Potentiometer/variable resistor Question

Thread Starter

bigjoncoop

Joined Feb 1, 2019
158
Hey guys,

Last night while trying to desolder a TrimPot / Potentiometer I accidentally scraped some solder mask off the PCB between 2 through holes. Now solder has bridged them and I cannot remove it.

But it looks like the two through holes are connected to the same Trace. I was wondering if you could take a quick look at this schematic of where the potentiometer is in the circuit and let me know your thoughts. Are two of the pins connected together? basically one of those variable resistor schematic symbols where the middle leg is not attached to a Trace. Pictures below.

Potentiometer looks like this
s-l300 (1).jpg
And this is where it's located in this schematic

received_318420369903745.jpeg
 

Thread Starter

bigjoncoop

Joined Feb 1, 2019
158
Your schematic shows a VR not a potentiometer, but I can't answer the rest.
It is definitely a blue trimpot like in the picture I posted. But if it is being used like a variable resistor is it possible that two of the legs are tied together? Or maybe one of the legs isn't attached to anything
 

AnalogKid

Joined Aug 1, 2013
9,248
That is *very* poor schematic technique. Better to show all three pins, even if one of them is floating, and all connections.

It is entirely possible that one of the end pins is connected to the center (wiper) pin with a trace. It does not affect the rotation-versus-resistance curve in any way, but it can prevent possible problems. For example, in a high shock-and-vibration environment, the wiper can actually bounce off of the resistive element. With the unneeded pin floating, the resistance seen by the rest of the circuit jumps from whatever to infinity. With the unneeded pin tied to the wiper, that resistance jumps from the set value to the pot's max value. This can be a critical difference in servos, motor controls, power supplies, etc.

ak
 

BobaMosfet

Joined Jul 1, 2009
1,768
Hey guys,

Last night while trying to desolder a TrimPot / Potentiometer I accidentally scraped some solder mask off the PCB between 2 through holes. Now solder has bridged them and I cannot remove it.

But it looks like the two through holes are connected to the same Trace. I was wondering if you could take a quick look at this schematic of where the potentiometer is in the circuit and let me know your thoughts. Are two of the pins connected together? basically one of those variable resistor schematic symbols where the middle leg is not attached to a Trace. Pictures below.

Potentiometer looks like this
View attachment 239837
And this is where it's located in this schematic

View attachment 239836
@bigjoncoop

Potentiometers can work as rheostats, but rheostats CANNOT work as potentiometers.

  1. Potentiometers are 3-terminal connected items (both ends of internal resistance + wiper - a v-divider), and
  2. Rheostats are 2-terminal connected items. (one-end of internal resistance + wiper)

In other words, if you short a wiper and one-end of a potentiometer together, you have a rheostat, which sounds like the way they are using your trimming potentiometer.
 

Irving

Joined Jan 30, 2016
1,496
It's common practice to join the wiper to an end pin to create a 2-pin devce from a 3-pin device. The Tenma 3-channel bench supply I have has around 20 presets and all are wired this way and show as rheostats on the schematic. See the picture below...

IMAG0911.jpg
 

Tonyr1084

Joined Sep 24, 2015
5,966
As others have already said - it's possible there's a trace from the center pin to one of the two outer pins (assuming your trim pot wiper is the center pin as shown below): If you've lost the solder mask on the center and right hole (in picture) then you have not created a short. IF you've bridged between the LEFT hole and the center hole then you've created a short. But that should be easy to clear with several different desoldering techniques. One word of caution - don't keep putting heat cycles to the joints. That weakens them and can easily lead to a lifted trace or pad.
1622303761710.png
 

Thread Starter

bigjoncoop

Joined Feb 1, 2019
158
@Tonyr1084 @Irving @BobaMosfet @AnalogKid @ElectricSpidey

thanks guys for your help/input.

Later this evening I will see what happens and report back.

I'm pretty sure all is okay, since it seemed like the two pins were connected to the same trace. It was just hard to tell for sure. But since you guys have clarified that having the middle pin connected to one of the outer pins is common in this scenario, i feel confident that I will be ok.

But that should be easy to clear with several different desoldering techniques. One word of caution - don't keep putting heat cycles to the joints. That weakens them and can easily lead to a lifted trace or pad.
I'm fairly experienced when it comes to Soldering/Desoldering, but even after using a solder sucker and desoldering wick / braid the trimpot come out. Finally After after a lot of frustration and heating and prying i finally got the little sucker out of there. But then after I reapplied some fresh solder to try to use some more desoldering wick to finally clear the through holes completely, a small about of solder (a microscopic layer) attached it self in the small scratch in the solder mask and no matter what I've tried will remove it. (it basically "tinned" the copper just below the solder mask and is touching both through holes!!) But fingers crossed that it doesn't matter anyway

897897.jpg
 

Tonyr1084

Joined Sep 24, 2015
5,966
Thanks for the picture, but that doesn't help. We need something much more "Macro" and clear so we can at the very best - GUESS as to what's going on. From your picture we can tell nothing other than you have a boost converter that is rated for "One Thousand Five Hundred WATTS. That's enough to run an electric car on (I think) (1.5KW) My home generator produces 2.8KWh. And that's not a big generator.
 
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