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| Really struggling with limiting potentiometer range |
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| Pentoad:
I am trying to limit the range of a trimmer, R3 so that when the supply voltage is 4.2 and its wiper is closest to R4, or 0%, the output voltage is 1.225v. Additionally, when the supply voltage is 3.81v and the trimmer is set closest to R2, or 100%, the output voltage should be 1.225. I've tried several different calculations and methods but none have given anywhere near the desired results in the simulations. How would I determine what resistors to use for R2 and R4 to give me that 3.81 to 4.2 adjustment range? The 1.225v is the voltage of my reference and when the output of R3 goes even a millivolt above 1.225v, it triggers a comparator and so the current on the output would only be a few nano amps. |
| magic:
Pretty simple. You want the output/input ratio to vary between 1.225/3.81 = .3215223097 1.225/4.2 = .2916666666 0.29 is the lower limit and the proportion of the whole divider that goes permanently into R2. Then 0.32-0.29=0.03 is adjustable and goes into R3. The rest is R4. Then all numbers need to be scaled proportionally, for example by 333 times to use a 10Ω potentiometer. |
| Pentoad:
I tried your suggestion and it is not working right and I am beginning to think there may be an issue with the simulation program. As you can see, I can get the lower threshold to work well but the higher threshold is needing 56.5 volts to give 1.225. |
| mikerj:
--- Quote from: Pentoad on January 25, 2020, 08:22:29 pm ---I tried your suggestion and it is not working right and I am beginning to think there may be an issue with the simulation program. As you can see, I can get the lower threshold to work well but the higher threshold is needing 56.5 volts to give 1.225. --- End quote --- The resistor values are correct, so I share your thoughts about the simulator. I notice you have the pot set to 99.9% for the case that works correctly; is there a reason for not setting 100%? i.e. does the simulator have problems with 100% and if so does it also have issues with 0%? |
| Pentoad:
The simulator doesn't allow 100%, it only allows you to get as close as possible to 100. 0% works fine so I think I will submit a bug report to them. |
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