Question about guitar pots wiring

IVRZ

Maybe someone can give me a clear explanation as ive been unable to find one on the net.
A guitar pot (potentiometer) has 3 lugs. ! input, 1 output, and 1 what?
I understand that lug 1 is the input (thats the first lug on the right looking at the pot with the knob up), lug 2 (the middle lug) is the output.
What does number 3 do? It seems to often be soldered to the casing of the pot.

All help and explanations appreciated
Thanks:)

Fingers Mcginty

A pot (potentiometer) is a variable resistor. Eg a 10K pot which has 3 lugs would be wired as follows. One lug is connected to one side of the internal resistor the other lug wired to the opposite end of the resistor. If you were to connect a DVM to these two lugs you will always measure 10K ohms. The other lug which is sometimes the middle lug but not always is the wiper which is the variable part of the resistor so between this and either of the other 2 will vary depending on the shaft position.

IVRZ

Thanks Fingers, i think i follow that. But, why do we need two "non-variable" lugs,( if im putting this correctly)? Couldnt we just have the one in and the wiper out, since all we need is variable resistance?

Also, how do I find out which lug is which?

ciaranmac

IVRZ wrote: »

Thanks Fingers, i think i follow that. But, why do we need two "non-variable" lugs,( if im putting this correctly)? Couldnt we just have the one in and the wiper out, since all we need is variable resistance?

Also, how do I find out which lug is which?

The middle one is the wiper.

Hope I'm right about this bit: The resistance between the middle and either end is at its minimum when the knob is turned towards that end and vice versa. If your circuit needs the '1' to be low resistance and the '10' to be high resistance, you connect the wiper and the lug on the '1' side, and you leave the '10' lug alone.

There are three lugs because there are lots of circuit designs that use both ends of the fixed resistance.

IVRZ

Brill Ciarnmac, makes sense to me thank you. Any idea why the unused lug is usually wired to the body of the guitar pot?

Fingers Mcginty

I think the reasoning behind the 3 terminals is because in a standard electronic circuit it is used more of a potential divider ie say for instance 10V and 0v are applied to the fixed resistor and then one can tap off any voltage desirable via the wiper but this voltage can be referenced to 0V or 10V . That's how I understand it anyway but if you're really curious you could google it?

To find out which lug is which you'll have to connect a dvm to a pair at a time. When you find the pair which doesn't indicate any resistance change when you move the shaft of the pot well then these two are fixed resistor. The other lad will be the wiper. Hope this makes sense mate!

Fingers Mcginty

ciaranmac wrote: »

There are three lugs because there are lots of circuit designs that use both ends of the fixed resistance.

Correct!

IVRZ

Im nearly there guys:). Last bit to go. Why is one fixed resistor soldered to the pot body in guitars or attached by one of those little green square things?

By the way you two leave Google in the shade!

Paolo_M

IVRZ wrote: »

Im nearly there guys:). Last bit to go. Why is one fixed resistor soldered to the pot body in guitars or attached by one of those little green square things?

By the way you two leave Google in the shade!

One lug is the input, one lug is the output and the final lug is ground.

A voltage can only be formed in reference to another and ground is used as reference voltage.

The back of the pot should have a connection to the output jack and bridge of the guitar. The connection to the amp provides the ground reference as it's internally connected to the neutral of the plug, which of course is in turn actually physically wired to earth.

Earth is used as you are standing on the earth. With the fuse in place now any time a voltage becomes present other than earth (your voltage) then the fuse will blow before it can shock you. Current only flows where there is a difference in voltage.

Ground also provides screening against noise for reasons too complicated to get into here.
Ground saves your life and makes your guitar sound good, it's real important.

All three lugs have to be used on a potentiometer for it to function as a potentiometer. The voltage to ground present at the output (wiper) compared to ground will be a fraction of what is present at the input (again compared to the same ground). That fraction depends of where you turn the pot.
Remember voltage only exists in reference to another. In guitar world that's almost always in reference to ground, as in the voltage level of the earth.

Someone mentioned that you can use two lugs. This is true but that forms a variable resistor or rheostat depending on it's exact usage, it does not perform the function of a potentiometer.

Fingers Mcginty

Paolo_M wrote: »

One lug is the input, one lug is the output and the final lug is ground.

A voltage can only be formed in reference to another and ground is used as reference voltage.

The back of the pot should have a connection to the output jack and bridge of the guitar. The connection to the amp provides the ground reference as it's internally connected to the neutral of the plug, which of course is in turn actually physically wired to earth.

Earth is used as you are standing on the earth. With the fuse in place now any time a voltage becomes present other than earth (your voltage) then the fuse will blow before it can shock you. Current only flows where there is a difference in voltage.

Ground also provides screening against noise for reasons too complicated to get into here.
Ground saves your life and makes your guitar sound good, it's real important.

All three lugs have to be used on a potentiometer for it to function as a potentiometer. The voltage to ground present at the output (wiper) compared to ground will be a fraction of what is present at the input (again compared to the same ground). That fraction depends of where you turn the pot.
Remember voltage only exists in reference to another. In guitar world that's almost always in reference to ground, as in the voltage level of the earth.

Someone mentioned that you can use two lugs. This is true but that forms a variable resistor or rheostat depending on it's exact usage, it does not perform the function of a potentiometer.

the big guns have arrived

IVRZ

Holey Moley!
So the third lug (not input or wiper output) is connected to ground, making a connection between me and the earth, via the amp?
Is that right?

And now maybe i need to ask What does "potentiometer" do that a variable resistance doesn't do?

IVRZ

Oh well, maybe that doesnt matter. Im off to try and wire up me guitar now. Thanks for all the help everyone and wish me non-electrocution !:eek:

Paolo_M

IVRZ wrote: »

Holey Moley!
So the third lug (not input or wiper output) is connected to ground, making a connection between me and the earth, via the amp?
Is that right?

And now maybe i need to ask What does "potentiometer" do that a variable resistance doesn't do?

That's right, it's what keeps you safe. You're probably already connected to earth anyway unless you're levitating while playing!!

A potentiometer divides the input signal by a fraction set by the rotation of the pot, and sends this reduced signal to the output.
Variable resistance simply varies the load on the source. This can function to reduce the gain of the source making it appear to work like a volume control but not what's really happening.