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xinz
Joined: 01 Jan 2008
Posts: 9
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 03 Sep 2008 17:27 newbie need help on capacitor |
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I'm newbie to electronic please help:
my output circuit requiered 5V 1Amp.
I'm using IC 7805 as volt regulator, followed by buffer capacitor, how much capacitance do I need to keep my output current & voltage stable?
is there any formula to calculate this?
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jwfan
Joined: 13 Nov 2005
Posts: 68
Helped: 10
Location: Earth
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| 03 Sep 2008 18:02 newbie need help on capacitor |
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| How about checking Datasheet of the 7805 you are using? Typically, there is an application circuit and recommendation of output cap
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xinz
Joined: 01 Jan 2008
Posts: 9
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| 04 Sep 2008 16:24 Re: newbie need help on capacitor |
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the reference circuit from datasheet for output cap is only 0.1uf.
But I see somebody schematic put larger cap ( 100uf - 1000uf), why?
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rvionics
Joined: 22 May 2005
Posts: 41
Helped: 4
Location: India
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| 04 Sep 2008 17:22 Re: newbie need help on capacitor |
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| For stable operation of regulator IC (Feed back loop stability) use 22uF~47uF capacitor.
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xinz
Joined: 01 Jan 2008
Posts: 9
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| 06 Sep 2008 16:37 Re: newbie need help on capacitor |
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thanks rvionics.
about your advice for capacitance, how could you know for sure it will be suffient?
is there any formula to calculate that? or it's based on your experience?
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tohbas
Joined: 31 Mar 2005
Posts: 24
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| 29 Sep 2008 3:45 Re: newbie need help on capacitor |
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i saw in the switching power supply . they always put big L and c to supply the current during curve down..
and make it constance.
bigger L with bigger current.. during peak output..
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Sadegh.j
Joined: 29 Nov 2006
Posts: 694
Helped: 32
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| 01 Oct 2008 4:53 Re: newbie need help on capacitor |
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| I also think you can find the information you are looking for in the data sheet of the IC
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GrandAlf
Joined: 09 Mar 2002
Posts: 285
Helped: 13
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| 01 Oct 2008 7:21 newbie need help on capacitor |
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| You need the largest capacitance to be on the input side of the regulator i.e the PSU feeding the regulator. Value depends on load etc, but a good starting point would be around 4700uf at a suitable voltage rating greater than the input PSU voltage.
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kspalla
Joined: 19 Aug 2008
Posts: 141
Helped: 23
Location: Bangalore
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| 03 Oct 2008 13:15 Re: newbie need help on capacitor |
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In general higher the capacitance higher the smooth filtering.
But there should be limit on this higher value right?
If the manufacturer says 0.1uF then this capacitor it self is sufficient to fulfill the needs as per specification.
Any thing you need more means further re-design.
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GrandAlf
Joined: 09 Mar 2002
Posts: 285
Helped: 13
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| 03 Oct 2008 16:24 newbie need help on capacitor |
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Typical for a 7805 would be 0.1 ceramic from centre ground pin to input pin as close as possible, same from centre pin to output pin, again as close as possible. Input capacitor in parallel with the 0.1 would be typically 2200 or 4700uf. On the output side across the 0.1, value would be in the order of around 220uf. The electrolytics do not need to be very close, but the 0.1s do, as they are there to stop high frequency instability.
Added after 1 minutes:
Just another point, if you want to draw the full 1 amp, you need a decent heatsink as well.
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