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Voltage regulator (from power supply) capacitor values?

  1. Jul 28, 2011 #1
    Hello our doctor gave us a project to design a power supply which looks like this
    http://i541.photobucket.com/albums/gg395/healing1000/powersupply.jpg [Broken]
    however at the voltage regulator part (78xx and 79xx) there are 2 capacitors (the 2 on the right) which I dont know how to find their values
    he asked us to make the power supply in a way that the ripple voltage is 0.01 and the voltage outputs are
    +5 +9 -5 -9
    so i will get 4 voltage regulators but how do i find out the values of those 2 capacitors on the right? he told us the 2 capacitors share the same value and we have to look them up and i tried googling and couldnt find out anything helpful so i came here in hope u guys can help me
    if the drawing isnt clear here is a similar voltage regulator i found online

    note: i know we are supposed to show our work but i really tried searching and couldnt find out how to figure the capacitors values help plz
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jul 28, 2011 #2


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    Staff: Mentor

    Welcome to the PF.

    It is easy to get that low output ripple from a linear regulator, as long as the input voltage to the regulator does not go below the minimum input voltage specification for the regulator (check their datasheets for those numbers).

    So the input storage capacitors between the diode bridge and the input to the 3-terminal linear regulator need to be big enough to hold up the input voltage to the regulator when the AC waveform is swinging through the non-conducting part of the cycle (when the bridge rectifier is off).

    To calculate the size of the input capacitance needed, you use the output current to the load, and the time that the bridge is off for each cycle, and use the traditional equation that relates the voltage of a capacitor as current is drained from it.

    Can you use those hints to start working on the solution?

    BTW, the output capacitance for the linear regulators that you mention is not real critical. You should be able to find some typical values in their datasheets.

    Also, for your Google searches to find more information, try this:

    LM7805 +"application note"

    Last edited by a moderator: May 5, 2017
  4. Jul 28, 2011 #3
    thanks for the welcome note
    i checked again it was ripple factor that is 0.01 not the ripple voltage
    To clarify things I dont have those parts im supposed to do this theoretically first

    There isnt any output current provided in the drawing the doctor gave us and also i dont know time that the bridge is off for each cycle

    The data sheets i find give me alot of info that i dont know what they are useful for but none of them show me what the capacitors values are

    If i can know how to get the values of the 3 capacitors in the drawing then all my life problems are solved

    by the way im extremely "beginner" its the first electronics course i take and we only are learning basics
  5. Jul 28, 2011 #4


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    Staff: Mentor

    Your sketch of the circuit appears to indicate setting the output current at 25mA, no?

    Also, note that on the regulator you show 78xx or 79xx. The LM7805 is for a +5V output, and the LM7905 is for a -5V output. So you could not put a LM7905 regulator into that circuit with the polarities of the capacitors as shown...

    Did you do the Google search that I suggested? That will be the best way for you to learn about how to calculate the values of the input storage capacitors for the linear regulators...
  6. Jul 28, 2011 #5
    Edit: I have been searching alot using different methods now and for the input capacitance some sites tell me its 220 micro farad and some 0.22 micro farad for the output capacitance 100 micro farad and some 0.1 micro farad so can u tell me which one of those work please?

    Also assuming the upper drawing is for the 78xx i draw one for the 79xx can you tell me if i did something wrong
    http://i541.photobucket.com/albums/gg395/healing1000/capacitance2.jpg [Broken]

    and any other hints on how to find out the value of the capacitor on the left? right next to the bridge? (dont know whats the time between each cycle)
    Last edited by a moderator: May 5, 2017
  7. Jul 29, 2011 #6


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    Staff: Mentor

    No, the 79xx drawing is not correct, and looking closer, some of the same problems are in your original sketch.

    The LM78xx and LM79xx are 3-terminal regulators. You are not showing the ground connection in either sketch. Also, you are not connecting the bridge rectifier correctly. Please double-check how a bridge rectifier works.

    And on the sizing of the input storage caps between the bridge output and the linear 3-terminal regulator, you need to calculate their size based on the output current drawn from the regulator, and on the peak output voltage of the transformer (and whether the AC mains frequency is 50Hz or 60Hz).
    Last edited by a moderator: May 5, 2017
  8. Jul 29, 2011 #7
    Well thats how the electronics doc drew it on the board (but your right i forgot the ground connectionthough) and asked us to get the values there is only one regulator in each supply but we are supposed to get 4 regulators for the +5 +9 -5 -9 but i will check again when i see him again

    Alright so can you tell me if the 2 capacitors values are 220/100 mf or 0.22 / 0.1 mf?
    and the relation im supposed to use for the capacitor on the left? I can get the rest of the info
    Last edited by a moderator: Jul 29, 2011
  9. Jul 29, 2011 #8


