Why is the prefix used in values? - Page 1
Forum section: electronics / beginners

author: tron9000
author_rank: Frequent Contributor
timestamp: November 05, 2015, 10:23:31 pm
content: I get asked frequently "why are the prefixes in some component values?"e.g. 1R0, 2u2, 3k3 etc.I usually reply that maybe its so you can easily identify them on a schematic, as if the copy is bad, or poorly scaled, then the decimal place could be hard to see and you could mistake 2.2uF for 22uF. Not sure if that's true or not?Anyone know?

author: AndyC_772
author_rank: Super Contributor
timestamp: November 05, 2015, 10:35:29 pm
content: That's exactly it. 3.9k is easily misread as 39k, but 3k9 is unambiguous even on a bad copy.

author: jolshefsky
author_rank: Regular Contributor
timestamp: November 05, 2015, 10:39:01 pm
content: As far as I know, basically that's it.It is a way to concisely specify unit and decimal point—useful especially when silkscreen space is tight (and often using monospaced fonts so the decimal point is one whole character width.) Thus, you can identify whether it's a resistor or capacitor as well as its value in 2 characters (e.g. 8u vs. 8K).It also alleviates the problem of the obscured/missing decimal point. And because it's a little unusual to the eye, it may make someone look a little more carefully.I do find it a little frustrating in numerical notation to not be able to identify significant zeroes. Is 100R ±10? or ±1?? At least 1K00 makes it clear that you mean 1000? ± 10?.

author: tron9000
author_rank: Frequent Contributor
timestamp: November 05, 2015, 10:44:48 pm
content: Nice one, Thanks!I know what you mean, I have seen 1K39 on silk screens next to resistors. anything that says 10R for example I assume that they are using standard 1% tolerances, unless I suspect that it is part of a more sensitive circuit, in which case I'm on your side jolshefsky

author: c4757p
author_rank: Super Contributor
timestamp: November 05, 2015, 10:56:58 pm
content: Quote from: jolshefsky on November 05, 2015, 10:39:01 pmSignificant digits are stupid. How do you show 2% in significant figures? Just give the tolerance separately.

author: rgawron
author_rank: Contributor
timestamp: November 05, 2015, 11:39:24 pm
content: I was told that it's because of bad quality of the first computer printers.

author: crispy_tofu
author_rank: Super Contributor
timestamp: November 06, 2015, 12:43:25 am
content: +1it was because the decimal place could be difficult to read or get rubbed off.

author: mswhin63
author_rank: Frequent Contributor
timestamp: November 06, 2015, 12:50:41 am
content: As an offshoot but most probably not anticipated in some values the length of the characters to make the component value are longer. It also reduces size of the screen printing on the board.

author: con-f-use
author_rank: Unknown
timestamp: November 06, 2015, 08:51:32 am
content: Usually it's bad practice to have a design that relies on precise values. One should avoid that for many reasons (cost, flexibility, sourcing...). It is rare to have a real requirement for tight tolerances above what the significant digits suggest.As for the "4k7" notation instead of "4.7k". Well space in schematics is sometimes rare. It's one character less, and also one that is hard to read in fine print (as many said). Mostly the unit is clear to, because there is a reference as well. If the component is named "R5" its clear that 4k7 is a resistor. Seldom do you see inductors with 4.7 kH or capacitors with 4.7 kF.