Default units for capacitors - Page 1
Forum section: electronics / beginners

author: level6
author_rank: Regular Contributor
timestamp: February 28, 2020, 03:03:39 am
content: I'm reading a diagram of a circuit where the capacitor value units are not indicated, not in the notes, nowhere. It just shows a value of 0.02. Is there a default implied unit for capacitors?

author: xrunner
author_rank: Super Contributor
timestamp: February 28, 2020, 03:07:00 am
content: Look over the entire diagram for a note which will state the units. If no note, it's probably microfarads (uF). But, can't be 100% positive.What type of circuit is it - what is it's function - i.e. audio or rf?

author: ejeffrey
author_rank: Super Contributor
timestamp: February 28, 2020, 03:51:44 am
content: How old is the circuit diagram? It's probably microfarads. Back in the day it was common (especially in the US) for capacitors to only be designated in microfarads or picofarads, not milli or nano and you could usually tell by the type. .02 doesn't make sense as picofarads so probably micro.One minor possibly is that this is a "prototype" filter i.e., a generic Nth order filter of a specific type. In that case the resistances and capacitors are dimensionless and you have to scale by the target frequency and possibly impedance to get physical values.

author: TimFox
author_rank: Super Contributor
timestamp: February 28, 2020, 03:56:05 am
content: There used to be a standard that polarized (electrolytic) capacitors listed as above 1.0 were in microfarads (e.g. 22, 470).Non-polarized (paper, film, mica, ceramic) above 1.0 were in picofarads.Fractional values (e.g. 0.47, 0.02, 0.022), were in microfarads.In all cases, unless otherwise specified.

author: level6
author_rank: Regular Contributor
timestamp: February 28, 2020, 04:01:30 am
content: Quote from: xrunner on February 28, 2020, 03:07:00 amIt's for audio. Here's the datasheet that it's on. I looked all over and couldn't find any indication of units. I think I'll send them an email and ask.http://www.alfarzpp.lv/eng/sc/AS3350.pdf

author: TimFox
author_rank: Super Contributor
timestamp: February 28, 2020, 04:10:21 am
content: One capacitor is listed as “220 pF”, so the other capacitors, all listed as “0.02” are almost certainly in microfarads.

author: I wanted a rude username
author_rank: Frequent Contributor
timestamp: February 28, 2020, 05:09:54 am
content: This tradition might explain why people sometimes use fractional uF even when specifying the unit, e.g. 0.1 uF instead of 100 nF. And also using uF for values ≥ 1 mF.

author: level6
author_rank: Regular Contributor
timestamp: February 28, 2020, 03:35:12 pm
content: I just got a reply back from the manufacturer. The units are micro farads.

author: TimFox
author_rank: Super Contributor
timestamp: February 28, 2020, 04:01:28 pm
content: On vintage equipment, watch out for “mf” or “mfd”, which were used for “microfarads”. Modern usage is “mF” for “millifarads”, but this can be confusing. Similarly, “mmf” or “mmfd” meant “micro microfarads” (now pF for picofarads), often pronounced “micky-mikes”. If I remember correctly, “nF” became common in USA sometime in the 1960s.

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 28, 2020, 08:13:52 pm
content: Default units are uF (microfarads) and pF (picoFarads).uF's are used with a decimal dot ("1.0" for example).

author: TimFox
author_rank: Super Contributor
timestamp: February 28, 2020, 08:25:41 pm
content: Watch out for small capacitors, such as 4.7 pF.