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

[Tim Shoppa]

Barry N4BUQ writes:
> 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.

Most of the LM317 data sheet dates from the 70's back when
electrolytcs weren't so low ESR.

And back then the dominant logic family was TTL which insanely short-circuits
Vcc to ground whenever there's a logic transition.

47uF and 100uF tantalums aren't so expensive, if you buy them for 6.3V.

Still, tantalums went out of style for TTL bypassing back about 1973 :-).
With a little care in PC board layout they discovered that small ceramic
capacitors (by the late 70's, monolithic ceramic capacitors) right at the
TTL package was way way better and cheaper than big tantalums.

>  My application will not 
> usually require a high current drain (< 1 amp in general) and should
> not pull heavy current spikes either.

1 Amp is a high current drain for a LM317. If the voltage drop is anything
more than a few volts you'll need heatsinking.

> 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?

Conceptually you've got it right, it's a bypass capacitor.

As a small hint, if you're paying 20 times for a bypass capacitor for a device than
the device itself costs, something's out of whack in the design. It's like discovering
the steering wheel for your Ford Escort costs half a million dollars and is solid gold.

Hobbyists, especially audio hobbyists, place way way way too much emphasis
on honkin big bypass capacitors when commercially they discovered much better
filtering methods like 70 years ago.

Design-wise, if you have a design that needs honking big low ESR bypass caps then
the design has high pulse currents. Unless the purpose of the circuit is to have
high pulse currents (think "coin shrinking"), this is an indication that it's a poor design.

Some regulators have stability issues without the right bypassing. LDO's are well
known for breaking out into oscillation if you use too low an ESR cap!

Tim N3QE