprises of transformer driving the single diode in series with resistor. Before introduction of solid-state electronics equipment utilized valves (vacuum tubes) that needed high voltages; power supplies used step- up transformers, rectifiers, and filters to produce one or more direct voltages of some hundreds of volts, and low alternating voltage for filaments. Only the most advanced equipment utilized expensive and bulky regulated power supplies. The unregulated power supply is one whose dc terminal voltage is affected considerably by amount of load. As load draws more current, dc terminal voltage becomes less.

Regulated Power Supply:

The voltage generated by unregulated power supply will differ depending on load and on variations in AC supply voltage. For critical electronics applications a linear regulator may be utilized to set voltage to the precise value, stabilized against fluctuations in input voltage and load. Regulator also really decreases ripple and noise in output direct current. Linear regulators frequently give current limiting, protecting power supply and attached circuit from over current. Adaptable linear power supplies are common laboratory and service shop test equipment, letting output voltage to be adjusted over a range. For instance, bench power supply utilized by circuit designers may be adaptable up to 30 volts and up to 5 amperes output.

AC/DC Supply:

In past, mains electricity was supplied as DC in some regions, AC in others. Transformers can't be utilized for DC, but simple, cheap unregulated power supply could run directly from either AC or DC mains without using transformer. Power supply comprised of rectifier and filter capacitor. When operating from DC rectifier was really a conductor, having no effect; it was included to permit operation from AC or DC without modification.

Switched-Mode Power Supply:

Switched-mode power supply (SMPS) works on the different principle. AC input, generally at mains voltage, is rectified without use of mains transformer, to get a DC voltage. This voltage is then switched on and off at high speed by electronic switching circuitry that may then pass through high-frequency, therefore small, light, and cheap, transformer or inductor. Duty cycle of output square wave increases as power output requirements increase. Switched-mode power supplies are always regulated. If SMPS uses properly-insulated high-frequency transformer, output will be electrically isolated from mains, necessary for safety. Input power slicing takes place at very high speed (typically 10 kHz- 1 MHz). High frequency and high voltages in this first stage allow much smaller transformers and smoothing capacitors than in power supply operating at mains frequency, as linear supplies do. After transformer secondary, AC is again rectified to DC. To keep output voltage constant, power supply requires sophisticated feedback controller to monitor current drawn by load.

SMPSs frequently comprise safety features like current limiting or crowbar circuit to help protect device and user from harm. In event that abnormal high-current power draw is detected, switched-mode supply can assume this is the direct short and will shut itself down before damage is done. For decades PC power supplies have given a power good signal to motherboard whose absence prevents operation when abnormal supply voltages are present.

SMPSs have the absolute limit on their minimum current output. They are only able to output above certain power level and can't function below that point. In no-load condition frequency of power slicing circuit increases to great speed, causing isolated transformer to serve as Tesla coil, causing damage because of resulting very high voltage power spikes. Switched-mode supplies with protection circuits may temporarily turn on but then shut down when no load has been detected. Very small low-power dummy load like a ceramic power resistor or 10-watt light bulb can be joined to supply to permit it to run with no primary load attached.

Programmable Power Supply:

Programmable power supplies permit for remote control of output voltage through analog input signal or computer interface like RS232 or GPIB. Variable properties comprise voltage, current, and frequency (for AC output units). These supplies are made up of a processor, voltage/current programming circuits, current shunt, and voltage/current read-back circuits. Extra features can comprise over current, overvoltage, and short circuit protection, and temperature compensation. Programmable power supplies also come in variety of forms comprising modular, board-mounted, wall-mounted, floor-mounted or bench top.

Uninterruptible Power Supply:

Uninterruptible power supply (UPS) takes its power from two or more sources simultaneously. It is generally powered directly from AC mains, while simultaneously charging a storage battery. Should there be the dropout or failure of mains, the battery immediately takes over so that load never experiences an interruption. Such a scheme can supply power as long as battery charge suffices, e.g., in the computer installation, giving operator enough time to effect an orderly system shutdown without loss of data. Other UPS schemes may use the internal combustion engine or turbine to incessantly supply power to the system in parallel with power coming from AC mains. Engine- driven generators would usually be idling, but could come to full power in matter of few seconds to keep important equipment running without interruption. Such scheme used in hospitals or telephone central offices.

High-Voltage Power Supply:

High voltage refers to the output on order of hundreds or thousands of volts. High-voltage supplies use the linear setup to produce the output voltage in this range.

Extra features available on high-voltage supplies can comprise ability to reverse output polarity along with use of circuit breakers and special connectors intended to minimize arcing and accidental contact with human hands. Few supplies give analog inputs (i.e. 0-10V) which can be utilized to control output voltage, effectively turning them in high-voltage amplifiers although with very limited bandwidth.

Components of the DC Supply Unit:

Typical dc power supply comprises of five stages:

1757_Components of a DC Supply Unit.jpg

Transformer: Its job is either to step up or (mostly) step down ac supply voltage to go well with requirement of solid- state electronic devices and circuits fed by dc power supply. It also gives isolation from supply line-significant safety consideration.

Rectifier: It is circuit that uses one or more diodes to convert ac voltage in pulsating dc voltage.

Filter: Function of circuit element is to remove fluctuations or pulsations (known as ripples) present in output voltage supplied by rectifier. Of course, no filter can, in practice, provides output voltage as ripple-free as that of dc battery but it approaches it so closely that power supply performs as well.

Voltage Regulator: Its major function is to keep terminal voltage of dc supply constant even when (i) ac input voltage to transformer differs (deviations from 220 are common) or (ii) load varies.

Generally, Zener diodes and transistors are utilized for voltage regulation purposes. Again, it is not possible to get 100% constant voltage but minor variations are suitable for most of jobs.

Voltage Divider: Its function is to give different dc-voltages required by different electronic circuits. It comprises of number of resistors joined in series across output terminals of voltage regulator. Obviously, it eliminates requirement of giving separate dc power supplies to different electronic circuits working on different dc levels.

All that is really needed for conversion from ac to dc is a transformer and rectifier (in fact, even transformer could be eliminated if no voltage transformation is needed). Filter, voltage regulator and voltage divider are simple refinements of the dc power supply though they are necessary for most applications except for battery charging and running small dc motors etc.

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