How to choose filter capacitors for power supply

you hey what's happening guys today I want to answer a question I get asked a lot but like how do you choose a specific capacitor for the job I'm not talking about whether you want to choose a ceramic or an electrolytic or anything like that let's talk about choosing the right size capacitor well kind of keep it easy but this is going to get a bit technical so if this ain't your thing I understand if you don't want to watch it anyway let's say that you need to make yourself a power supply the voltage is positive negative for an op amp project so here's just something simple I drew up we have our AC coming in right here there's our rectifier this could be a single IC full bridge rectifier it really doesn't matter then we have our two main filter caps here each of four thousand microfarad then we have a positive voltage regulator a negative voltage regulator smaller filter caps on the outputs of those couple diodes to keep transients and stuff down and then we have our output and of course you know across their output here would be our load cuz nothing matters without a load right because otherwise they're just not any information so we have our filter capacitors there to help us control ripple so what is ripple well ripple is just the the fluctuation of the voltage mean you know what AC looks like you know up and down DC is supposedly a straight line well when you rectify AC to DC you're going to have some ripple so let's let's roll down here a little bit everything in there mm-hmm so let's say this is what our unfiltered DC looks like coming from the bridge rectifier what that rectifier has done it has taken those negative going peaks and flipped them up and in doing so it has doubled the frequency our 60 Hertz is now 120 Hertz but now we have this space here between our peak and our trough and that is the ripple is it bad well it depends on what you're doing if you're driving a digital circuit from a 5 volt supply your variation should be no more than 5% or you know 1/4 of a vote if you're doing a small analog circuit they can be pretty finicky and it might require less than 1% so you want to keep it ripple down as much as you possibly can and you do that by using a filter capacitor so here's our filter capacitor don't even imagine this is our V in here this this will correspond to what's on the other side of these right here so there's our filter capacitor there's our current and there's our load so this is what it is looking like after the filter capacitor so instead of whoops you know we look like this now we have this look here and what's happening don't draw this out more this area here is what the capacitor has basically sucked up and is changing but we still have this small area here and that's our ripple so we want to make sure that we have big enough capacitors to do the job we need to know how to find the ripple so that we can choose the calculator or the the capacitors to do the job so how do we do that well there's a formula of course there's a formula in here so it is a C is equal to DV over DT DV is our is our ripple in RMS so basically it is the difference between the peak and the trough and DT is our time this is kind of a hard to calculate so this is an approximation that I've used over the years 1 over the frequency and then we can get you know like eight point three nine times ten to the third and that will allow us to calculate our ripple all right so let's test out our formula let's say we have a five volt supply and one amp load and a 4700 micro farad capacitor so if you plug those numbers into our formula you're going to get five hundred and ten millivolts now that would be over the 0.25 millivolts that we're looking for for a nice steady supply but one thing you have to keep in mind with is that when you're using voltage regulators they often have a ripple rejection factor and it's often given in DB but you can figure that out relatively easily let's say you 7815 has a ripple rejection characteristic of 60 DB so you can figure that out you can say - 60 DB of course is log 20 log 10 over the in ovary out that gives you what minus 3 log 10 you know V in out or 10 to the minus 3 so that is a factor of 1,000 now your factor of 1,000 if you plug into that 510 is going to get end up giving us watching we want to put the decimal place in a long point I'll get you out at 0.5 1 millivolts yeah so that is just a great way to choose your capacitors now what you want to do is you just want to plug different capacitance values or her formula into the formula until you get what you need I know kind of bit of hitting this kind of a try and fail see what you get but you know it's kind of way it works all right hope you guys enjoyed this if you did please give me a thumbs up feel free to comment and share and don't forget to subscribe please please take care of yourselves limit your exposure to sick people wash your hands god bless you all that's it I'm out peace I want to thank you all for watching and spending time with me today a community like this is something that we can all be very proud of so again thank you very much for all your support of learn electronics please feel free to check out the patreon page dollar month is all I ask and really helps keep the channel alive we also have an Amazon shop where you can buy most of the items that you on here and there's a link to it down below

Channel: learnelectronics

Views: 33,517

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Id: oNW371DQQqA

Length: 8min 0sec (480 seconds)

Published: Mon Mar 09 2020