Simple resistor in parallel with pot makes taper wierd?

[Triffid]

I am simply trying to decrease a pot value using a resistor in parallel with pot lugs 1 and 3.  The total resistance (mm on 1 and 3) reduces as expected, as does the resistance with the pot maxed and mm on 1 and 2.

The problem i am having is that the taper has a wierd drop right and the end as pot is maxed out.  So the taper actually looks something like....

Code Select Expand


              /\
            /    \
          /
        /
      /
    /


Maxing the pot turn doesn't actually max resistance, i have to back it off a bit to get max.

I hope my problem makes sense.  Anyway... this is for the volume on one of my effects so that drop is not desirable... am i doing something wrong?

Thanks in advance

[aaronkessman]

eh you shouldnt be getting that. might be a bad pot, or somehow damaged. I've done it recently without a problem.

if you have Excel, do this for a 10ohm pot whjich you want to make 5ohms with a 10ohm parallel resistor:
type in a column of 1-10 - these are the resistances on your make believe pot. in the next column, type in =1/(1/D8+1/10) and control-D it to fill the column (D8 corresponds to you "1" cell, and that 1/10 is for the second parallel resistor.

graph what you get and you'll see how the tape changes. use whatever values you want for the pot and resistor.

eh, sort of a confusing description, but you're using R =1/(1/r1 + 1/r2) for parallel resistors...

shouldnt give any sort of thing like youre getting.

[Skreddy]

Actually this is what I would expect if you're using a resistor across lugs 1 and 3.  If you're trying to alter the taper and/or lower the value of a pot, the correct placement of the parallel resistor is across the wiper and whichever end you're trying to reduce the resistance at (e.g.; across lugs 1 and 2 in this instance).  With the resistor across lugs 1 and 3, the resistance goes up as the wiper travels near the middle and back down again towards the ends.  If the resistor doesn't do what you expected across lugs 1 and 2, then try it across lugs 2 and 3 instead.  One of those two ways will do what you want.

[Triffid]

I thought it should be ok too but I tried it with 2 different pots (one radio shack and one from small bear).  

I wasn't quite sure what you meant with excel... but I charted the values i got at about 8 turns of the pot and it looks like it is doing a reverse log up a point, then curving back down at the end, like described before.  

I have tried log pots and linear pots with the same effect.

I am just using random value resistors... 50k pot with 22k resistor for example... does that matter?

Thanks again

[Triffid]

Responding to Skreddy...

Thanks for you help... that is what i tried from the beginning... but doing it that way made the total resistance (across 1 and 3) fluctuate as the pot turned too... .i think i need that value to be constant.  Does that make sense?

Thanks again

[Skreddy]

Oh.  Then I guess you just have to get a pot that's the value you're seeking instead of modding one with external resistors.   What value do you need?

[Triffid]

Well, its more of a matter of switching one value pot with another on the fly.  I have 500K and I want 100K....  so 125K should do it.   I wanted to experiment with a switch to put he 125K resistor in parallel.  I haven't really tried yet, i might just try it and see how it sounds.  Thanks for your help.

[ezanker]

Try out this calculator: http://www.diystompboxes.com/analogalchemy/emh/emh.html

basically yout use 2 resistors, 1 from lug 1 to 2, and the other from lugs 2 to 3.  The calculator will give you the 2 resistor values needed for the new pot value.

Erik

[Triffid]

Awesome... great link man... i'll try it out soon

[Skreddy]

Try out this calculator: http://www.diystompboxes.com/analogalchemy/emh/emh.html

basically yout use 2 resistors, 1 from lug 1 to 2, and the other from lugs 2 to 3.  The calculator will give you the 2 resistor values needed for the new pot value.

Erik

Sweet.

[Triffid]

Thanks again, that worked great

[petemoore]

Put a knob on the pot, turn it half way, take a reading.
 Use the DMM to see whappens to the values and taper as you go, adding R's to change taper.
 From lug 1 to 3 [pot's value]
 from lug 1 to 2 and 2 to 3 ['shows' taper]