Re: [N8VEM-S100:5179] Tantalum capacitor replacement?

[David Riley <frave...@gmail.com>]

On Sep 9, 2014, at 3:19 PM, Don Caprio <ilv...@gmail.com> wrote:

> Man you guys are brains :^)
> 
> I'm not in the industry, just a weekend hobbyist at best.
> 
> This topic may have been discussed before so excuse me if it has. Since were on the topic
> of capacitors and such I've wondered why most of the S-100 designs call for Tantalums for
> the filter caps on the power source (LM7805). Is there a good reason to use tant's? Tant's
> perform better and from what I've read have better ESR characteristics. Can the
> radial aluminum electrolytic be used in their place without concerns? Like Tant's they use a 
> liquid (electrolyte) and can lose capacitance as they age. Heat will shorten life as well. 

The aluminum electrolytic compounds are comparatively drier than the tantalum compounds, which makes them less prone to the effects of drying out (obviously, they're not totally immune, and there are other aging effects to keep in mind, mainly chemical).  They also withstand overvoltage somewhat better; my general rule of thumb for tantalums is that you should use ones rated for about twice your working voltage, whereas with aluminum, I'm usually satisfied with whatever the next step up is (say, 6.3v for 5v circuits).

There are better aluminum ones these days; the aluminum polymer caps I mentioned generally have the reliability advantages of aluminum caps (and are substantially more durable, since the electrolyte doesn't dry out) with near-tantalum ESR.  For vintage boards, they're usually not a drop-in replacement, not least because they're usually only surface-mount these days, but they're very popular on modern computer motherboards, where they're typically advertised as "solid state capacitors".

As Vince mentioned, the ESR for the bulk capacitors on an S-100 motherboard isn't as relevant, because each board is regulated.  That can smooth out a lot of the spikes (though not really the dips); bulk capacitance on the motherboard is usually only used to cover up poor board design on the cards, though.  There is some plausible benefit to having local bulk decoupling near the slots, but you really mostly want that done on the board, right near where the power comes in.


- Dave