Author Topic: A very basic potentiometer question  (Read 1423 times)

lborl

A very basic potentiometer question
« on: March 19, 2018, 08:17:03 AM »
I'm almost embarrassed to ask this but I can't seem to work it out myself.

Say you have a layout that calls for a 100k pot - let's say for gain - and you want to leave that pot out of your build so that the gain will be always maxed. From what I understand on other people's questions on the subject, you'd just replace that pot with a fixed 100k resistor because potentiometers are just variable resistors, right?

But. If the potentiometer is 'maxed' in the sense that guitarists mean, with the gain pot turned right up, surely it's providing no resistance at that point? Doesn't the 100K indicate the maximum resistance the potentiometer can present, that is to say when it's turned right 'down'?

Sorry for asking such a basic question but I haven't been able to find it answered anywhere else

antonis

Re: A very basic potentiometer question
« Reply #1 on: March 19, 2018, 08:28:38 AM »
It depends on the way pot is wired..

If it's wired as variable resistor with lugs 1 & 2 shorted, then there is no resistance at max set..
If it's wired with lugs 2 & 3 shorted, then there is full resistance at max set..
If it's wired like "pure" pot, then you have to consider output resistance according to lugs taken of.. 

because potentiometers are just variable resistors, right?
Nope..
Rheostats are just variable resistors and act as current restrictors..
Potentiometers act as voltage dividers but they can ALSO be wired as variable resistors.. :icon_wink:

http://www.differencebetween.net/technology/difference-between-rheostat-and-potentiometer/


P.S.
Don't be embarrassed till your next query about the reason for shorting pot lugs instead of leaving then open.. :icon_wink:
« Last Edit: March 19, 2018, 08:45:34 AM by antonis »
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

Joncaster

Re: A very basic potentiometer question
« Reply #2 on: March 19, 2018, 08:56:19 AM »
That "maximum resistance" is in reference to ground.

The signal has two paths: one towards the output, and one towards ground (no volume)
If the way towards ground is blocked by 100K, it'll go out in full glory, and if the output is blocked by 100k, it'll go to ground.
Half way, it gets softer cause half of it goes to ground, half of it out.

What I did for replacing a volume pot with fixed resistors was to mimick the pot being turned down to about 3 o' clock (rather than maxed at say, 5 o' clock).
For a Log pot that would mean roughly a 50/50 split in resistance value. Lin pot would be 80/20 split.
So a 500K pot can be replaced with two 250k in series, take the output from the middle of that.

So just a regular voltage divider then (used to great effect in tube amps between gain stages)

This changes the output impedence and offers some high frequency roll-off (or is it a shelf? can't remember) if you need it.
Found it a bit nicer than a bleed cap across the output if I wanted less treble. Maybe cause of the filter curve?
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GibsonGM

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Re: A very basic potentiometer question
« Reply #3 on: March 19, 2018, 08:58:26 AM »
No embarrassment!   Welcome to the forum.

Look at the typical volume pot this way...the signal 'goes in at the top', and comes out from the wiper, the middle terminal of the pot.  If the pot is all the way "down", the wiper is very near to, or touching, ground...so there would be 100k or whatever value the pot is in the signal path, AND the wiper is at or very close to ground, so the signal flows to ground - when it is shorted, you get no output.

At middle settings, you might have 50k "above" the wiper, and 50K "Below" it.....this VOLTAGE DIVIDER action means that 1/2 the signal gets to the output, and 1/2 to ground.   
Calculated by.......        (R2/R1+R2) * Vin   

When the wiper is all the way "up", there is a straight path out, and 100k between that point and ground, which prevents 'leakage' of the signal to ground (tho this LOADING does have effects....we'll ignore there here).   It is easier for the signal to go 'out' than to get thru that resistance, so out it goes.     Make sense?  :)
MXR Dist +, TS9/808, Easyvibe, Big Muff Pi, Blues Breaker, Guv'nor.  MOSFace, MOS Boost,  BJT boosts - LPB-2, buffers, Phuncgnosis, FF, Orange Sunshine & others, Bazz Fuss, Tonemender, Little Gem, Orange Squeezer, Ruby Tuby, filters, octaves, trems...

antonis

Re: A very basic potentiometer question
« Reply #4 on: March 19, 2018, 09:15:43 AM »
Welcome to the forum.
Ahaaaa...!!!

So now we wellcome people even after their 4th post...!!!

P.S.
I've a feeling you answer mostly for wellcoming and less for quering definition, Sir..
(but I presume it's quite normal behavior for such a scorching climate guys, like you..) :icon_redface:
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

lborl

Re: A very basic potentiometer question
« Reply #5 on: March 19, 2018, 09:54:35 AM »
Interesting. Thanks, that certainly makes some more sense.

So, sorry, just to see if I've understood, with that same 100k Gain control on a layout:

- If lugs 2&3 are joined a 100k resistor will be the same as that pot as max setting

- If lugs 1&2 are joined you can leave the resistor out completely

- Otherwise you need something in between 1k and 100k (or whatever) to hardwire a specific setting?

antonis

Re: A very basic potentiometer question
« Reply #6 on: March 19, 2018, 09:59:28 AM »
True and Correct..!! :icon_wink:
(if by max setting you mean full clockwise..)
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

GibsonGM

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Re: A very basic potentiometer question
« Reply #7 on: March 19, 2018, 10:05:37 AM »
Welcome to the forum.
Ahaaaa...!!!

So now we wellcome people even after their 4th post...!!!

P.S.
I've a feeling you answer mostly for wellcoming and less for quering definition, Sir..
(but I presume it's quite normal behavior for such a scorching climate guys, like you..) :icon_redface:

I get bored, Antonis.  I note that most other people provide "Other explanations", too, so I figure that the more information, the better :) In fact, many times I answer something, and someone later says the same thing, without realizing someone has 'been there'.  Oh well.

Please come back in and discuss pots further, there is plenty of room for discussion!

If I haven't said 'welcome', I probably will do so up until about 5 posts, so lborl is lucky, he just made it!

I leave you in Antonis's hands, lborl...
MXR Dist +, TS9/808, Easyvibe, Big Muff Pi, Blues Breaker, Guv'nor.  MOSFace, MOS Boost,  BJT boosts - LPB-2, buffers, Phuncgnosis, FF, Orange Sunshine & others, Bazz Fuss, Tonemender, Little Gem, Orange Squeezer, Ruby Tuby, filters, octaves, trems...

lborl

Re: A very basic potentiometer question
« Reply #8 on: March 19, 2018, 11:20:47 AM »
Awesome. Cheers everyone