How Can You Determine Unknown Resistance in a Circuit?

[ee1215]

If I have a circuit similar to this: http://i.imgur.com/JaUGb.png

I am reading in voltage as an input from Vin, when switch is closed, and am wanting to find out the unknown resistance R. Similar to dmm...what would be the best approach? Reading the voltage, calculate current to find R?

 

[berkeman]

If I have a circuit similar to this: http://i.imgur.com/JaUGb.png

I am reading in voltage as an input from Vin, when switch is closed, and am wanting to find out the unknown resistance R. Similar to dmm...what would be the best approach? Reading the voltage, calculate current to find R?

Use a PIC uC with an on-chip ADC...

 

[vk6kro]

The voltage at Vin will be proportional to the resistance R.

So you can say V_in = R / (R + 1000) * 5 volts

Once you measure the value of V_in, you just put it in the above formula and work out the value of R.

For example, if the value of V_in was 3 volts, you would move the 5 volts and the (R + 1000) to the left of the equation and say 3 volts / 5 volts = 0.6

So, 0.6 * ( R +1000) = R
so 600 ohms = R - 0.6R = 0.4 R

R = 1500 ohms

Now check, 1500 / 2500 * 5 volts = 3 volts
So, the answer is right.

 

[ee1215]

thanks vk, going to give it a try with my module.

 

[alphysicist]

There are a few different approaches you could take to find the unknown resistance in this circuit. One option would be to use Ohm's Law, which states that the voltage (V) across a resistor is equal to the current (I) multiplied by the resistance (R). In this case, you could measure the voltage (Vin) and the current (I) using a multimeter and then use those values to calculate the unknown resistance (R = V/I).

Another approach could be to use Kirchhoff's laws, which are fundamental principles in circuit analysis. Kirchhoff's Voltage Law (KVL) states that the sum of the voltages around a closed loop in a circuit is equal to zero. In this case, you could set up a loop that includes the voltage source (Vin), the unknown resistance (R), and the switch, and then use the voltage readings from the multimeter to solve for the resistance.

Alternatively, you could use the fact that the voltage across a resistor is directly proportional to the current passing through it. This means that if you measure the voltage and current at two different points in the circuit, you can calculate the resistance between those two points. For example, you could measure the voltage across the unknown resistance (Vout) and the voltage across the known resistor (R1) and then use those values to calculate the unknown resistance (R = Vout / I).

Overall, the best approach will depend on the specific details of your circuit and the equipment you have available. It may be helpful to try multiple methods and compare the results to ensure accuracy. Additionally, it is important to keep in mind any potential sources of error, such as resistance in the wires or connections, and take steps to minimize them.