[R-390] OT: Wet or Solid Tantalum?

[Barry]

> Barry N4BUQ writes:
> > It's for the output on an LM317 power supply.  As I
> understand it, the tantalum's properties are called for in this > application (low ESR, frequency response, etc.) and
> a "solid" tantalum is specified in the schematic.
> >Believe it or not, a 47uF/50V solid tantalum runs
> over $20 with Mouser and I assume the same for other e-tailers.
> It's just a LM317. It does not need a fancy-pants $20 tantalum (wet or
> dry). I don't care what the schematic says or what the application is.
> Run-of-the-mill electrolytics these days are surprisingly low ESR, compared
> to what was on the shelves in the 60's and 70's. And low-ESR-spec
> electrolytics (small price premium) are even better. The reason for the
> current emphasis on low ESR is the vast use of switching supplies
> everywhere, they were not so common back in the 60's and 70's.
> Almost all LM317 applications need nothing more than a ceramic disc (or
> monolithic ceramic, very very common today) at the output. If there are
> very large current draw spikes (think 70's era TTL) then a modern low ESR
> electrolytic or a much smaller (e.g. 1uF) tantalum in addition to the
> ceramic cap will be fine. Modern TTL series do not draw huge current
> spikes. Analog circuits will almost never draw big current spikes, if they
> do then honestly there's a more fundamental design problem.
> Tim N3QE

In the National spec sheet for the LM117/317, some of the designs - particularly the ones for high-current applications - 
show 47uF and even 100uF tantalums (wonder where you find one of those?) across the output.  My application will not 
usually require a high current drain (< 1 amp in general) and should not pull heavy current spikes either.

I guess I don't understand the purpose of this cap.  Is it to supply momentary heavy current draws that the regulator (or 
other supply components) would have trouble delivering?  If not, then what is it for?

Thanks,
Barry - N4BUQ