Hi there

I am doing a bit of repair work building a new power supply for an apple airport express as the old one exploded... i'm using the following layout

components
fairchild ka278r05 - 5V 2A low dropout voltage regulator
http://www.fairchildsemi.com/ds/KA%2FKA278R05C.pdf

tsc ts1086 - 3.3V 1.5A low dropout voltage regulator
http://www.maplin.co.uk/Media/PDFs/Module%2046058.pdf

my plan is to have a standard plug dc psu supply power to the ka278r05 with a 47µF cap on its output to ground (as per the datasheet) have this go to the 5v on the airport express and also act as the input to the ts1086. then have a 10µF cap on the output of the ts1086 (as per datasheet). the ts1086 will provide the 3.3v line needed by the airport express

i only have basic knowledge of electronics so the things i am unsure of...

are the caps values correct ...because i have that cap in the middle of the 2 regulators should its value differ at all?

should i have a cap on the input to the ka278r05?

in the ts1086 datasheet it says i should use a tantalum cap for the 10µF cap
.... do i need to really - i dont have any lying about i have a 10µF 400V rated alu electrolytic handy from the blown psu

reading up on tantalum caps got me all confused with the 2 arguing groups about whether they are good.

if i should go tantalum as suggested by datasheet...does that mean the 47µF should also be a tantalum?

thanks for taking the time to read this over guidance is much appreciated

 

Tantalums are much more stable than electrolytics, but also more expensive. I would go with the electrolytic caps. These should work fine. Is the psu you are using regulated? If so, you don't need a cap on the input. But if not, or if the psu unit is some distance away, use the cap. It won't hurt anything to use a cap in either instance. If you are using a wall adapter, I would definitely use a cap on the input. Don't forget to put a small value cap in parallel to help filter out the RF (a .1uF mica cap will work fine). Use these where ever you put the electrolytic caps. So, my recommendation would be to go with 47.1uF on input of 5V reg, 10.1uF on output of 5V reg and 10.1uF on output of 3V reg.

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thankyou very much for the reply, it's just the answer i needed.

with regards to the mica capacitors - i take it optional but highly recommended... in my parts salvage box i have a psu from a computer it has capacitors that are yellow boxes labelled "100nJ63"

see picture for a blurry view (yellow thing in centre and bottom left)
http://www.flickr.com/photos/tim_programmer/2765651282/

would this be the kind of cap i am after?

EDIT: is this cap type polarised? if so what's its identifying attribute

 

Those are polyfilm caps 100nf 63V. They will work.

 

Oblong is right. Those should work fine.

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"Remember, you're special.....just like everyone else."

 

Great!

I really appreciate that... thanks for taking the time to help!

Tim