why no resistor on 5v regulator

Discussion in 'The Projects Forum' started by Gadersd, May 21, 2013.

  1. Gadersd

    Thread Starter Member

    Dec 8, 2012
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    I just bought a 5v regulator for use with my ics. I looked at some 5v regulator circuits and noticed that they did not use a resister in series with the capaciter at the 5v regulator's input. Wouldn't this cause a short circuit when the capaciter is first charging?
     
    Last edited: May 21, 2013
  2. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    No. A resistor in series will simply limit how fast the capacitor can charge.
     
    Last edited: May 22, 2013
  3. wayneh

    Expert

    Sep 9, 2010
    12,100
    3,034
    Actually yes, but the regulator survives the small amount of power. If you made that cap really, really big, it could cause a problem. But even 1,000µF is no issue, in my opinion. I'd be curious to learn where the line is.
     
  4. tubeguy

    Well-Known Member

    Nov 3, 2012
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    The capacitor acts like a short for an instant when power is first applied, then begins charging. A resistor in series limits the initial current, but is not necessarily needed. With the capacitor it forms a low pass filter and can further smooth the input voltage.
     
  5. Gadersd

    Thread Starter Member

    Dec 8, 2012
    98
    1
    What size resister would you recommend?
     
  6. #12

    Expert

    Nov 30, 2010
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    Based on not knowing exactly which regulator chip you are using and not knowing how large a capacitor you intend to use, and no mention of how much current this resistor has to allow under normal operating conditions, and no idea what voltage you are starting with or what the source of that voltage is, I'd say zero ohms.

    All the 3 terminal regulators I know automatically limit their current so they don't get hurt by large capacitors, so I'll have to guess, "zero".
     
    Last edited: May 21, 2013
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The OP said the capacitor in question is on the input. Some of the answers given seem to be referring to a capacitor on the output.
    Gadersd, are you concerned about the capacitor appearing as a short circuit to the rectifier(s)?
     
  8. wayneh

    Expert

    Sep 9, 2010
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    Guilty! :eek:
     
  9. tindel

    Active Member

    Sep 16, 2012
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    A regulator input resistor also has the added benefit that it helps spread out the heat dissipation - so that the heat dissipation is not just in the regulator. I have used this tactic successfully several times in my designs. Another added benefit is that you don't take any added efficiency hit when you do this. You do have to be mindful that you don't drop below the regulator input drop-out voltage.

    As #12 wise(-arse)ly pointed out - it is difficult to suggest a resistor value if you don't provide details about your circuit. We'd need input voltage, output voltage, input capacitor, load current, and output capacitance to make an input resistor suggestion.
     
  10. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    It always amazes me how far a thread could go with no circuits, when one is actually needed.
     
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