Components with multiple possible values - how to designate on schematic?

Question

Quite often it happens to me that values of resistors or capacitors don't play such a significant role. This is mainly the case with decoupling capacitors and e.g. LED series resistors. Often in decoupling caps their ESR and other characteristics are more important and I usually specify them globally as X7R/X5R. If I were designing more analog, this would not be as applicable, as in analog, component values/properties are much more strictly defined.

I, as a circuit designer, presumably have to be specific about the whole project so that the other people can take it and produce a working board. But this means that I put constrains on the purchasing department. If for some reason 4-resistor arrays of \$100\Omega\$ were not available, that could result in a delay, when the purchasing person reaches back to me with the issue.

Wouldn't it be fun, if the schematic defined more possible values for that component? For example it could say, that 80, 100, 120, 180 Ohms are all OK. Or simply define a range 80-180 Ohms.

But how to mark it on paper? Or more generally, is it worth it?

schematic

^{simulate this circuit – Schematic created using CircuitLab}

You really do not want to be giving purchasing departments any sort of engineering responsibility. It's up to you to verify the availability of parts before you design them into a circuit. And for generic parts like resistors and capacitors, availability should never really be an issue anyway. — Dave Tweed, Apr 12 '15 at 16:57
@DaveTweed Yeah the _responsibility_ argument is enough for me to remain specific. Thx — Dzarda, Apr 12 '15 at 17:06

score 4 · Accepted Answer · answered Apr 12 '15 at 16:54

Well first of all decoupling cap values do matter :). But to answer your question you should manage that with your bom. Let's say you have a 100 ohm resistor, give it an internal part number. Now under that part number on the bom you can assign multiple manufacturers part numbers as acceptable alternates.

I have a system that manages this for me in orcad CIS/CIP but you can keep track of it in excel.

Specing multiple values for a component sounds like a bit of a mess in production if you ask me, and a risk to the design. Maybe there's a combo that will cause an issue. I wouldn't allow it in my shop.

If you want to be more free with purchasing you can specify that substitutions are allowed if parts meet all of the same requirements or 1% ok where 5% is specified. I would specify that you must approve all substitutions though. After all the success of the design is your responsibility :)

Also unless they own the design, factories don't like to be making design decisions like let me out a 10k in here where he has a 1k.

Yeah you're right - It's just me trying to be too smart sometimes :) Doesn't pay off. — Dzarda, Apr 12 '15 at 17:05

score 1 · Answer 2 · answered Apr 12 '15 at 16:53

I think I've seen (and have used) "Select on Test" as a part value, perhaps with a note nearby giving the likely range of values.

I might use "See BOM" (BOM = bill of materials = parts list) with the BOM listing possible alternative parts.

Of course, what you choose to do as a hobbyist may be quite different from what can be done in a large company with Approved suppliers and parts.

Components with multiple possible values - how to designate on schematic?

2 Answers2