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In my circuit , I need decoupling capacitors , but my local shop doesn't have some values like 300nf so I need to put in parallel 3 of 100nf . the equivalent CAP would be 300nf but with multiple paths to GND (each is 100nf) . So dose the equivalent CAP would work or it must be 300nf as one piece ?

NOTE : this decoupling capacitors are needed for Vi of 7805 REG .

Also I need a 15pf decoupling noise capacitors for DP,DM for USB but I only have (10pf , 5pf) and 12pf (dose 12pf work as a proximately value ? or this value is critical ?).

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In general, 3 100 nF capacitors could be a bit better than a single 300 nF capacitor.

Basically one of the major unwanted properties of capacitors is equivalent series resistance of the capacitor. When you have resistances in parallel, the equivalent resistance is decreased and same is true for ESR.

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The 300nF cap you believe you need is in fact 330nF (standard part) and this is probably a minimum recommendation so choose 4 x 100nF. Despite this, Fairchild (in their spec) indicate that Ci is 100nF (on the 7805 but not other 78xx devices) and National semiconductor specify 220nF for the 7805.

For the 15pF I'd use what is asked for. I'd like to say try the 12pF but you might get the odd data corruption now and then and never really be sure whether it was the 12pF fitted.

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protected by clabacchio May 23 '13 at 14:42

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