Understanding ceramic caps values

I am currently building an SMR voice card for Ambika based on the starter kit.

According to the BOM, I need:
C26, C27 Ceramic 22p
C4, C10 Ceramic 100p
C14, C22 Ceramic 220n (100n replacement is acceptable)

In my bag of parts I found three pairs of ceramic caps marked as 101, 180, 224 and now I am wondering which is which?

Fra@nk, although someone can probably just tell you the answer (and I can’t), I would suggest looking at the build instructions for other products like the Anushri. I had built an Anushri prior to the Ambika and found some of the information I learned in through those build instructions very helpful with the Ambika. I recall that the Anushri instructions more clearly identified which caps are which. That should give you your answers.

ÜberUsefull™ Link

Yep, already did this but did not come to a clear solution. I build a couple of Shrutis but have not ran into the 180s and 224s yet. So I decided to ask before proceeding. :slight_smile:

Thanks for the link Frank … I was acutally using this one
So the confusing ones are C26 and C27: the pcb says 20p, the BOM says 22p and the kit included 18p (marked 180).
This leads me to the question: can I replace C26 and C27 with 18p?

I guess I can. :slight_smile:

These two capacitors, together with the quartz, provide the clock pulses which ‘drive’ the ATMega. The quartz is designed to work together with caps of a certain value, so depending on which quartz you use (or goes into the kit) you need to use the corresponding value of caps. This is why the value printed on the pcb can differ from what you find in the kit: when the pcb was designed it was planned to use a different quartz. If you build from a kit, you can be relatively sure that it contains caps that will work with the included quartz. If you source yourself, you have to look at the specs of the quartz you are buying to know what caps you need.

A look into the Schematics shows C26 and C27 are the Caps for the Quartz Oscillators RC Circuit. So the correct value doesn’t depend on whats written on the PCBs, its rather in the Specs for the Quartz. Different Quartzed need different Values for the Caps (something between 16p and 32p is common), and you cant always souce the same Quartzs. Be ashured, if 18p is included with the Kit Olivier does this on purpose to match the Quartz.



You know what: I learned something today

Thanks for your feedback!

@fcd72: There are collaboration tools to avoid that. Live preview and so on. Or just a popup saying ‘warning, someone else is typing something witty right now’…

Oh well, at least we both told the same story…

Note that using 22pF with a quartz rated for 18pF, or the opposite usually works perfectly fine :slight_smile: