Limiting a 50K pot to 35K - Page 1
Forum section: electronics / beginners

author: Stonent
author_rank: Super Contributor
timestamp: November 07, 2013, 09:03:06 pm
content: I've got a 50K linear pot but I really need a 35K. It will be used as a voltage divider. I won't ask you to figure out the resistor I need in order to do that. I think I can probably figure that out. I'm just wondering if by doing that will I end up with a lop-sided resistance? What I mean is I'll have a range from 0 to 35K but will the center be 17K5 still, or will I have a 0 to 35K range but an uneven resistance across the range?

author: Lightages
author_rank: Unknown
timestamp: November 07, 2013, 09:08:13 pm
content: You will have a very non linear pot in the end. Depending on the configuration you choose you will end up with a semi-log pot or and inverse semi-log. It is not a practical thing to do if you want linearity.

author: JoeyP
author_rank: Frequent Contributor
timestamp: November 07, 2013, 09:09:46 pm
content: If you use two 58.3k resistors - one from the wiper to each end of the pot, then the mid point will be 17.5k. However, it will not be linear across the entire range. If you need full linearity across the entire range then this won't work.

author: Simon123
author_rank: Regular Contributor
timestamp: November 07, 2013, 09:16:57 pm
content: What if you rewind wire wound potentiometer?

author: Clint
author_rank: Regular Contributor
timestamp: November 07, 2013, 09:25:28 pm
content: Depending on the construction of the particular Pot you are using and how much control you need you could reduce the wipe however if you Pot has a 270 degree wipe trying to lose 81 degrees is too much (dont forget the method of losing the wipe need to be wrapped tin so it makes contact and can be soldered to the leg to make it stable. However if you could find a 40k ohm pot that would be quite achievable. Now if you had sliders.... easy peesy - any other way you will make it not have the ability to go min or max.

author: Stonent
author_rank: Super Contributor
timestamp: November 07, 2013, 09:27:11 pm
content: The final intention was to be the amplitude adjustment on a xr2206 based func. gen.I may try with just the 50k and see what it looks like. It is just for personal use so I can live with any faults if I have to. I guess I could insert a physical stop if needed.It is a standard panel mount pot with a 1/4" split shaft.

author: Jebnor
author_rank: Regular Contributor
timestamp: November 07, 2013, 09:31:56 pm
content: You can use the Parallel resistance formula to find the parallel equivalent resistance. Rpar=1/(1/35k - 1/50k) ~= 117k.The resultant is fairly linear. I attached a graph. The Correleation factor between them is 0.995. So, pretty linear.

author: Stonent
author_rank: Super Contributor
timestamp: November 07, 2013, 09:41:15 pm
content: Hmm that's not bad.

author: alanb
author_rank: Frequent Contributor
timestamp: November 07, 2013, 09:42:28 pm
content: As the pot is being used as a voltage divider you could probably substitute the 50k for the 35k, provided that the loading from the next stage is not too great there should not be a problem. If the ends of the pot are in series with other resisters you will need to adjust their values accordingly, multiplying them by 50/35 should do the trick.

author: Zbig
author_rank: Frequent Contributor
timestamp: November 07, 2013, 09:44:16 pm
content: When I broke a transmitter for a RC helicopter once, I was surprised when I discovered that they used some weird pots with reduced min-to-max angle for reading the gimbal deflection. They prove impossible to find. I ended up disassembling a normal pot and painting a part of the resistive part with a conductive, silver-based lacquer used for repairing the heating of the car's rear windshield.

author: edpalmer42
author_rank: Super Contributor
timestamp: November 07, 2013, 10:19:02 pm
content: Quote from: Stonent on November 07, 2013, 09:03:06 pmCan you show us more of your circuit? Depending on what's around it, you might be able to end up with a linear pot. For example, a 120K resistor across the two ends of your pot will make it look like a ~35K pot. If one end of the pot is the input and the other end is grounded, the output on the wiper will be linear as long as you're not drawing significant current out of the wiper. Depending on the function you're looking for, a different configuration might work better.Ed

author: Stonent
author_rank: Super Contributor
timestamp: November 08, 2013, 05:59:20 am
content: Quote from: edpalmer42 on November 07, 2013, 10:19:02 pmI don't have it handy at the moment but I think it was just the two outer pins going straight to the pins on the main chip and the wiper going to ground.

author: dr_p
author_rank: Frequent Contributor
timestamp: November 08, 2013, 07:52:04 am
content: Quote from: Stonent on November 08, 2013, 05:59:20 amThat's the symmetry adjustment, IIRC. The amplitude has some more connections.Anyway, I think you can use the 50k instead of the 35k with no problems. Just try it out.

author: grumpydoc
author_rank: Super Contributor
timestamp: November 08, 2013, 08:25:52 am
content: Quote from: dr_pWell, in the original question Stonent said"It will be used as a voltage divider"so it shouldn't matter at all that it's 50k since it's the ratio of the two limbs that matters, not the absolute value.Unless, that is, the pot itself is part of a bigger voltage divider in which case it might be necessary to adjust the resistances around it to maintain the same voltage across the 50k pot (or the same current through it).EDIT: Ah, I see alanb already said that!

author: Zero999
author_rank: Super Contributor
timestamp: November 08, 2013, 10:38:08 am
content: Yes you should be able to alter the value of the other resistors in the potential divider in order to achieve the correct ratio with a 50k pot.The problem is potentiometers tend to have a very poor tollerance (+-20% or worse) so it's still not reliable. To get round this, you could put a trimmer in series with one of the other resistors in the divider.

author: Stonent
author_rank: Super Contributor
timestamp: November 08, 2013, 11:00:47 am
content: Ok a couple of things, my memory was off on this and yes it was symmetry and not amplitude. I've been looking at different schematics and it looks like there have been different values of pots used from 20K to 35K some straight to ground, some with a 1K between the wiper and ground.I think I'm having such a hard time remembering the details off the top of my head is my version will have features from a few different schematics. Some only go to 100KHz, some go to 2MHz. I want 2MHz. One has a nice filtered input with reverse voltage protection. I want that. Another goes down to 0.1Hz, I'm going to add that. And another has an input function which I'm not sure exactly what it is for but the chip is capable of it, so I'm going to end up with it.Maybe I'd better print all the schematics out and come up with my own schematic and go off of it.