help please

Discussion in 'General Electronics Chat' started by vipper, May 21, 2014.

  1. vipper

    Thread Starter New Member

    May 21, 2014
    1
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    i have a circuit diagram in front of me. on this diagrm is a l7808 8v regulater, on each side of this is a non polarized capasiter 10uf but it does"nt show the volts. what voltage should be use for each? the circuit is powered by 12v and i do know that u are supposed to times it by 1.3 and round off, thats all in good for the front of regulator but what about the output side. if anyone would have some input on this, i would be greatfull
     
  2. #12

    Expert

    Nov 30, 2010
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    Theoretically, you can use just as much voltage rating as you need, a 10V cap on an 8V output.

    However, theoretically, the regulator never goes bad. I would use a 16 volt capacitor in both positions because I have read the educational stuff that says increasing the voltage rating UP TO twice the applied voltage gives the best life time for the capacitor.

    This only applies to aluminum electrolytic capacitors. Tantalums would be safe at 15V or 16V. Ceramic capacitors often have huge over voltage ratings with no ill effects, so a 50 volt ceramic capacitor would be right at home in a 12V circuit.
     
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  3. prashanth58

    New Member

    Jan 30, 2010
    8
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    U hv nt mentioned4what purpose u intend2use it4.If it is4a basic circuit then rule of thumb suggest that u use filter capacitors of about 470-2,2200uF(lesser value4circuits consuming a few milliamps&larger4circuits4powering>500-1Amp or so. In either case It isn,t strictly very necessary 2use a non polar cap.As4the voltages pwr reg,s req Minof>3voltsthan the input, in ur case 8v means 12v-25 volt caps will be ideal. Remember2ground the Negative to the -or ground(center pin of the reg)The opt cap can be min10v or more. in all cases remember to feed Bridge or at least a full wave rectified voltage between 12-18vMax.Avoid feeding from a half wave rectifier. this wont count if u r using 12v dc.You may do away with the 10uF cap in both cases, or put it close2the reg in paralell with the abovementioned capacitors if its really makes u happy. All the best...Bye:)-)

    Moderator Note:

    Just so as you know #12 has the right of it. Text speak is strongly discouraged here, we don't expect perfect English, but we do expect the effort. English can be hard enough without adding to the obscurity.
     
    Last edited by a moderator: May 22, 2014
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  4. #12

    Expert

    Nov 30, 2010
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    prashanth58

    I disagree. The intended use of this chip is as a regulator. The datasheet will show small capacitors intended to avoid oscillation. vipper is not asking how to design the 12 volt supply that is getting regulated so he is not asking about filter capacitors in the hundreds or thousands of uf.

    I would also like to mention that text-speak is not approved for this website. Please use real words and real punctuation.
     
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  5. wayneh

    Expert

    Sep 9, 2010
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    Touché for reading it carefully enough to tell one way or the other. I just didn't have the energy; if they can't take the time to write, why should I take the time to read?
     
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  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I prefer to use a 25v rated electro on a "12v" input, instead of a 16v rated cap.

    The 12v input might be a 12v battery application which can be 14.5v on charging and possibly have spikes etc above that.
    :)
     
  7. #12

    Expert

    Nov 30, 2010
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    If we want to speculate, the 12 volt system might be a car, and they can have 60 volt spikes. Then the capacitors would have to be rated for more than 60 volts and the chip would need protection to keep it from blowing up.

    I tend to think this is a project with a transformer circuit rectified to make 12V DC or an alkaline battery until the O.P. says it isn't ordinary.
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    What if the "12v" is coming from a plugpack? I have plugpacks here labelled "12v DC" that actually put put about 17 or 18v with no load. ;)
     
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