XX below Resistor meaning? - Page 1
Forum section: electronics / beginners

author: Sirgeorge
author_rank: Contributor
timestamp: September 24, 2020, 03:08:34 am
content: Not a question I could find the answer to just by searching. Does anyone know what XX below a resistor means? Found it right next to a ground and assuming its to "eliminate" any extra charge. Also, a number without an SI unit attached can just be taken to mean the base unit right? So 233 without any SI unit name can just be taken to mean 233 Ohms, correct?

author: ledtester
author_rank: Super Contributor
timestamp: September 24, 2020, 03:38:12 am
content: A picture would help a lot.It sounds like you're talking about a schematic."233" on a SMD resistor could mean that it's a 23K ohm resistor.

author: Sirgeorge
author_rank: Contributor
timestamp: September 24, 2020, 05:33:01 am
content: Here you go. This should be what you asked for.[ Specified attachment is not available ]

author: Sirgeorge
author_rank: Contributor
timestamp: September 24, 2020, 05:34:54 am
content: Quote from: Sirgeorge on September 24, 2020, 03:08:34 amLinked in above post. Still getting used to this site's setup.

author: Mr.B
author_rank: Unknown
timestamp: September 24, 2020, 06:16:57 am
content: It looks to me that R751 is the schematic designator for the resistor, not its value.The value of R751 appears to be 47 ohms.The value of R752 looks to me like the engineer either forgot to put a value in or has not calculated it yet.XX is probably just a placeholder... "I will come back and work that out later".I do it all the time when I have not yet determined exactly what LED I will use in an application, therefore cannot yet calculate the current limiting resistor.

author: exe
author_rank: Unknown
timestamp: September 24, 2020, 07:22:11 am
content: I think I've seen this for unpopulated parts.

author: srb1954
author_rank: Super Contributor
timestamp: September 24, 2020, 08:03:39 am
content: The XX could also mean "select-on-test" or "adjust-on-test" where the technician selects the value of the resistor to obtain the desired circuit performance.This technique was much more common in the olden days when labour was cheap and parts were relatively expensive. However, it is not very common these days when labour is expensive and parts are cheap. Also it is not really compatible with efficient manufacturing of SMD PCBs.

author: Sirgeorge
author_rank: Contributor
timestamp: September 24, 2020, 09:15:47 am
content: I guess I should add that this is right before ground.

author: radiolistener
author_rank: Super Contributor
timestamp: September 24, 2020, 11:52:47 am
content: It means "unknown value"

author: tkamiya
author_rank: Super Contributor
timestamp: September 25, 2020, 12:51:19 am
content: Giving some context will help. But.... I've been playing with KiCad. On KiCad, R<some number> will be a label. It distinguishes one register from next. XX would be a value. When I build a schematic in KiCad, or any Cads really, it would assign label but you also have to update it with the value. It's up to the user to do it. If you forget it, I'd get ?? but on some other system would be XX or even nothing.Looks like someone forgot to update it.I know you are new to this site. Everyone is very willing to help. More detail you give, more you get back.