How do you bypass pots?

[cruisemates]

I started building pedals about a year ago using the Vero layouts at TagboardEffects.com, but for some D**M reason potentiometers still trip me up in theory. I have read RG's "Secret Life" and the "Beavis" tutorial  --- but I still have questions.

RG - much of it over my head, meaning the one basic thing I needed to know was not there although I learned a lot of stuff that will help in the future.

Beavis was simpler, but I am still confused.

Okay a simple rheostat I can understand - lug 1 gets current - the resistance is varied by the position of wiper 2 (depending on the amount of resistive material) - so - whatever is between 1 and 2 is the resistance I have working at the moment. However, in many of rheostats they also connect lug 3 to the wiper - as a "backup". Beavis says "now, the resistance between lugs 1 and 3 never changes" so, if I have a 100k pot - it sounds like he is saying the resistance will always be 100k between 1 and 3 no matter what.

So - if when using the pot as a rheostat between 1 and 2, if 2 fails - then the resistance goes up to full value, correct? in a 100k pot, that will always be 100k because the wiper is no longer shorting the signal path. Is this correct?

So, here is my question. Doing vero projects (not pre-made PCBs) gives me more flexibility, so I would like to be able to just take certain pots out of the circuit - but I do not know how to do that in any given situation.

Let's say I want the circuit act as of the pot is always full on. Do I merely jumper the connections where the rheostat would have gone, or do I put in a resistor of the same equivalent value as the pot? Now, I am guessing you will say "it is conditional" - in some cases the pot is full on when there is NO resistance (The #2 trim is at the same point as the incoming signal) - in some cases (a rheostat) the pot is full on when the incoming signal and wiper are at opposite ends.

Is there a quick and easy logic table to know how to bypass any given potentiometer?

• If the put is full on when it has the most resistance you.... (use the equivalent resistor)


• If the pot is full on when there is no resistance you (use a jumper)


• If you like the sound somewhere in the middle of the rotation you (measure the R and use that value resistor)

Is it that simple? I have not yet seen an article on "how to bypass pots in circuits"

I guess a simple answer would be "just set the pot where you want it, measure the resistance at the point and use that" - but really, I am guessing and I would rather a more definitive answer. Thank you in advance.

[Brisance]

I guess a simple answer would be "just set the pot where you want it, measure the resistance at the point and use that"

That's also the best and correct answer, since you need it set at something, when used as a variable resistor, replace with a suitable value resistor, when used as pot, two appropriate resistors.

[merlinb]

So - if when using the pot as a rheostat between 1 and 2, if 2 fails - then the resistance goes up to full value, correct? in a 100k pot, that will always be 100k because the wiper is no longer shorting the signal path. Is this correct?

Yes. The idea is that it is better for a fault to leave you with 100k than infinity ohms.

So, here is my question. Doing vero projects (not pre-made PCBs) gives me more flexibility, so I would like to be able to just take certain pots out of the circuit - but I do not know how to do that in any given situation.
Now, I am guessing you will say "it is conditional"
Is there a quick and easy logic table to know how to bypass any given potentiometer?

• If the put is full on when it has the most resistance you.... (use the equivalent resistor)


• If the pot is full on when there is no resistance you (use a jumper)


• If you like the sound somewhere in the middle of the rotation you (measure the R and use that value resistor)

Is it that simple?

Yes, it is.
So yes, it is conditional, and no there isn't a table for this sort of thing. A pot is a simple device -it's just a resistor with a wiper. If you wire it as a variable resistor (rheostat as you call it) then its simpler still. You're only option is to set the pot where you want it, then measure and replace it with resistor(s) that replicate the pot at that particular setting.

As a professional engineer I still find myself tack-soldering a trimpot into a circuit, adjusting it, then removing it and measuring the result. Then I pick the nearest standard value resistor and solder that in permanently. There's no shame in pragmatic design.

[cruisemates]

Thank you both - I find myself wanting to eliminate pots quite a bit in builds (and no shame at all, in fact, double checking everything is a habit I try to maintain).

I get it now. thanks. I guess some simple things are not explained because more advanced people see them as being so logical.

[PRR]

While we use the same 3-leg part for either, do NOT confuse a rheostat for a potentiometer.

You have the idea that a rheostat is one variable resistor, true.

It is not clear that you understand that a *potentiometer* is TWO resistors, permanently connected together, both variable, but the *sum* of the two resistors is always a constant (the nominal value).
_______

> Beavis says "now, the resistance between lugs 1 and 3 never changes"

And you believed that??

Don't believe anything on sight.

There are many fools on the web (also a lot of semi-fools in books).

Beavis is no fool. But even wise-ones can think one thing and type another. Or they know the subject so well they omit details you are not aware of. As you say: "more advanced people see them as being so logical".

This fool says: The 3-leg pot, when you buy it, will be 100K from 1 to 3 no matter what you do with the wiper. BUT once you connect the wiper to either end, your results WILL vary. Beavis' statement could be ambiguous, depending which way you look at it.

