Another question form my Macintosh IIci schematics

[geokov]

Really basic question. What would the voltage be for these capacitors with the red arrows? For example, why don't the 0.01 uF capacitors show voltage when the other capacitors to the left show both capacitance and voltage? Thanks.

 

[David Cook]

Those capacitors are connected to the +12V line. The working voltage (WV) aka rated voltage that is printed on the capacitors or specified for them in the datasheet must be higher than 12V. Depending on the chemistry of the capacitor, the rated voltage should be noticeably higher. For example, at least 16V for electrolytic and at least 25V for tantalum. Ceramic is a bit weirder, in that it could be rated 16, but it will then be below the stated capacity because it declines as the voltage approaches the rating.

To answer your second question, for such low capacity usually the rated voltage is quite high (50V or better) because it easily fits in the package. It is easy to manufacture a 0.001 capacitor in standard sizes, so the manufacturer doesn't have to squeeze the voltage rating down to fit that capacity. If you find a replacement of the same size and capacity, it would be really surprising if it didn't have a rated voltage well in excess of 12V. So, they didn't bother listing the rating.

 

[geokov]

Those capacitors are connected to the +12V line. The working voltage (WV) aka rated voltage that is printed on the capacitors or specified for them in the datasheet must be higher than 12V. Depending on the chemistry of the capacitor, the rated voltage should be noticeably higher. For example, at least 16V for electrolytic and at least 25V for tantalum. Ceramic is a bit weirder, in that it could be rated 16, but it will then be below the stated capacity because it declines as the voltage approaches the rating.

To answer your second question, for such low capacity usually the rated voltage is quite high (50V or better) because it easily fits in the package. It is easy to manufacture a 0.001 capacitor in standard sizes, so the manufacturer doesn't have to squeeze the voltage rating down to fit that capacity. If you find a replacement of the same size and capacity, it would be really surprising if it didn't have a rated voltage well in excess of 12V. So, they didn't bother listing the rating.

Thank you David for taking the time to reply with such a helpful and educational reply. I am working on an updated set of IIci schematics for my personal use. I am repairing a IIci for my husband so am recreating the Bomarc schematics in KiCad as a way of learning about the IIci before tackling the actual motherboard.

 

[bigmessowires]

From what I've seen (not just in Apple's schematics) the voltage rating of a capacitor is often omitted when the capacitance is roughly 0.1 uF or less. I've always interpreted this as a "don't worry about it" message from the designer. But if you want to be extra sure, do as David suggests and examine the schematic to see what voltage will be present on that capacitor and make sure the specific capacitor you buy has a rating that's well in excess of that.

I just did a quick scan of 0.001 uF ceramic capacitor options at DigiKey, and virtually all of them have a voltage rating of 50-100V or more, except the very first search result is only 16V: https://www.digikey.com/en/products/detail/murata-electronics/GRM033R71C102KA01D/2543501 But that's in a ridiculously tiny 0201 package that you will accidentally inhale while attempting component placement.

 

[geokov]

From what I've seen (not just in Apple's schematics) the voltage rating of a capacitor is often omitted when the capacitance is roughly 0.1 uF or less. I've always interpreted this as a "don't worry about it" message from the designer. But if you want to be extra sure, do as David suggests and examine the schematic to see what voltage will be present on that capacitor and make sure the specific capacitor you buy has a rating that's well in excess of that.

I just did a quick scan of 0.001 uF ceramic capacitor options at DigiKey, and virtually all of them have a voltage rating of 50-100V or more, except the very first search result is only 16V: https://www.digikey.com/en/products/detail/murata-electronics/GRM033R71C102KA01D/2543501 But that's in a ridiculously tiny 0201 package that you will accidentally inhale while attempting component placement.

Thank you for helping out. Those capacitors won't get more than 12V so clearly any of the appropriate uF ceramic capacitors will be just fine. I don't need to replace them, I am just trying to make an accurate schematic for myself and learn all about the IIci.