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When placing a passive component such as a cap or resistor on schematic, most people usually place the relevant rating (usually power for resistors, voltage for caps, and current for inductors) next to the symbol in addition to the actual component resistance, capacitance, or inductance value.

However, as the design solidifies and actual physical components are chosen, the rating for the physical part may be different than (and hopefully, exceeding) the design requirement. For example, I may need a 10V cap on a certain line, but there is a cap that is cheaper or perhaps already stocked by my contract manufacturer that is a 16 V cap but otherwise identical. It would make the most sense to spec out the already-stocked part.

In such a case, what value should be shown on the schematic - Vmax = 16V or 10V? On one hand, I feel that schematic should communicate design requirements, and part substitutions should be decided elsewhere. On the other hand, the schematic also serves as a reference for the actual built circuit - so the actual part rating is more useful from that perspective.

An additional complication is that the BOM is usually generated from the schematic, and thus displaying the design requirement but then also spec'ing a higher-rated part (i.e. for stocking reasons as mentioned above) can lead to confusion if someone compares the specified requirements to listed parts.

I've read this question and the answers linked to it and haven't found an answer to the specific question.

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My usual practise is to place actual purchasable values on the parts in the schematic with (where appropriate) a note giving any relevant formula. There are usually less then 5% of the parts where there is enough maths to matter.

This is kind of important if the assembly BOM is to be exported from the CAD tools (And you REALLY want BOM generation to be a fully automatic thing), nothing worse then having to hand hack a BOM (Except possibly hand hacking the XY placement data!).

My libraries are set up so that there are distinct libs for 0402/0603/0805/1206 parts which means the layout guys get the footprints from the schematic database.

In terms of the amount of data to include in a component, for jellybean stuff I usually give values, power/voltage ratings and package details, maybe dielectric, but leave the manufacturer as generic. Then in my BOM template I list acceptable vendors for the generic parts, "Or equal and approved" is a key phrase as it means you will be asked when they want to sub something insane.

Generally, while all this metadata is there, I turn off things like power rating and dielectric in the default view, folks can get them if they need to know, but it is not helpful for 90% of the reasons people look at a schematic ("4k7" is helpful, "4k7,1%,0.063W,0603,50V,Thick film,Vishay Dale" is just noise most of the time).

Like everything else in this game, the right answers here probably vary depending on what you are drawing and who the audience is.

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  • \$\begingroup\$ Dan: Although your answer is enlightening, I don't think it directly answers my question. To be direct: when you select an "actual purchasable value" on the schematic, are you placing the value of the actual part you selected or the minimum purchasable requirement? i.e. for a regulated 5V line, a 16V cap requirement is common. But I have a 50V one in stock that will work. Both values are purchasable. Do you put 16V or 50V on your schematic? \$\endgroup\$ – rothloup Oct 23 '18 at 13:13
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    \$\begingroup\$ I would place the 16V part if that is what I wanted for production, maybe with a BOM note that higher voltage caps were an acceptable substitution. The BOM should include a notes page for things like this, also detailing which 'generic' manufacturers you are prepared to accept without a new prototyping round and the like. Of course this applies mainly to jellybean parts and is mostly about the BOM, it is a whole other game when the component actually really matters, but even there the printed schematic might just say C21 100pF while the BOM says C21 100pF ATC100B NO SUBSTITUTE or the like. \$\endgroup\$ – Dan Mills Oct 23 '18 at 13:57
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For passives you usually only specify requirements and not the actual part number (unlike ICs). So one way is to keep a per-order BOM. That is - every time you order a batch of boards you copy the BOM and add another column with the actual part placed. This should be greatly simplified by using resource management systems.

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    \$\begingroup\$ Where I've worked we always specified the actual part numbers, even for passives. It keeps Purchasing from inadvertently putting some crap bypass cap in a spot where a high-quality cap is needed to set a filter characteristic, or other similar mistake. \$\endgroup\$ – TimWescott Oct 22 '18 at 15:26

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