UNTRANSLATED .. 2.4

Measure resistance by comparison

Objective

Learn to apply Ohm’s law to find the value of an unknown resistance by comparing it with a known one. Voltage across a resistor is given by V = IR . If same amount of current is flowing through two different resistors, the ratio of voltages will be the same as the ratio of resistances, I = U_{A1}/R_2 = (U_{PV1} − U_{A1})/R_1.

_images/res-compare.svg

Procedure

  • Connect the unknown resistor R from PV1 to A1.
  • Connect 1~k\Omega (R_2) from A1 to Ground.
  • Set PV1 to 4 volts.
  • Measure voltage at A1. Calculate the current I = U_{A1}/R_2. Value of R_1 = (U_{PV1} − U_{A1})/I.
  • Select Electrical->Plot I-V curve from the menu to get an I-V plot

Discussion

What is the limitation of this method ? How do we choose the reference resistor ? suppose the unknown value is in M\Omega, what will be the voltage drop across a 1~k\Omega reference resistor ? Our voltage measurement is having a resolution of 1/4096.

We will use this method later to measure the resistance of solutions, using AC.