78L09 setup

[vince]

Hey guys,

I am building a power supply based on a 78L09 regulator. Most schematics using it have cap. values of over 100 uF.
But the typical application page in the datasheet is as follows:



Is there any explanation for choosing higher cap values than specified?  The time constant is definitely higher, which means less ripple. BUT.. I am thinking 0.33 uF and 0.1 uF must already be more than enough.

Thank you
Vincent

[Paul Perry (Frostwave)]

C1 value depends on where the unregulated DC is coming from.
If it is coming from a wallwart with a decent sized (say 500uF) cap inside it, then a small one near the reg is OK.
But if you are running AC into the box & rectifying it, then 500 or 1,000uF might be needed, depending on current load.

[Crevil]

A good rule of thumb is using a cap at the approximated amount of current you are going to neat.

If you are going to use 800mA then a cap on 800uF will normally do, then if you are going to make it more silent, put a 100nF after the 78LXX, if you can get some Sibatit caps that will be perfect, they are really good at killing noise, but they are also often hard to get.

This is just a rule of thumb, not something specific, and used most when speaking of supplyes using AC.

[Sir H C]

The output one is too small in my opinion.  That capacitor is to handle spikes which are too fast for the regulator, so you usually would want at least 1uF there, not too big here though.

And what does Note 2 say?

[vince]

And what does Note 2 say?

NOTE 2. Bypass Capacitors are recommend for optimum stability and transient response and should be located as close as possible
to the regulator

We already knew that ! 

The transformers are rated 60 mA I believe, so there won't be a lot of current going through the regulator.
I guess a 500 uF capacitor for the input would be enough, right?
But I'm not exactly sure of the purpose of the output capacitor. Should I use a very low value, like 100 nF as suggested and also a bigger one, like 10 uF ?

Thanks
Vincent
P.S. Sorry if this topic was brought back from the dead, but I was working on another project

[Steben]

Wohow! 800 and 500? Ar we talking "micro"farads? That are big whoppers!

[vince]

Wohow! 800 and 500? Ar we talking "micro"farads? That are big whoppers!

Yeah, we're talking micro Farads. Good power supplies generally have capacitors from 500 to 1000 uF after their diode bridge, to reduce the ripple to minimum. When I was in university, we once built a precision power supply, and we used 10 000 uF capacitors! Those were huge!

But in this case, they only have to be rated 16V, so they won't be very big.