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    Staff: Mentor

    Well, it's important to get it drawn correctly. Please use Google Images and search on this:

    diode bridge and linear voltage regulator

    You will see how it is supposed to be connected. Please also look at the datasheets and application notes for the LM7805 and LM7905. That will give you more information on how to calculate the capacitor values.
  10. Jul 29, 2011 #9
    This will be a stupid question but in my original sketch is the regulator 3 terminal or 2 terminal? (is the capacitor on the left for the regulator or the bridge :D)

    im sure now about the values of the 2 capacitors on the right if its 2 terminal its 0.22 micro farad and 0.1 micro farad if 3 terminal its 0.33/0.1
    the only thing left after i find out the q above ^^^^^^
    is the capacitor on the left value the data sheets give the info for the 2 capacitors on the right not for the one on the left connected to the bridge
    i still dont know how to find out the capacitor on the left value
    thanks for your patience so far i appreciate it ^^
    Last edited by a moderator: Jul 29, 2011
  11. Jul 29, 2011 #10


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    Staff: Mentor

    There is no such thing as a 2-terminal regulator. Those are 3-terminal linear voltage regulators. Have you read through their datasheets some? Have you been able to find some application notes for them?
  12. Jul 29, 2011 #11
    yes and some data sheets from some sites say 0.22 0.1 and some say and some drawings i found online and some sheets 0.33 0.1 and most of the data sheets i found didnt have the capacitor values only little had trust me i really searched and i arent just posting here and not doing anything until you reply

    there are alot of weird info that i have no use of in the sheets here is how one of the sheets i downloaded look like for example
    http://i541.photobucket.com/albums/gg395/healing1000/datasheet.jpg [Broken]
    (full of useless things that i never heard of before in my life and never studied which show no way of finding the values of the capacitors im looking for..)

    I dont know how anything in there can help me finding the capacitor values?
    and even if i get the 2 values on the right (which im confused if 0.33/0.1 or 0.22/0.1 cause different sites gave me different values) i still have no idea how to get the one on the left

    there is a semi engineer that works at the university who told me the 2 capacitors at the right should be found online and i have to look them up and the one on the left has a separate way to calculate thats why i keep separating them

    so what do i have to do now to find out the values? since different sites gave me different answers and the one on the left I have no idea how to find?
    Last edited by a moderator: May 5, 2017
  13. Jul 29, 2011 #12


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    Staff: Mentor

    In the part of the datasheet that you linked to, the Dropout Voltage Vd is definitely part of what you use to calculate the minimum value of the energy storage capacitor between the bridge and the linear 3-terminal regulator's input.

    I did a Google Images search on lm7805 application note +transformer, and got some good hits. Do that search, and click on the first result -- it shows a full circuit with both the LM7805 and LM7905 together to generate +/- 5V output rails.

    For simple standard dropout voltage (like the Vd = 2V shown in your datasheet picture), the output capacitance value is not critical. The circuit that you drive with your power supply output will often have extra capacitance on it anyway. The values that you list for the output capacitances are fine (doesn't matter much whether it is 0.1uF or 0.33uF or whatever. That is just there to improve the transient response of the output circuit. Just choose whatever value you want, and link to the datasheet that shows it in your answer.

    If you were using a "Low Dropout Regulator", which can have Vd much lower like 0.5V, then the value of the output capacitor is more important, and is described in detail in their datasheets.

    For the input energy storage capacitor, as I said before, it has to be big enough to hold up the input voltage to the linear regulator while the diode bridge is in the non-conducting portion of the AC input waveform. The input voltage to the linear regulator has to be >= the output voltage plus the dropout voltage Vd. This is discussed in most application notes about applying linear regulator ICs.

    I also Googled basic linear power supply design tutorial, and looked through a few of the hits. This one:

    http://technology.niagarac.on.ca/courses/etec1120/Files/Unit12.pdf [Broken]

    is pretty good, as it explains how the diode bridge and the energy storage capacitor (they call it a filter capacitor) work together to provide the input voltage to the linear regulator circuit.
    Last edited by a moderator: May 5, 2017
  14. Jul 29, 2011 #13
    Thanks that was a really good post , i noticed you are active here when its night for me
    so its pretty late here i will read the article tomorrow therefore
    but I will have to finish before you log in tomorrow so this will be my final post regarding this cause it will be too late by this time tomorrow for more search

    again thanks alot if you have any other notes or hints you can post them and i will read them tomorrow when i wake up,
    Last edited by a moderator: Jul 29, 2011
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