Should voltage ratings of caps always be in the default comment for a part? - Page 1
Forum section: electronics / general

author: Pack34
author_rank: Frequent Contributor
timestamp: November 09, 2017, 01:37:39 am
content: Same as the title. I think the schematic looks a lot better, but I hate hidden information.What's the general consensus here?

author: helius
author_rank: Super Contributor
timestamp: November 09, 2017, 02:22:34 am
content: For generic bypass caps on logic, it isn't really necessary. The voltage across them will always be within delta of a known value.For everything else, it is helpful to print the W.V. since it makes it clear how the element is expected to function in the circuit (the expected differential node voltage must be less than the W.V.), and it is required by the technician in order to replace any caps that got destroyed.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 09, 2017, 02:27:16 am
content: What's a schematic?It's a written document, it conveys information.Does the information need to be there? Avoid omissions, and avoid redundancy.Obviously, you can't put everything on the schematic. Some of it is going to end up somewhere else: a BOM, for example. If it's better placed elsewhere, then... why not?In EDA, I tend to use only one parameter. The data is accessible elsewhere, whether it's an Altium Smart PDF, or in the BOM, or if you have the luxury of viewing the source files.In written schematics, I've been known to indicate whatever is important: a component value of course, but also voltage or current, a recommended part series, limiting ESR/ESL, etc.Whether you want to put that on the (EDA) schematic as well, is up to you. I don't, because I think the added clutter and space taken up isn't worth the slight increase in availability.I've seen schematics with secondary and tertiary parameters, usually V/A/W rating and footprint/package size. I've also seen (a vast majority of) schematics clearly without an aesthetic design behind them. I suspect most engineers don't actually want to be engineers, and self-flagellate with ugly schematics, UGLY_NAMESWITH_CAPS and so on. Like this stuffshouldbe hard, and that difficulty should be taken out on themselves, and anyone else who might happen across those documents.But then, I'm pretty sure managers feel the same way, and they are the ones making purchasing decisions on software. So you have it, that almost all professional software is ass, too.Uh, anyway, cynicism aside, that's my opinion.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 09, 2017, 02:47:30 am
content: On the subject of information priority:I'm rather disinclined to do capacitor voltages anyway, because they're so utterly meaningless. Tantalums should be double to triple; ceramics might be okay at rating, or they might need 3 or 5x overrating. What's the point? Any other part you might replace it with, has an equally wide scatter; do you just hope for the best?So I might be inclined to specify electrolytic capacitor voltage ratings, or resistor wattages, because those are fairly reliable at least, but not for the other types. (I write the above as a generalization, because most statistically speaking, most capacitors are ceramic.)Or inductor saturation currents -- ah, but then you need additional terminology on the schematic to indicate that that's what you're referring to, that the inductor needs so-and-so DC current and inductance spec. If you just write "2.2uH 10A", most people are going to think DC current rating, when it might actually be a pulsed application for example, and that's the required saturation current!You could just as well hide a note in the corner saying "inductors and capacitors must be within xx% at the labeled voltage", but service techs aren't going to pay attention. Or if they do, they're likely to disregard it, because who the hell knows what value a capacitor is -- anyway, 0.1uF is 0.1uF, right? Right? Us engineers know better, but most techs won't, and knowing where to find that information is part of our job. I mean, some manufacturers don't even provide C(V) curves, and most that do, tuck it neatly away on their mess of a website.On that subject: ferrite beads are off, not just by modest factors, but whole orders of magnitude. It's a seriousdesignproblem, and even less well known than ceramic capacitor C(V). Example: a 390 ohm 2A 1206 ferrite bead typically saturates around 200mA -- one round order of magnitude! HI0603P600R-10 is -3dB at 1/20th the DC rating. You need to be aware, not just what you might be replacing a part with, but what you're choosing in the first place!Tim

author: tszaboo
author_rank: Super Contributor
timestamp: November 09, 2017, 02:47:39 pm
content: No, I dont place voltage rating on the schematic. unless it is very different from the other ones. I place voltage rating for the 1KV caps going between isolated DC-DC converters. Or somewhere, where it was the wrong value on previous boards.For the 100nF capacitors I even stopped placing capacitor value. And silkscreen on the PCB. I mean I'm going to use 50V rated 0603 100nF anyway, because I cannot be bothered to select a lower voltage rating for no reason.