gdjacobs wrote on 2020-05-11, 05:49:
The performance characteristics are too different to make a general recommendation like that. In particular, series inductance and leakage characteristics may require wide band ceramic caps or multiple capacitors (i.e. ceramic and poly) for equivalent performance. Of course, some applications didn't need the performance of a tant in the first place in which case a wet lytic or poly cap may be fine.
While puzzling over capacitors used on the ISA slots of a 384/386 motherboard from around 1992 (in this case what I'm pretty sure are black plastic packaged, SMD tantalum capacitors, though aluminum poly caps come in similar packages).
Those are 10 uF capacitors (marked 106) and I think are used on both the 12 and -12V rails (I just had to remove one that failed short on the -12V rail). And there's no additional ceramic capacitors to complement them.
A couple other boards with through-hole tantalum caps and no complementary ceramics also appear to be 10 uF, but the Hedaka and PCChips 286 boards (D60 chipset, M205 and Hed 988) both use 47 uF wet electrolytics along with 104 marked ceramic caps (10nF) smattering of other value ceramics on the Hedaka board. (at least I think the 10nF ones are ceramic, they're little sky-blue suqare shaped things)
Actually, those 10nF blue ones are probably multilayer ceramics.
So it seems like significantly higher capacitance values are used for electrolytic capacitors in the same position as tantalum capacitors for power supply decoupling purposes. (maybe in part to account for drying out with age and heat, but probably also the need for wider tolerances in other areas, like ripple current tolerance, charge and discharge rate, etc)
Plus higher value electrolytic capacitors are fairly cheap and compact, so if the specific purpose works fine with extra capacitance values that's fine, too. (a higher margin for overvoltage might be wise)
I tend to see a lot of electrolytics that cut it close to the minimum voltage needed for a given circuit, like a lot of 16V ones for 12V lines and sometimes ones that actually undercut potential supply voltage ranges: there's some 10V power decoupling caps in the Sega Genesis console's I've opened in power supply filtering/decoupling circuits (prior to the pair of 7805s) that can sometimes be above 12V, I've seen 14 in some cases when open circuit, though that usually drops to around 9 or 10 volts with the system switched on. There's also some diodes in that portion of circuitry, which could account for some voltage drop but it still seems to cut it closer than typically advised.
OTOH the cap that failed on my board looks to be a 25V rated one, though there's also some 16V ones in similar positions on alongside other ISA slots.