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.

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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.