Why do we do that?

[Kipper4]

Is there a convention on pots wired as variable resistors? If that's the right terminology.

In reply 93 here Mr Lyman connects ground to wiper

http://www.diystompboxes.com/smfforum/index.php?topic=114186.80

Does it matter if he had connected signal (top) to wiper?

Are there conventions? Rules? Reasons?

Do the electrons care?

Thanks for any insight
Rich

[Mark Hammer]

It's simply good practice and "insurance".  Should the wiper ever lose contact and continuity, the max resistance never exceeds the value of the pot.  In many instances, that probably doesn't matter, but in others it might mean oscillation, loss of power, or other undesirable outcomes.

Why would the wiper lose contact?  Pot lugs are generally "rivetted" into place.  I don't know about you, but I've had plenty of pots where one or more solder lugs could be wiggled, because the rivet had become loose for whatever reasons.  It is also the case that, since wipers essentially scrape the resistive strip on a continuous basis, even if you do your best to keep external dust and dirt out of the pot, it may be busy creating its own internal grime buildup that makes it scratchy.  And scratchy pots are simply those that don't maintain consistent continuity with the wiper across their entire rotation.

So, bottom line: you don't have to do it, but it does't cost anything to do it, and there can be instances where you're sorry you didn't.  Like I say, just good practice.  Myself, I live dangerously, so do what I say, not what I do.    Good on designers for drawing them that way, though, and at least planting the seed.

[GGBB]

As Mark has said, the only convention I know of is to tie the wiper to the "unused" outer lug so as to prevent signal loss or intermittence in the case of a wiper contact failure. Beyond that, all I know is that the electrons surely don't care which way around the pot is used. For the same reason we don't distinguish between the ends of a simple resistor either. My convention is, when doing a layout, to use whatever orientation fits the layout better.

[Kipper4]

Thanks Mark
Just to clarify the question is top to wiper, bottom to wiper?
Does it matter etc?

[Kipper4]

Good point Gordon my convention on layouts is keep it simple. Have one wire to the pcb and solder tie the lugs at the pot with a wire. This also helps if I got it wrong and it needs reversing.
I figure one less trace one thing less to go wrong.

[GGBB]

Thanks Mark
Just to clarify the question is top to wiper, bottom to wiper?
Does it matter etc?

It matters if by top you mean lug 3 and by bottom you mean lug 1. Lug 3 to wiper causes resistance to increase with clockwise rotation. Lug 1 to wiper cause resistance to decrease with clockwise rotation. The circuit itself will dictate which is correct. But which way around the pot goes doesn't matter when used as a variable resistor.

[Kipper4]

All good points thanks guys.
Perhaps the time has come for me to learn my clockwise to anti-clockwise.
If anyone cares to explain I'm listening.
Maybe this will help future understanding of my schematic.
And also be a good lesson for me.
Maybe there's a thread containing this info already.
Cheers

[Ben Lyman]

here's one way to look at it (at least with all my Alpha pots)

[GGBB]

Love that!

Expanding...

The CCW side is lug #1, wiper is #2, CW side is #3. That relates to schematic symbols as follows.

These ones are obvious:


Sometimes clockwise direction is indicated using only an arrow:


If lug numbers or an arrow aren't show, then usually (but not guaranteed), when "looking" down in the direction of the arrow, left is counterclockwise (lug #1) and right is clockwise (lug #3). Think of it as if the arrow were a car and the pot was the steering wheel. Examples:

From left to right - 1,2,3:


These two are in opposing directions:

[Kipper4]

Thanks guys that's brilliant.
I couldn't find much definitive with a Google or forum search.
That's really clear.
Kudos to you.

[Ben Lyman]

That is some very cool info GGBB. I've always wondered about all those symbols and also when there's no symbols, thanks!

expanding further: when I look at a schematic I (try) to discern what more or less resistance is doing and which way I want my pot to turn in order to create the desired effect.

In my trem/vibe I wanted more resistance on the CW turn to increase the tremolo effect (fig. A)
For the speed and depth pots, I wanted less resistance on the CW turn to increase the speed and depth (fig. B)

so if 1,2,&3 are CCW,W,&CW respectively

[Kipper4]

I'm loving the post it notes Ben my old buddy.

[Ben Lyman]

Cool Rich, here's a couple more for ya! It's just a way to visualize and try to understand the pot schematic symbol... at least inside my weird brain 

[MrStab]

here's something related to this i've sometimes wondered about:

when i'm making a PCB with a pot wired as a VR, i like to make the trace split off to either lug instead of just having em connect right next to each other, in the off-chance the trace breaks close to the pot and both connections are severed. like this:



overkill, no doubt, and i wouldn't do it if i was tight on space, but probably no more paranoid than connecting using the unused lug in the first place. anyone else do this? i'd imagine, at the very least, the trace should meet the outermost lug to get the full benefit of the "full resistance" fail-safe.

[duck_arse]

ben, in this dia:


full ccw would be "short" circuit, not open.

[edit :] ahh, geeze, full CW would be short, not open.

[Ben Lyman]

oh, thanks duck. ya, I still don't know all the terminology yet. So, short circuit it is. In my brain, the wiper represents a jumper wire moving over the resistor. Then I know which way to connect the pot lugs to get the desired effect.

[PRR]

> in the off-chance the trace breaks close to the pot and both connections are severed. like this:

Sometimes two nails is enough.

Sometimes you should use a bigger board.

_I_ submit that, if you fear pads and traces breaking, you should leave more copper there. And avoid sharp corners where cracks start.



A limit comes when your iron can't heat all that copper. Links into ground-planes often need some copper-gap around them to limit heat-suckage and thermal strain in use. But most pot-pads don't need all that much copper.

Yes, some stupid layout software does not make arbitrary pad shape easy. I say that's a software fault (does not do what the user/buyer needs). Fight or switch is your decision.

[Kipper4]

Whats the difference?

both get signal at lug 3.

[duck_arse]

turn the pot in the left hand circuit "up". which direction did you turn? now turn the pot in the right hand circuit in that same direction, and describe the resistance change.

[Ben Lyman]

Only difference I see is the direction you turn the pot to increase/decrease resistance.
Figure 1 will lower the signal as you turn the knob up (CW=more R)
Fig 2 will raise the signal as you turn the knob up (CW=less R)
I think...?

EDIT: beat by duck!