Since this is not coming from a hardware guru. With reference to .brb and .sch files for power shield and asset tracker, a few of the capacitors are mentioned as “10uF 50 V”, that’s easy to order since voltage ratings are mentioned. For the rest, anything close to 20 Volts should be fine ? Or as less as 10 Volts would also work ? At times capacitors of certain voltage ratings are not readily available from local vendor, just asking to make sure if precise " Voltage" ratings is going to be a deal breaker.
Secondly, tolerance. Does that have to of a specific value or there is a bare minimum we should look at ?
For caps you should take 3-5 times the expected voltage for their purpose to cope with potential spikes.
Since I don’t know which caps your are actually looking at, this is only a rule of thumb, but when they are located anywhere “behinde” the 5V regulator, you should be fine with 15V-25V rated caps.
If you can be sure that there won’t be “dangerous” spikes on that cap, you can go for 2x 5V too.
As said, this is only a “rule of thumb” statement, so @mohit will be the authority with exact statements in this particular case.
Thanks guys @ScruffR and @kennethlimcp … i did asked a generic question. I was looking for something like a “thumb rule” as you shared, just to estimate. l’ll wait on @mohit … and then go ahead with the ordering …
Capacitor voltage ratings really depend on what part of the schematic it is used in. A capacitor on the power line may need to be rated at 2-3x of the max input voltage, while a capacitor on a regulated 3.3V can get away with 1.5x the rating.
As you pointed out, you can’t always find capacitors with exact voltage ratings you want, in which case its safer to go on the higher side. Buy what is readily and cheaply available to you. For example, 10V rating on a 5V line works great but it might be more expensive than, say 20V rated cap, in which case the later would work just fine. Another point to note is that higher the voltage rating, fatter the capacitor - so allow enough room in your design.
Tolerance: As far as tolerance goes, the caps used on power shield and asset tracker are rated at 10%. Unless your design involves use of tuned circuits (filters,oscillators,etc), you can get away with lower tolerances.
As a former component engineer and failure analyst for electronic circuits/assemblies, I’d like to comment on this topic. What’s being discussed here is known as voltage derating, and for aluminum electrolytic capacitors (the most common power supply bulk filtering capacitors) derating of more than 20% of maximum continuous voltage is rarely necessary unless the capacitor is likely to see frequent surges in excess of its surge rating. Even in high temperature applications it’s hardly ever necessary to derate more than 50%. So, for example, a capacitor on a 12V input (that might see occasional excursions to 15V) would require 15V/0.8 = 18.75V (a 20V part would be appropriate) for 20% derating, and at 50% derating a 35V rating is more than enough (15V/0.5 = 30V).
Selecting a part with significantly higher voltage is acceptable if larger size, higher ESR, and higher dissipation factor (with its accompanying higher leakage current) are also acceptable. The aluminum electrolytic capacitor’s dielectric is a very thin aluminum oxide layer that is maintained by the applied voltage, so after a short period of operation a 100V part operated at 20V will have essentially the same dielectric thickness (and strength) of a 25V part operating at the same 20V. There’s generally no need to select a part rated for more than twice the highest anticipated long-term DC voltage.
If it is necessary to guard against momentary high voltages spikes/surges, zener diodes and TVS (transient voltage suppressors) are much better choices.
