I can't figure out what resistance values to use....

In summary, the conversation focused on a problem involving a potentiometer and the confusion on how to compute voltages in a circuit. The individual had mistakenly assumed that the potentiometer was wired as a variable resistor and had used incorrect values in their calculations. It was clarified that the potentiometer value remains constant and its position changes the fraction of resistance above and below the wiper. The correct approach is to model the circuit as two resistors and use the potential divider equation to calculate the voltage output.
  • #1
Megatronlol
6
1
Thread moved from the technical forums, so no Homework Template is shown
BCk2QVg
I started an electronics program recently and I am a month into it so I am quite new at this. I have a problem involving a potentiometer and I don't understand how to compute the voltages because I don't know what I am supposed to use for my resistance values.

I know that in series the total resistance is the sum of the individual resistors. I assume that since R1 = 330Ω and R2 = 470Ω that I am taking the pot from 1Ω to 1000Ω and using a minimum value of 801Ω and a maximum of 1800Ω and using my voltage divider formula but that answers that I get don't match the measured values from the lab. In the lab I measured a maximum voltage of 8.13V and a minimum voltage of 2.65V. Can anyone offer any insight as to what resistor values I would be using for my voltage divider formula and why those values? Thanks.

https://imgur.com/a/BCk2QVg here is a link to the problem and circuit

upload_2018-10-7_17-19-16.png
 

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  • #2
When the pot is at the top of its travel, the upper resistance of the divider is 330R and the lower 1470R So the output voltage will be 10 x Lower/(Upper + Lower).
When the pot is at the bottom of its travel, the upper resistance is 1330R and the lower resistance is 470R, and the output voltage can be calculated in the same way.
 
  • #3
You need to show us your calculations before we are allowed to help.
 
  • #4
anorlunda said:
You need to show us your calculations before we are allowed to help.

Normally I would agree but they have shown they don't understand how the pot is working in this circuit with the following, so their calculations are going to be totally wrong.

Megatronlol said:
I know that in series the total resistance is the sum of the individual resistors. I assume that since R1 = 330Ω and R2 = 470Ω that I am taking the pot from 1Ω to 1000Ω and using a minimum value of 801Ω and a maximum of 1800Ω and using my voltage divider formula

That's wrong. The pot is NOT wired up as a variable resistor.

The value of the pot doesn't actually change in this circuit. It's value is a constant 1000R but when you move the wiper the fraction "below" the wiper (which is the output node) compared to "above" it changes. So for example with the wiper in the middle the pot (alone) can be modeled as two 500R resistors.

The whole circuit could then be modeled as (a different) two resistors which can be used in the potential divider equation.

330+500 = 830R
and
470+500 = 970R

Obviously your numbers would be different because they don't ask for the voltage with the wiper in the middle.
 

1. How do I determine the appropriate resistance values for my circuit?

The resistance values for a circuit can be determined using Ohm's Law, which states that resistance (R) is equal to voltage (V) divided by current (I). This means that for a given voltage and desired current, the resistance can be calculated by dividing the voltage by the current. Additionally, the resistance values of individual components can be found on their datasheets or by using a multimeter.

2. What factors should I consider when choosing resistance values?

When choosing resistance values, it is important to consider the desired current, voltage, power dissipation, and tolerance of the resistors. It is also important to consider the compatibility of the resistors with other components in the circuit and to ensure that the resistors can handle the expected heat and current load.

3. How do I know if the resistance values I have chosen are appropriate?

To determine if the chosen resistance values are appropriate, you can use a circuit simulator or perform calculations based on the circuit's voltage and current requirements. Additionally, you can test the circuit with the chosen resistors and make adjustments if necessary.

4. Is there a standard range of resistance values that are commonly used?

Yes, there are standard ranges of resistance values that are commonly used in circuits. These include the E6, E12, E24, and E96 series, which contain resistors with values that are multiples of 10, 12, 24, and 96, respectively. These standard series make it easier to find and use resistors with commonly used values.

5. What should I do if I can't find resistors with the exact values I need?

If you can't find resistors with the exact values you need, you can use a combination of resistors in series or parallel to achieve the desired resistance. Additionally, you can use a variable resistor or potentiometer to adjust the resistance to the exact value needed.

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