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Capacitor specs for use with 7805 regulator

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loosewire

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I will be using some 120 VAC -> 9 VDC wall adapters for powering some simple classroom experimenter boards. The adapters have a rated output of 600mA. (Jameco part# 100175) I plan to clean and regulate this down to 5 VDC with a 7805 regulator.

The 7805 datasheet shows an example circuit with a .22uF cap for the input side and a .1uF cap for the output side. I have seen other circuits using 1000uF and 10uF for the input and output though. Why is there such a wide variance? And what are the pros & cons of using the different sizes? (These would be electrolytic and rated at 16v in all cases.)

Also, there is a similar wall adapter that provides 6VDC. But it is my impression that I should regulate these cheap adapters AND that the 7805 wouldn't work well with such a small difference between the input and output voltages. Are these impressions correct?

Thanks.
 
Around 50-100uF on the output you should be OK. The wall adaptor will already have a fairly large capacitor on the rectifier so 10-100uF on the input will do.
 
loosewire said:
I will be using some 120 VAC -> 9 VDC wall adapters for powering some simple classroom experimenter boards. The adapters have a rated output of 600mA. (Jameco part# 100175) I plan to clean and regulate this down to 5 VDC with a 7805 regulator.

The 7805 datasheet shows an example circuit with a .22uF cap for the input side and a .1uF cap for the output side. I have seen other circuits using 1000uF and 10uF for the input and output though. Why is there such a wide variance? And what are the pros & cons of using the different sizes? (These would be electrolytic and rated at 16v in all cases.)

Also, there is a similar wall adapter that provides 6VDC. But it is my impression that I should regulate these cheap adapters AND that the 7805 wouldn't work well with such a small difference between the input and output voltages. Are these impressions correct?

Thanks.
The .22uF and .1uF caps are for decoupling the input and output rails at RF. Leaving them out or just using electrolytics can give rise to problems. I once fitted an .022 cap in error and wondered why the output voltage rose to over 6v. I 'scoped it and found RF ripple at the output - the chip had gone bananas. If space is at a premium, then tantalum capacitors work will very well there because of their low ESR.

A bog standard 7805 needs an overhead of about 2v if memory serves me right, so you should have a main reservoir cap big enough so that during the negative peaks of the ripple, the input stays above about 7v. You should not need an electolytic at the output.
 
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