Author Topic: Trimmer resistors pin not connected?  (Read 3218 times)

0 Members and 1 Guest are viewing this topic.

Offline MoriambarTopic starter

  • Supporter
  • ****
  • Posts: 490
  • Country: it
Trimmer resistors pin not connected?
« on: August 20, 2023, 09:30:05 am »
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 driver
1855657-0

When 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!
 

Offline langwadt

  • Super Contributor
  • ***
  • Posts: 4412
  • Country: dk
Re: Trimmer resistors pin not connected?
« Reply #1 on: August 20, 2023, 11:02:45 am »
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 driver
(Attachment Link)

When I'm going to the pcb though, I have this worrysome warning:
(Attachment Link)
(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!

it is only a warning and it would still work, the connections are to one end and the wiper

if 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

 
The following users thanked this post: stryker, Moriambar

Offline MoriambarTopic starter

  • Supporter
  • ****
  • Posts: 490
  • Country: it
Re: Trimmer resistors pin not connected?
« Reply #2 on: August 20, 2023, 11:19:28 am »
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 driver
(Attachment Link)

When I'm going to the pcb though, I have this worrysome warning:
(Attachment Link)
(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!

it is only a warning and it would still work, the connections are to one end and the wiper

if 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
ah ok, thanks, I was worried that the connection would make my resistor fixed.
thanks
 

Online Doctorandus_P

  • Super Contributor
  • ***
  • Posts: 3338
  • Country: nl
Re: Trimmer resistors pin not connected?
« Reply #3 on: August 20, 2023, 05:56:48 pm »
it is only a warning and it would still work, the connections are to one end and the wiper

if 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
ah ok, thanks, I was worried that the connection would make my resistor fixed.
thanks


I 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.
 

Offline langwadt

  • Super Contributor
  • ***
  • Posts: 4412
  • Country: dk
Re: Trimmer resistors pin not connected?
« Reply #4 on: August 20, 2023, 06:04:01 pm »
it is only a warning and it would still work, the connections are to one end and the wiper

if 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
ah ok, thanks, I was worried that the connection would make my resistor fixed.
thanks
I don't know why langwadt thinks it will work.

I looked in kicad and the standard trim pot connects to pin 1 and 2
 

Offline Benta

  • Super Contributor
  • ***
  • Posts: 5860
  • Country: de
Re: Trimmer resistors pin not connected?
« Reply #5 on: August 20, 2023, 07:40:06 pm »
I don't know why langwadt thinks it will work.
Not 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.
 

Offline TimFox

  • Super Contributor
  • ***
  • Posts: 7936
  • Country: us
  • Retired, now restoring antique test equipment
Re: Trimmer resistors pin not connected?
« Reply #6 on: August 20, 2023, 08:45:12 pm »
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.
 

Offline langwadt

  • Super Contributor
  • ***
  • Posts: 4412
  • Country: dk
Re: Trimmer resistors pin not connected?
« Reply #7 on: August 20, 2023, 08:56:53 pm »
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.

you 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
 
The following users thanked this post: Doctorandus_P

Offline Benta

  • Super Contributor
  • ***
  • Posts: 5860
  • Country: de
Re: Trimmer resistors pin not connected?
« Reply #8 on: August 20, 2023, 09:08:09 pm »
Guys, you are into practical implementation again.
This about schematic symbols.
 

Offline langwadt

  • Super Contributor
  • ***
  • Posts: 4412
  • Country: dk
Re: Trimmer resistors pin not connected?
« Reply #9 on: August 20, 2023, 09:33:34 pm »
Guys, you are into practical implementation again.
This about schematic symbols.

it 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
 

Online Doctorandus_P

  • Super Contributor
  • ***
  • Posts: 3338
  • Country: nl
Re: Trimmer resistors pin not connected?
« Reply #10 on: August 20, 2023, 10:04:35 pm »
Guys, you are into practical implementation again.
This about schematic symbols.

Huh?
Schematic symbols do not live in a vacuum. And besides, from the opening post:

When I'm going to the pcb though, I have this worrysome warning:
...  And in fact I have no connection for Pin 3, making my trimpot useless, I guess.

And the "missing" connection for pin 3 is also from a PCB footprint.

https://xkcd.com/386/
 

Offline MoriambarTopic starter

  • Supporter
  • ****
  • Posts: 490
  • Country: it
Re: Trimmer resistors pin not connected?
« Reply #11 on: August 21, 2023, 06:13:36 am »
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!
 

Offline langwadt

  • Super Contributor
  • ***
  • Posts: 4412
  • Country: dk
Re: Trimmer resistors pin not connected?
« Reply #12 on: August 21, 2023, 06:41:29 am »
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!

you can make a copy of the foot print and change pin3 to also be pin2
 
The following users thanked this post: Moriambar

Offline MoriambarTopic starter

  • Supporter
  • ****
  • Posts: 490
  • Country: it
Re: Trimmer resistors pin not connected?
« Reply #13 on: August 21, 2023, 06:48:33 am »
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!

you can make a copy of the foot print and change pin3 to also be pin2

Wow, nice and simple. I would've never thought about it, thanks!
 

Offline John B

  • Frequent Contributor
  • **
  • Posts: 799
  • Country: au
Re: Trimmer resistors pin not connected?
« Reply #14 on: August 21, 2023, 10:57:07 am »
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.
 

Online Doctorandus_P

  • Super Contributor
  • ***
  • Posts: 3338
  • Country: nl
Re: Trimmer resistors pin not connected?
« Reply #15 on: August 21, 2023, 11:13:40 am »
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.
 
The following users thanked this post: Moriambar

Offline MoriambarTopic starter

  • Supporter
  • ****
  • Posts: 490
  • Country: it
Re: Trimmer resistors pin not connected?
« Reply #16 on: August 21, 2023, 11:36:42 am »
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.
Laziness. 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
 

Online Doctorandus_P

  • Super Contributor
  • ***
  • Posts: 3338
  • Country: nl
Re: Trimmer resistors pin not connected?
« Reply #17 on: August 21, 2023, 03:09:56 pm »
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.
 
The following users thanked this post: Moriambar


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf