[time-nuts] GC-1000 Clock Cap Choice?

[Dr. Frank Stellmach]

Hi Brooke,

Our electronics designers (@automotive electronics supplier) have to 
take lot of care concerning voltage stabilizers, that the cap provides 
sufficiently low ESR, which electrolytics deliver only at much higher 
capacitance value; and a series resistor is introduced to limit loading 
currents of the Tantalum. Otherwise, the Tantalums might fail, either 
open, or they might burn, which already caused total damage of vehicles 
(Mercedes).

I assume, latter aspect has been overseen in your device. This is the 
main fault modus of Tantalums.

Electrolytics dry out over the years, especially when stored 
non-operating, Tantalums do not at all.

You have to chose higher cap value and higher environmental temperature 
grade (>= 105°C) for the ElCo, to get a reliable replacement.
I commonly do not recommend polymer elcos, they have some further 
disadvantages, one being the price, the other (perhaps) being CN 
compounds in the polymer, which may be poisonous in case of fire.

High cap value Ceramics are available since years, therefore , if price 
is no argument, I'd prefer to take them for values up to ~ 100µF; X5R 
ceramics should be good enough. With some luck, they (SMD type) fit to 
the existing pads on the solder side.

Otherwise, you have to analyse the (misdesigned) circuitry for possible 
high pulse currents, and replace the bad tantalums by new ones in series 
with a limiting resistor. Several tantalums in parallel might also 
improve withstanding to current pulses. Check the  tantalums datasheets. 
epcos has got some detailed application hints, as far as I remember.

regards Frank