# Why are capacitor voltage ratings such strange values?

This is a similar type of question to this question about the use of the number 47.

I was looking at ordering some parts just like any other day and until now I've never really thought about it much, but I realised that the voltage ratings for capacitors seem really strange (to me at least).

The standard voltage ratings seem to be:

• 6.3 V
• 10 V
• 16 V
• 25 V
• 35 V
• 50 V
• 63 V
• 100 V
• 160 V

I've taken these values and plotted them on a graph to see if there was any correlation and it seems like there is an exponential correlation.

My actual questions are:.

1. How were these values initially decided upon?
2. Was/Is the voltage rating dictated by the material used and has just stuck over the years?
3. Are there any other components out there that use the same voltage ratings?
• Well, one thing pops right out to me because I used to use vacuum tubes. The 6.3V was one of the common filament voltages, though that was an AC rms figure. Lots of transformers included a secondary for that. – jonk Aug 17 '16 at 8:06
• @jonk Not only it was AC, but you're also not supposed to pick a cap rated at the exact voltage you have. – Dmitry Grigoryev Aug 17 '16 at 8:23
• @dmitry: Of course. The whole thing is only about the number itself reminding me. Not about suggesting anything like you seem to imagine I did. I was addressing #3 of the OP's question. I took that to mean ANY. – jonk Aug 17 '16 at 8:25

It's mainly all about preferred numbers: -

See this wiki page and, see also this stack exchange answer about fuses values.

It's all about splitting the difference between 1 and 10. For instance $\sqrt{10}$ is ratiometrically half way between 1 and 10 and the cube root splits into the numbers 2.154 and 4.642. Multiply by 2.154 again and you get 10.

So, if you split the range 1 to 10 into 5 chunks you get 1, 1.5849, 2.5119, 3.9811, 6.3096 and finally 10.

These values approximate to 1, 1.6, 2.5, 4.0, 6.3 and 10.

Like I said earlier, this is mainly the reason but I suspect the prevelence of 24 V systems may have caused the 35V capacitor to come about and, of course the 40 V capacitor is not that uncommon: -

• @agentL and that is precisely what i have delivered. Try again if u have a valid point that somehow I have missed. – Andy aka Aug 17 '16 at 17:39
• ops, you're right and I was wrong. However, your answer consists of nothing but a cryptic table and 2 links. Maybe if you could add more explanation it would not look like a table of resistor values. That's what I mistook it for. – Agent_L Aug 17 '16 at 18:01
• It woudl be nice if you would expand on it more. I certainly didn't understand anything about the answer until I followed the link, and even then 35V doesn't fit into the table. StackExchange prefers answers where the content is in the answer itself, not the links. That way they're not as affected when the links expire. – Cort Ammon Aug 17 '16 at 18:07
• @Meower68 see edit – Andy aka Aug 17 '16 at 20:03
• @CortAmmon see edit – Andy aka Aug 17 '16 at 20:03

Lots of components' values lie equally spaced on a logarithmic scale (i.e. are exponential, as you have observed). This makes a great deal of sense, because component values span multiple orders of magnitude. In my parts box, I have capacitors from 10pF to 2.6mF (over 8 orders of magnitude) and resistors from 1ohm to 1Gohm (9 orders of magnitude). You wouldn't want to linearly space component values. If you did resistors in increments of 1ohm, you would have poor resolution at the low end, but your product catalog would need to have a billion items in it! The values could be linearly spaced within a decade, i.e. do 0.01ohm increments for resistors between 1 and 10ohms, 10ohm increments for resistors between 1 and 10kohms, for example, but what makes the decade cutoffs significant? It's better to have the resolution of selectable values to be proportional to the value itself everywhere, rather than chunked into select ranges.

• OP asked about voltages, not values. – Agent_L Aug 17 '16 at 16:46
• Oops, true, but the same principal still applies. – Void Star Aug 17 '16 at 16:56
• @agent_l the exact same reasons apply for the value of the voltage as they do for the value of the capacitance (or resistance, or whatever) – mbrig Aug 17 '16 at 18:03
• @mbrig Yes, I get it. But if it was that obvious, OP wouldn't have to ask, wouldn't he? Plus, with voltages it's bit different, because you're not looking for rating exactly as operating voltage, but for anything over your voltage (+ safety margin). Overspeccing here is possible while with value is not. Unlike values, there are typical voltage that majority of circuits run at. And the materials that capacitors are made of have different characteristics when handling voltage or overvoltage. There is much more to OP's voltage ratings question than to a value question. – Agent_L Aug 17 '16 at 18:41
• Well, to points 2 and 3 I say, yes to point 2, materials have some bearing on voltage ratings but you can always change the thickness of the dielectric, so you can achieve whatever rating you want within reason, and no I don't think so to point 3, because other passive linear components are spec'd on current or power dissipation, capacitors being the exception due to their limited dielectric strength, and semiconductor product lines tend not to have complete ratings/specs matrixes like the passive linear parts, for those you get to choose one parameter and then just overspec the others. – Void Star Aug 19 '16 at 16:34