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in this video i'm going to talk aboutlinear voltage regulators and show you how to use one to charge 5 volt usbdevices like phones and mp3 players. so what is a voltage regulator? a voltage regulator is a device that takesan unregulated input voltage that could be fluctuating over time, and spits out aperfectly regulated constant voltage. for example here i have a twelve voltbattery that will be 13.8 volts fully charged, and around 11 volts fully discharged.

this 5 volt regulator will ensure thati get a constant five volts regardless of the input voltage. and the capacitors in the circuit willmaximize the stability of the regulator's output. i'll show you the circuit on the breadboard later on. in the meantime take a look at this. inan earlier video i showed you how to make unregulated dc power supplies. right now the output of that unregulatedsupply is about 16 volts with about 2 volts of ripple on it. but if i add a 12 volt regulator iget a constant 12 volts out

regardless of what's happening on theinput. here's the same thing with a 9 volt regulator... and a 5 volt regulator. so you can buy voltage regulators to getany voltage you want. and it's really not much morecomplicated than that. basically you have a higher inputvoltage that could change at any time and the linear voltage regulator justclips all of that off leaving you with a lower but veryprecise dc power supply. even a terrible linear voltage regulator

will give you an output that is accurateto within five percent. and newer voltage regulators have athermal shutdown feature which means i can't even show you one catching on fire! pfft... progress... now how do i find a voltage regulator? most voltage regulators begin with a fewletters, then "78", and then they have two digits indicating output voltage. so here i have an l7805cv and the "05" indicates that it's a5 volt regulator.

the lm7809 is made bya different manufacturer and it's a 9 volt regulator. and on the right we have a 12 volt regulator. you can buy these anywhere that sellselectronics... radio shack, jameco, maplin and a million other places becausethey're such a basic component. just search for voltage regulator. you can also salvage them from oldelectronics. check it out... i found a 3.3v, 5v, 6v, and 9v regulator

all on one pcb from an old dvd player. and i'm not going to lie, i didn't know these were all voltageregulators until i googled the part numbers written on them. as always with engineering, google isyour best friend. okay so let's say you've got a voltageregulator. let's say you've got an lm7805 for 5 volts and want to wire it up. well that's really easy. all you need is the regulator and threecapacitors.

10 microfarads on the input, 10 microfarads on the output, and finally a 0.1 microfarad ceramic capacitor on the output. these capacitors help ensure thestability of the voltage output, and although you don't always need them, if you have them on there the circuit ispretty much guaranteed to work every time. the values don't have to be exact so ifyou use slightly more or less capacitance it'll probably be fine. so let's give it 12 volts in and at5 volts out you can see that it works.

right now this circuit would be good forloads of up to 100 milliamps but since i want to handle more currenti'm going to put a heatsink on it. okay now i promised to show you how tomake a 5 volt usb charger right? well first get a usb cable like the oneyour device needs. get it from a dollar store so you don'tget ripped off. cut the head off the cable and stripthe insulation. next, clip off everything but the red andblack wire. the red wire goes to your +5 voltline and the black wire goes to your

circuit's ground. double check your regulator is working, then plug it all in. that was easy wasn't it? now you can be even more angry whenpeople sell you car chargers for thirty dollars. just make sure you keep an eye on thetemperature of the heatsink and use a bigger one if necessary. now there's two more things that youshould know about linear voltage regulators.

1) they're not very efficient. 2) they have what is called a dropout voltage. the power wasted in a linear regulatoris given by (vin - vout) x output current. so if i take the 16 voltunregulated power supply that i built, use a 5 volt regulator, and draw 300milliamps it'll generate 3.3 watts of heat... which is a lot for a tiny device! so a big heatsink is important. now let's see what happens when i useanother input voltage that's closer to

the 5 volt output that i want. this is a 7.2 volt nimh battery and when it's fully charged it'll supply8.4 volts. if i draw the same 300 milliamp load,i get a power dissipation of 1 watt, so i can get away with no heat sink at all. in general you want to avoid high inputvoltages with linear regulators because the higher the input voltage, the lowerthe efficiency. now the dropout voltage that i mentionedearlier is related to the minimum input voltage they have to feed your regulatorto guarantee a regulated output.

most of the time your input voltage willhave to be at least a volt or two above the output voltage of the regulator. so for a typical 5 volt regulator you'llprobably need a minimum input of 7 volts. check this out... here i am dropping the input voltagefrom twenty volts all the way down to 7 volts and the output voltagedoesn't change. now watch what happens when i drop it alittle lower... the output voltage starts to drop, and your circuit won't get five voltsanymore.

the exact amount of headroom you need iscalled the dropout voltage and it will always be given in the datasheet. you can also buy regulators that have alower dropout voltage than normal, for example this is an lm2940 regulator and it has a dropout voltage of 0.5 volts. this means that for 5 volts out, your inputcan drop as low as 5.5 volts before you lose regulation. low dropout regulators are slightly moreexpensive than standard linear regulators, but they can be useful if your inputvoltage drops really close to the output

voltage that you are expecting. for example if you're trying to power adevice with batteries, think very carefully about how lowyour battery voltage will go. so that's it! voltage regulators are easy. inputvoltage, output voltage, a few capacitors and a heat sink and you're done. thanks for watching!