Ignoring the obvious jokes about AC/DC; including bipolar DC currents, bistable multivibrators, and all other possible smutty electronics terminology innuendos; in this article we’ll be taking a look at how the high-voltage Alternating Current (AC) electricity that comes from the power plug and is transported to your computer becomes the low-voltage Direct Current (DC) electricity that runs your computer.
I use your computer in the above paragraph as it’s the most obvious choice; considering the fact that you’re probably reading this article on your computer; and if not have probably printed it off from a printer attached to a computer. In fact it’s not just computers that have to change the AC mains voltage into a usable working DC voltage. Most if not all mains-powered electronic devices have to do this: Even to a certain extent CRT monitors and televisions, although these also utilise the high-voltage AC current of mains electricity in addition to stepping the voltage both up and down, as well as converting AC into DC in some of their internal circuitry. For the purpose of this article I’ll be using the example of a computer (PC) power supply unless otherwise indicated.
In comes the power lead carrying between 217 and 254 volts of AC electricity in the UK (Depending upon the time of day and the geographical location in the UK.), or around 110 volts AC in the USA. The power supply’s job is to convert that voltage into three separate DC voltages; 12 volts, 5 volts, and 3.3 volts. Due to the high wattage requirements of some of the circuitry in the computer; these supplies; particularly the 12 and 3.3 volt supplies, have to also be able to supply very large currents, measured in Amperes. (Amps.)
The relationship of electrical current (Amps) to electrical power (Watts) is defined in Joule’s Law as P = IV. (Power in watts is equal to the sum of amperes in amps multiplied by voltage in volts.) Therefore if you were to have a power supply supplying 12 volts at 12 amps; the available wattage would be 12 volts X 12 amps = 144 watts.
(This equation can also be reversed to show the inverse of this: -
I = P/V (Amperage = power in watts divided by voltage in volts.) and V = P/I Voltage = power in watts divided by current in amps.))
For the purpose of this article, we’ll ignore the large currents to the greatest possible extent; and rather we’ll concentrate on the basics of changing high-voltages into low-voltages, and AC into DC.
There are 4 main component blocks in a (single-output) power supply; those being: -
A transformer is a single electrical component consisting of two or more coils of wire formed around a core of varying density depending upon the type of transformer. It works by the electromagnetic field induced in the primary coil or input coil by an AC electric current affecting the secondary or output coil and causing a proportional electric current to flow within that coil. The ratio of the two or more AC currents in question is dependant upon the construction of the transformer itself.
Recently; in electronic equipment that requires very low electrical current; the transformer has been replaced by a high-wattage resistor/ AC potential divider circuit. This has the effect of dropping the voltage by using electrical resistances rather than the electromagnetic induction principles of a transformer. Since resistors are [usually] smaller and lighter than transformers, as well as being cheaper; this type of voltage-dropping circuit is commonly used wherever possible these days.
Its advantages are reduced cost and reduced weight. Its disadvantages are that it can only output a small current: commonly considerably less than 1 amp, also that the load on the AC mains input of the circuit is always constant and unchanging; whether or not the circuit’s output is being used to power anything.
A [bridge] rectifier typically consists of four diodes connected in a certain configuration end-to-end. (see diagram.)
The action of the bridge is to use the component diodes ([Rectifier] Diodes will only allow electricity to flow one way through them dependant upon their connected polarity.) to sort the component parts of the AC wave-cycle into positive and negative; therefore changing the oscillating AC waveform into a crude type of DC current.
The crude “DC current” output of the rectifier stage of the circuit isn’t anything like pure DC electricity: It’s very unstable and resembles its former state to some extent.
A large-value capacitor placed in its path helps to iron out the remaining inconsistencies and reduce the inherent instability somewhat.
No matter how much smoothing is applied to the output of the rectifier by capacitor(s), it can never be transformed into a totally stable DC current by this method alone. A voltage-regulator IC (Integrated Circuit) is placed in-circuit at this stage to finally stabilise any residual waveform-ripple and set the exact output voltage before the current can finally be outputted to run an electronic circuit.
- So very basically; that’s how it’s done. There’s more; a huge amount more, to be learned. – But these are the very basic basics of it.
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