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| Really struggling with limiting potentiometer range |
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| bson:
--- Quote from: Nerull on January 26, 2020, 06:18:59 pm ---SPICE also isn't case sensitive, you'll have the same M/m problem in LTSpice. SPICE requires a 5M resistor to be written as 5MEG. --- End quote --- That's the because the punch cards it was designed run off of didn't have mixed case. ::) |
| Pentoad:
--- Quote from: SiliconWizard on January 26, 2020, 04:10:30 pm ---You don't need a simulator for that. (But Easyeda sucks anyway. Online tools are for the birds. ;D ) Just put it in equations: 4.2*R4/(R2+R3+R4) = 1.225 3.81*(R3+R4)/(R2+R3+R4) = 1.225 Solving this yields: R2 = 2068*R4/889 R3 = 13*R4/127 If you want R3 to be the fixed value from which to derive the two others (which makes sense as you'll have a lot less choice for pot. values): R4 = 127*R3/13 R2 = 2068*R3/91 (Then of course you'll have to select the closest values that exist in a given E series.) --- End quote --- Hello all, been away from the project for a long time due to other stuff coming up. I am still really struggling with the maths here, for example, I have no idea where 2068 is coming from. Is there a for dummies way this can be explained or a resistor calculator I could use? Sorry for being so dense. |
| ve7xen:
Wolfram Alpha can solve the system of equations for you, if you've forgotten your high school algebra ;). Where this is coming from is rearranging one of the equations for R3 (put all R3s on LHS) and the other for R2, and then substituting them into each other and simplifying, so they both become dependent only on R4. Once you do that you will arrive at the equations SiliconWizard gives. If you want to solve this with WolframAlpha, you enter it like this: --- Quote ---4.2*R_4/(R_2+R_3+R_4) = 1.225, 3.81*(R_3+R_4)/(R_2+R_3+R_4) = 1.225 for R_3 --- End quote --- It can also solve for the resistor values too if you add that constraint: --- Quote ---4.2*R_4/(R_2+R_3+R_4) = 1.225, 3.81*(R_3+R_4)/(R_2+R_3+R_4) = 1.225, R_3 = 100000 --- End quote --- |
| Zero999:
I would start by working out the voltage across the potentiometer, then the current and determine the other resistor values from that. Vupper = 3.81V Vlower = 1.225V VR2 = Vupper-Vlower = 3.81-1.225 = 2.585 IR2 = V/R = 2.585/500 = 0.00517 = 5.17*10-3A = 5.17mA IR1 = IR2 = IR3 as they're all in series. VR1 = V1- Vupper = 4.2-3.81 = 0.39V R1 = V/I = 0.39/5.17*10-3 = 75.435R VR3 = Vlower = 1.225V R3 = 1.225/5.17*10-3 = 236.943R I would set R1 and R3 to lower values, to account for component tolerances, especially the potentiometer which probably has a tolerance of 10% or worse. I'd go for 220R for R1 and 68R for R3. Another thing you could do is connect a 100R 1% tolerance resistor in parallel with the potentiometer and recalculate all of the other values. The 1% resistor will reduce the overall tolerance of the potentiometer. |
| Pentoad:
--- Quote from: ve7xen on March 10, 2020, 08:47:28 pm ---Wolfram Alpha can solve the system of equations for you, if you've forgotten your high school algebra ;). Where this is coming from is rearranging one of the equations for R3 (put all R3s on LHS) and the other for R2, and then substituting them into each other and simplifying, so they both become dependent only on R4. Once you do that you will arrive at the equations SiliconWizard gives. If you want to solve this with WolframAlpha, you enter it like this: --- Quote ---4.2*R_4/(R_2+R_3+R_4) = 1.225, 3.81*(R_3+R_4)/(R_2+R_3+R_4) = 1.225 for R_3 --- End quote --- It can also solve for the resistor values too if you add that constraint: --- Quote ---4.2*R_4/(R_2+R_3+R_4) = 1.225, 3.81*(R_3+R_4)/(R_2+R_3+R_4) = 1.225, R_3 = 100000 --- End quote --- Thanks! Wolfram done the trick! --- End quote --- |
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