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eplanet
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« on: February 19, 2012, 02:34:03 PM »

hi friends
please see this attachment.
can you tell me why use c1 and c2 for lm7805 and why in other config we use capacitors with different values.
this pictures use for pic micro controller and LCD power supply.
thanks
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Hero999
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« Reply #1 on: February 19, 2012, 04:10:41 PM »

The value isn't critical. I'd also recommend adding a 100nF ceramic capacitor to the output as it has better high frequency characteristics. The addition of capacitors is known as bypassing or decoupling and it touched on in the LM7805 data sheet.
http://www.ti.com/lit/ds/symlink/lm340-n.pdf

A more detailed explanation can be found on the LM317 data sheet.
http://www.ti.com/lit/ds/symlink/lm117.pdf

Here are a couple of links with more information:
http://www.ti.com/lit/an/snva020a/snva020a.pdf
http://www.ti.com/lit/an/snva558/snva558.pdf
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eplanet
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« Reply #2 on: February 21, 2012, 10:34:24 AM »

what is the different between c1 and c2 will be 100uf instead of 10uf?
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audioguru
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« Reply #3 on: February 21, 2012, 01:00:30 PM »

The datasheet recommends 0.22uF input and 0.1uf output ceramic disc capacitors.
The LM7805 might oscillate with 10uF or 100uF electrolytic capacitors.

Of course if the input was rectified AC then a huge main filter capacitor is needed in addition to the 0.22uF ceramic capacitor.
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Hero999
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« Reply #4 on: February 22, 2012, 03:45:26 AM »

what is the different between c1 and c2 will be 100uf instead of 10uf?
You could have answered that yourself if you had bothered to read the first two links I posted previously.
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eplanet
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« Reply #5 on: February 22, 2012, 06:31:42 PM »

can you tell me more about this statement about these values

"The value isn't critical but if you make it too small you risk it being inadequate to damp oscillation. If you make it too big, at least with a single capacitor, you run the risk of the high frequency impedance of the capacitor being too high.  "

damp oscillation and high frequency impedance  ?

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Hero999
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« Reply #6 on: February 23, 2012, 03:41:36 AM »

All that means is that if the capacitor is too small, it won't be large enough to damp the oscillations but if it's a single, large value capacitor it won't be good either. Higher value capacitors tend to have poor high frequency characteristics and become inductive past the resonant frequency, the higher the value, the lower the resonant frequency.

Rather than using a single, high value capacitor, it's better to use a large capacitor and lower value capacitor in parallel with one another. The type of capacitor is also important; for example tantalum is better than electrolytic. I'd recommend a 1uF tantalum in parallel with a 100nF and 10nF ceramic capacitor on both the input and output. If you don't have a 1uf  tantalum, go with 22uF electrolytic.
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