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Homework Help: DC Power Supply

  1. Jun 22, 2007 #1

    I'm in dire need for help in my assignment. I have been asked , a), to research on a dual output DC power supply (Output = +15v and -15v) of not more than 15 components and to identify the different sub-sections that make up the power supply on the schematics. And, b), give an explanation on the operation of the power supply.

    I have not reached the topic on DC Power Supply yet so any information on this can help me very much.

    Thank you in advance,

    -Joaquin 'elOq' Ravee

    [ EDIT : I have, unfortunately, posted this in the wrong forum :( as my defense, this is my 1st post :) ]
    Last edited: Jun 22, 2007
  2. jcsd
  3. Jun 22, 2007 #2


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    Hi eloq and welcome to PF,

    Well the different sections (assuming a AC-to-DC power supply) are:
    4)Voltage regulator

    The first three are pretty simply, however 4) will be a little complicated since you need dual polarity. For a task such as this, I would choose two voltage regulators (example LM317 and 337). These voltage regulators (3 terminal) will provide a variable output. I believe that the 7815 would provide a symmetrical split at the output.

    This is actually very easy to build. But before we discuss the voltage regulator section in more detail. I want to know what is your understanding on the first three sections of the power supply.
    Last edited: Jun 22, 2007
  4. Jun 22, 2007 #3

    I know that you need a transformer to step down the mains voltage (to 15v) to the rectifier circuit.

    The rectification is performed with four diodes (1N4001?) and i am not too sure what happens next. (I guess the output is a pulsating DC)

    I am not too sure on the filtering but from what i understand it smooths the pulsating DC to a pure output voltage.

    -Thank you ranger :) i hope to learn a lot from here.
  5. Jun 22, 2007 #4


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    Well the transformer section is pretty straight forward.

    The second section is the rectifier, and as you've mentioned correctly - the output is a pulsating DC [any general purpose diode will do; or you can use a rectifier module that has the diodes packed into one package]. This configuration of diodes that you have in mind is called bridge rectifier. May I ask why you suggested a bridge rectifier instead of a full wave rectifier (2 diodes) or even a half wave rectifier (1 diode)?

    Moving on, the filter section is made of capacitors and inductors. A very simple filter consisting of a capacitor should suffice for this project. Using your knowledge of basic RC circuits, how does a capacitor smooth out of the rectified voltage? It may help if you draw the output waveform from the rectifier.
    Last edited: Jun 22, 2007
  6. Jun 27, 2007 #5
    hi! i seem to be doing this project too but do any of you have a schematic? its much easier to understand that way..

    *thanks in advance*
  7. Jun 27, 2007 #6


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    Hi kadzuki2,

    Yes, I do have a bunch of concepts that I can draw up. But me doing that will not help you in any way. In a sense, I will doing your assignment for you. Here at PF, we generally frown on such things.

    However, if you're willing to put much effort on your side, I [and others] will be most happy to assist/guide you through the thought process or whatever little help you will need along the way.
  8. Jun 27, 2007 #7
    erm ok.. here's the list that i have researched and found out so far... but i kinda have 0% knowledge.

    I found this quote "It acts as a reservoir, supplying current to the output when the varying DC voltage from the rectifier is falling.".. i pretty still much don't understand it.. but i know its a part of the capacitor smoothing process.

    The last part is the Voltage regulator which provides an extra smoothing of the output waveform. This Voltage regulator is connected across the DC output of the Rectifier.

    Am i correct on the Voltage regulator? I pretty much still don't understand about the part on the Regulator but i know that i'm to use LM7815 and LM7915 because of the +/- 15V
  9. Jun 27, 2007 #8


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    The part about the filter or reservoir as they call is true. In this case they are considering a simple capacitor filter. The capacitor smooths out the rectified waveform from the rectifier. The capacitor is charging to the rectified voltage; however, becuase of the nature of the rectified voltage (half a sine wave), one portion is cut-off. So because of this, just think of the half thats cut-off as a period where nothing is supplied to the capacitor. So now the capacitor "acts like a filter". Because nothing is supplied to the cap in that period, it discharges through the load; it could be a simple resistor. This repeated many times over the interval of a few seconds produces a final output waveform that has ripple voltage in it.

    The voltage regulator serves to smooth out this ripple and the output is a nice looking DC line. And example of a simple voltage regulator is zener diode. Are you familiar with how a zener diode works? But the voltage regulator connects across the filter.

    I do believe that its feasible to have a dual polarity [fixed] output using a pair of zeners. You just have to take the output for the right sections of the bridge rectifier/filter. This I have to think about a little. But if you're comfortable with the LMs, that should be no problem.
    Last edited: Jun 27, 2007
  10. Jun 27, 2007 #9
    Zener diode... ermm i did a search on wiki, so i did understand a little so its basically it works like a voltage stabilizer for low-current application right? ermm this is the schematic i drew... might be REALLY off... so learning from my mistakes =). *do correct me*

    - http://img292.imageshack.us/my.php?image=imagecm3.jpg *My Schematic*
  11. Jun 27, 2007 #10


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    Well the rectifier looks OK. The filter looks fine also. However, until you tell the values of capacitors, I cant be too sure. Also I didnt take a look at the datasheets for the LMs, so I guess mayb they have special filter requirements? I suspect that you're only using these capacitors as high pass filters, but I'm only guessing becuase of what I stated above.

    Well assuming that you've read the datasheet properly, and configured the LMs as they should be; the regulator looks pretty robust; especially with with the added caps on the outputs. You added the last four caps because.....you need a filter just in case?

    But conceptually your design should work.
    Last edited: Jun 27, 2007
  12. Jun 27, 2007 #11
    Thank you Mr. Ranger! Greatly appreciated =) I guess it should be o.k. So i can remove the 4 caps at the back right? The values of the caps are 1uF 35V Electrolytic Capacitor, is it o.k?

    Thank You lots Mr. Ranger! =)
  13. Jun 27, 2007 #12


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    Well I can't tell you yes or no. I need to know what was your intention in putting them there and I also need to know if the LMs need them to function properly. If you're using them as a filter, it'd be a good idea to leave it.
  14. Jun 27, 2007 #13
    Well actually we just need to design the DC Power Supply that has an output of +/- 15V. Thank you very much Mr. Ranger =)
  15. Jun 28, 2007 #14
    what is filter
  16. Jun 15, 2008 #15
    Hi, i am doing the project as well, but i don't really understand your explanation of how the capacitors act as a filter. Would you mind to elaborate?
    Thank you
  17. Jun 23, 2008 #16
    hi , i m doing the same question as eloq. do u mind giving me the explanation on the operation of the dual output dc power supply (output=+15v;-15v)
    and thankz!!!
  18. Jun 23, 2008 #17
    Umm a few questions about the sub sections. Is it rectifier or do I write it as diode? Is it filter or do I write it as capacitor?
  19. Dec 2, 2011 #18
    hiiii am doing engg. i want basics concept to design unregulated power supply circuit i know the component i need but when i draw schematics diagram i have no idea. how to calculate ,what formula i need?
    how can i chose rating of component

    load 30ohms o/p 5amp current
    transformer- power rating?
    diode-power rating
    capacitor-power rating

    can i calculate likes transformer+load

    or with transformer+diode+cap=load

    yet no body has define clearly and exact plz guide and explain with suitable example
  20. Dec 2, 2011 #19


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