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I have the following voltage divider circuit with a potentiomenter, and I need to find the value of the resistors.

schematic

simulate this circuit – Schematic created using CircuitLab

How can I find the values of R1 and R3? I believe both resistors could have the same value. That would be nice, if that is the case.

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  • \$\begingroup\$ You could start off by calculating the current in the series circuit since you have at least 1 resistance with a known voltage across it. \$\endgroup\$ – brhans Mar 31 '15 at 18:25
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    \$\begingroup\$ The potentiometer is hooked up in a way so it may as well be a resistor, why is it in there? \$\endgroup\$ – Samuel Mar 31 '15 at 18:26
  • \$\begingroup\$ Assuming there is no loading affect, then the potentiometer will give you a reference voltage between 10V when at one end and -10V when at the other. \$\endgroup\$ – Tom Carpenter Mar 31 '15 at 18:30
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Hints: -

  1. What current flows thru the pot when there is 20 volts across it?
  2. What current flows thru those resistors
  3. What is ohms law?
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  • \$\begingroup\$ the Current value does not matter, does it? Doesn't I (current) disappears in the equations, anyway? \$\endgroup\$ – Pototo Mar 31 '15 at 19:34
  • \$\begingroup\$ It only matters as a means to an end \$\endgroup\$ – Andy aka Mar 31 '15 at 19:36
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If you assume there is no loading from the wiper of the Pot, then you can do the following:

(1) Find the current through the resistors - as they are in series it will be the same current through each. You know that R2 is dropping V=(10-(-)10)=20V, and you know that R2=10k. Ohms law tells you the current.

(2) You know the voltage across R1 is V=(15-10)=5V, and you know the current through it (see (1)). So Ohms law tells you the resistance.

(3) Do the same as in (2) for R3. You will notice the voltage is the same and the current is the same, so R3=R2.

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  • \$\begingroup\$ 2.5K Ohms for R1 and R2? That is what I got \$\endgroup\$ – Pototo Mar 31 '15 at 19:36
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In a series circuit like this, the voltage across each resistor is proportional to its resistance, so you can solve it simply by looking at the voltages across each resistor, and the given value of one resistor - no need to work out currents, or otherwise involve Ohm's Law.

You have 20 volts across the 10K pot, so the resitances will be 2K per volt. There is 5 volts across each of the unknown resistors, so their value will be ??

Note that this assumes that there is no significant current drawn from the wiper of the pot.

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  • \$\begingroup\$ I believe that should be 0.5K per volt. \$\endgroup\$ – Tom Carpenter Mar 31 '15 at 20:02
  • \$\begingroup\$ @TomCarpenter: Arghh! You're right. 10K/20V = 0.5K per volt. \$\endgroup\$ – Peter Bennett Mar 31 '15 at 21:32

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