Trimmer resistors pin not connected? - Page 1
Forum section: electronics / general

author: Moriambar
author_rank: Unknown
timestamp: August 20, 2023, 09:30:05 am
content: Hi there.I'm quite a newbie with kicad so I have to ask, although it seems stupid.I have a trimpot going to gnd as in the image below, to fine tune a simple cc driverWhen I'm going to the pcb though, I have this worrysome warning:[ Specified attachment is not available ](which basically says "warning, no connection for symbol R8 Pin 3. And in fact I have no connection for Pin 3, making my trimpot useless, I guess.I used the standard trimpot symbol, what have I done wrong, and how can I fix it?Cheers!

author: langwadt
author_rank: Super Contributor
timestamp: August 20, 2023, 11:02:45 am
content: Quote from: Moriambar on August 20, 2023, 09:30:05 amit is only a warning and it would still work, the connections are to one end and the wiperif you want to get rid of the warning and be explicit, use the 3 pin symbol for a trim pot and either connect the third pin or mark it unconnected

author: Moriambar
author_rank: Unknown
timestamp: August 20, 2023, 11:19:28 am
content: Quote from: langwadt on August 20, 2023, 11:02:45 amah ok, thanks, I was worried that the connection would make my resistor fixed.thanks

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 20, 2023, 05:56:48 pm
content: Quote from: Moriambar on August 20, 2023, 11:19:28 amI don't know why langwadt thinks it will work.The resolution of your screenshot is very low, it does not show pin numbers, and the second attachment is apparently attached to something else, but not this forum.When in doubt in KiCad, just open the schematic editor and the PCB editor at the same time (dual screen or big monitor is really nice here) and then when you select something, the cross probing function will also highlight it in the other program. This is an easy way to verify connections in KiCad.

author: langwadt
author_rank: Super Contributor
timestamp: August 20, 2023, 06:04:01 pm
content: Quote from: Doctorandus_P on August 20, 2023, 05:56:48 pmI looked in kicad and the standard trim pot connects to pin 1 and 2

author: Benta
author_rank: Super Contributor
timestamp: August 20, 2023, 07:40:06 pm
content: Quote from: Doctorandus_P on August 20, 2023, 05:56:48 pmNot hard to understand. We're talking schematic symbols here, not PCB layout or specific parts."R_Trim" is just a variable resistor that you can... tadaa... trim (you know, screwdriver). It has two pins. It has no specific footprint attached to it, that's up to the designer.If you want a 3-pin symbol, "R_Potentiometer_Trim" is the right one.Fully consistent and correct.

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2023, 08:45:12 pm
content: In real life, there are very few small "variable resistors" with only two terminals.Far more common are "trimpots" with both ends and the wiper brought out to three terminals.It is very common to see one end connected to the wiper to make a two-terminal adjustable resistor.

author: langwadt
author_rank: Super Contributor
timestamp: August 20, 2023, 08:56:53 pm
content: Quote from: TimFox on August 20, 2023, 08:45:12 pmyou don't need to connect the wiper to the end, but it usually a good practice to it so that it doesn't go open if the wiper has a bad connection. And if possible arrange the circuit so that the pot goes to the max value nothing bad happens

author: Benta
author_rank: Super Contributor
timestamp: August 20, 2023, 09:08:09 pm
content: Guys, you are into practical implementation again.This about schematicsymbols.

author: langwadt
author_rank: Super Contributor
timestamp: August 20, 2023, 09:33:34 pm
content: Quote from: Benta on August 20, 2023, 09:08:09 pmit is about implementation, the symbol only has two pins, the footprint has three pins, the PCB is warning about that to make sure that is what you intended

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 20, 2023, 10:04:35 pm
content: Quote from: Benta on August 20, 2023, 09:08:09 pmHuh?Schematic symbols do not live in a vacuum. And besides, from the opening post:Quote from: Moriambar on August 20, 2023, 09:30:05 amAnd the "missing" connection for pin 3 is also from a PCB footprint.https://xkcd.com/386/

author: Moriambar
author_rank: Unknown
timestamp: August 21, 2023, 06:13:36 am
content: Guys I basically checked my chosen PCB footprint and the component datasheet: everything is fine and dandy.Pin 1 is connected and pin 2 is connected too (wiper). I know it's best practice to connect the wiper with one of the other pins, but I don't care, the important thing is that it IS a trimpot and resistance will be trimmed.OTOH: is there a way to connect the third pin to the wiper for good measure without changing the schematic?Thanks!

author: langwadt
author_rank: Super Contributor
timestamp: August 21, 2023, 06:41:29 am
content: Quote from: Moriambar on August 21, 2023, 06:13:36 amyou can make a copy of the foot print and change pin3 to also be pin2

author: Moriambar
author_rank: Unknown
timestamp: August 21, 2023, 06:48:33 am
content: Quote from: langwadt on August 21, 2023, 06:41:29 amWow, nice and simple. I would've never thought about it, thanks!

author: John B
author_rank: Frequent Contributor
timestamp: August 21, 2023, 10:57:07 am
content: Kicad doesn't have every part you need. I would create a schematic symbol and footprint pair and name them accordingly so you know what part they match.I would only change the footprint so that pads/plated holes have the same number if they are electrically connected.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 21, 2023, 11:13:40 am
content: I do not understand the reasoning behind the "without changing the schematic" part. But it's your project. And indeed if a footprint has multiple pads with the same pad number, then KiCad wants you to connect them together.

author: Moriambar
author_rank: Unknown
timestamp: August 21, 2023, 11:36:42 am
content: Quote from: Doctorandus_P on August 21, 2023, 11:13:40 amLaziness. Plain and simple. The schematic has been done with, and I just would like to move on to the pcb.Maybe, now that I think about it, choosing the other trimpot symbol (with three pins) and shorting the third pin to the wiper could take less than duplicating the footprint, mark it as new and make it short two pin together.Cheers

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 21, 2023, 03:09:56 pm
content: I guess it's not lazyness, but just acting on a (beginners?) assumption.But whatever way you choose, both ways are pretty trivial to do in KiCad, and as you get used to it, your toolbox will grow.