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THE POWER SUPPLY By Emiliyan Nikolov What is it? The power supply is a main building block of any electronic device. It’s purpose is to deliver the energy necessary for the components and circuits to operate and do the job they are meant to. Where does it take energy from? To deliver the energy, the power supply has to take it from a source. This can be: • the power grid • energy storage device such as batteries, fuel cells, supercapacitors, etc. • alternative source such as wind generator, solar panel, hydroelectric generator, etc. supercapacitors portable wind generator Then what? Every device needs a specific voltage and amount of current to do what it has to. So it’s up to the power supply to convert the input from the source to the exact voltage needed by the device with enough ant not too much current. Here we can differentiate two main types of power supply: •step-up power supply •step-down power supply the output voltage and max current of a laptop power brick Step-up power supply As the name of it may show, it steps up the input voltage to one higher than it, but the output current is lower than the input (the output power can’t be higher than the input). This kind of power supply is generally the more complicated one because when the input is DC a simple transformer won’t do the job and that is why these power supplies are always SMPS (we will see what this is later on). SMPS that takes 12 volts from a battery and outputs mains voltage to operate an ordinary 60W light bulb Why do we need it? Step-up power supplies are used when we need voltage higher than we have. Examples are power supplies for CRTs, for lasers, car inverters, etc. and even the stun gun can be considered one. CRT monitor stun gun powered by batteries that outputs several hundreds of thousands of volts car inverter Step-down power supply You guessed it! This simple step type of power supply down power takes a higher voltage supply and steps it down to a lower voltage. The output current can be higher or lower than the input. This type of power supply can be as simple as a resistor connected in series with the powered device and as complicated as a highly stabilized and precision laboratory test instrument. home made bench test power supply And why do we need this? Step-down power supplies can be found everywhere around us. Just look around! The charger to which your phone is connected, the power supply of your PC, the power brick which charges your laptop. There is one for almost every electronic household device – be it external as an adapter or internal as part of the device itself. DVD player internal power supply simple adapter PC power supply How does it work? Lets start with the step-down power supply. In old days when the electronic components where still too big almost all power supplies were based on transformers. There would be a big transformer converting the line voltage to a lower one and from there a rectifier and a filter are used to turn the AC into DC if needed. When the circuit won’t come in touch with the user, the voltage is just too low or simplicity is needed a resistor or something similar can be used to block the excess voltage and current dissipating it in heat. A voltage devider transformerless power supply used in household control switch – there is no electrical contact with the user As the electronic components got smaller and cheaper a new kind of power supply started to take the place of the previous two – the switchmode power supply (SMPS). This type of power supply works by taking the DC or rectifying the AC, converting it in high frequency AC and then with the help of transformer or an inductor stepping it down to the needed voltage. SMPS block diagram This type of operation has two main benefits. As the frequency is higher the used transformer can be several times smaller than an ordinary one and thus the whole power supply can be very small and lightweight. (when a transformer is used) on the left: 100 watt mains transformer – it is as big as it looks like on the right: 100 watt transformer for SMPS roughly 2 by 2 centimetres. The other benefit is that because it operates by turning the powered device on and off at a very high frequency rather than just burning the excess power off as heat it can be a lot more efficient than a simple resistive method. (when an inductor is used) power supply used in high-efficiency applications Step-up power supply operation If the input is AC a simple transformer and SMPS can do the job perfectly in almost the same fashion as step-down power supply. If the output is DC only the SMPS can step the voltage up. The principle of operation is the same as if the voltage is stepped down (by transformer or by inductor). inductor based SMPS operation Stepping up with an inductor The SMPS that steps the voltage with the help of an inductor is also known as boost converter. It uses an effect called back EMF which can produce very high voltages and currents. These voltages can be many times higher than the input and ultimately the output voltage can be several times higher than the input. Generally this kind of operation is used when the output is not much higher than the input otherwise a transformer based SMPS is used. here the back EMF occurs Unregulated vs. regulated If the powered device is more sensitive just a rectification and filtration of the AC may not be enough. If a higher current is applied or the line voltage changes the output of the power supply will also change. That is where a regulator is used. This, if there is one in the power supply, is the most complicated part of the whole unit. unrectified AC waveform rectified AC rectified, filtered and regulated AC (perfect DC) rectified and filtered AC (almost DC) The regulator By itself the regulator also can be consider a power supply and as such there are also linear (using resistance) or switch-mode regulators. It can be made of discrete components or it can be a dedicated integrated circuit as big as an ordinary transistor. A very popular linear regulator LM7812 linear regulator switch-mode regulator 2N5294 transistor in comparison – the sizes of both are exactly the same For the curious ones… Here is a list of the resources used and sites that can explain the matter in more details. • http://www.howstuffworks.com/search.php?terms=power+supply • http://www.hobbyprojects.com/block_diagrams/power_supply_units.html • http://www.rason.org/Projects/swregdes/swregdes.htm • http://www.circuitstoday.com/dc-power-supplies • http://www.allaboutcircuits.com/vol_3/chpt_9/2.html • http://sound.westhost.com/articles/power-supplies2.htm • http://www.electronics-tutorials.com/basics/power-supply.htm • http://www.ladyada.net/learn/powersupply/transformeracdc.html Hope you enjoyed it! THE END