After reading-- Check it. Think about it. Try to disprove it.

Some facts you just can't check on your own. What is the height of Mt Washington? It can be measured, but with traditional survey it needs many men and costly time.

Some facts CAN be checked at home for small effort. I'm building a garage. The sales-sheet says my car is 191 inches long. Is that true? Maybe there was a running change. Maybe my bumper has been bumped (out?). I can go in the driveway, hold a stick front and rear and make scratches in the dirt, then run a tape measure between.

The "action" of a pot's three terminals doesn't even require you to go outside. Just need a pot and an ohm meter. For fast fun, get two or three ohm meters so you can meter all points at once.
_______

> simple answer would be "just set the pot where you want it, measure the resistance at the point and use that" - but really, I am guessing

What guessing? Diddle a pot until happy. Take the pot out and measure *both* resistances. Say you get:
1-2 = 20K
2-3 = 80K

Look in your drawers and find a 22K and a 82K. Mount then at 1-2 and 2-3 just like your pot-notes. Done. Perfect.

Note that I picked "standard values" in the 10-15-22-33.... series. 10% off in value is usually nothing in audio. But do not pick 1-2 10% high and 2-3 10% low. That could be a 20% error in pot *ratio*, which is often the most important parameter of a pot-setting.

--- using my calculator, I see that 91K may be a better fit. Too much bench experience tells me it is rarely worth using the calculator. Also I am less likely to have a 91K than an 82K.

[cruisemates]

Interesting comments.  As far as beavis's comment goes -  I brought it up to ask for clarification which implies I was questioning, not believing. 

But in all Fairness to Beavis, I also understand that he is writing in a conversational tone, and that taking statements out of context is not an accurate portrayal of what he may have said.

[Brisance]

I made a drawing, which might help to better grasp it:


Also A potentiometer will inevitably be hard to grasp unless you understand the concept of voltage dividers, which is of course essential knowledge.

Maybe read this or similiar:
https://learn.sparkfun.com/tutorials/voltage-dividers

[cruisemates]

Yes - thanks Brisance - I know that concept but didn't fully appreciate the "two separate rheostats" idea before.

Voltage division make sense to me, plus, I can follow examples and use pots in layout (like at tagboardeffects.com), so I have no problem understand how to bias FETs, for example. I just think I really needed to understand more about pots in a circuit design perspective (not how pots are designed, but the different ways pots fit into a circuit's design).

One thing that still throws me off is "why do you ever need a reverse taper pot?" - can't you just use the other lugs so the gain goes down instead of up? Like you just said, one side goes down as the other goes up - so why do you need a reverse taper pot? But see, I am sure there is a good reason, or else they wouldn't exist.

It's hard to see some things being newbie, and sometimes the challenge is making sure what you "think" you know is actually correct. I am trying to avoid making false assumptions, so I swallow my pride and ask. It's like understand rectifiers in tube amps, I found out I was under the wrong impression about how they work, and I needed a tutorial, and I got what I needed by asking (that was in another forum).

[PRR]

> "why do you ever need a reverse taper pot?"

It is electrically the same.

The difference is the user interaction.

Setting a straight pot below 10% or 5% is tricky.

A wide-range volume control "wants to be" 100% at top, 10% near the middle, and adjustable to 1% without much fooling.

We do this by "tapering" the resistance element. Instead of 100K spread out evenly over the whole rotation, the "bottom half" is 10K and the "top half" is 90K. Now "50% rotation" is 10% electrical. 1% electrical is 5% rotation, a usable zone.

If we'd used a Linear, then 10% is "1" on a 0-10 dial and 1% is "0.1", which in practical pots is the crack between slim and none and much too twichy.

OK, we have a passable wide range volume control.

But there are two cases where we want the "reverse" of that. When gain is trimmed in the shunt leg of a NFB amplifier, with a rheostat (commonly implemented with 2 of the legs of a 3-leg pot), lower resistance is higher gain. If we try this with a "Audio taper" we can reverse the connections, but now "the knob rotates backward". This is utterly arbitrary human habit: while radio volume knobs go one way, the "flame volume" knob on a gas stove turns the other way (clockwise for less). However we can't break this clockwise-up habit when staring at audio knobs, so we demand "Reverse Audio" where the 10K and 90K parts are the other way. A similar thing for many Frequency knobs: either the bass range covers most of the dial and all the highs are bunched-up in the last crack, or we use Audio taper and mark the knob backward, or we demand Reverse Audio taper.

Sorry if I was too blunt.

[cruisemates]

Thank PRR - that was the perfect explanation. I hadn't thought about log pots - of course they need to be reversed to get the right direction, because as you said - everyone has expectations that things will always work the same way. One would be crazy to put out a pedal where the pots are reversed.

In fact - the passive tone controls on the Paul Cochrane Tim drive me crazy (they are full on when they are full CCW) - a simple fix would have been to use reverse taper pots. Few users would have even realized they're passive.