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

[Tim Shoppa]

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