How to Measure Resistance on an Electronic Circuit - dummies

How to Measure Resistance on an Electronic Circuit

By Doug Lowe

Resistance on your electronic circuit is measured in ohms, represented by the Greek letter Omega (Ω). Measuring resistances is similar to measuring voltages, with a key difference:

You must first disconnect all voltage sources from the circuit whose resistance you want to measure. That’s because the multimeter will inject a known voltage into the circuit so that it can measure the current and then calculate the resistance. If there are any outside voltage sources in the circuit, the voltage won’t be fixed, so the calculated resistance will be wrong.

Here are the steps for measuring resistance in the simple circuit:

  1. Remove the battery.

    Just unplug it from the battery snap connector and set the battery aside.

  2. Turn the meter selector dial to one of the resistance settings.

    If you have an idea of what the resistance is, pick the smallest range that’s greater than the value you’re expecting. Otherwise, pick the largest range available on your meter.

  3. If you’re using an analog meter, calibrate it.

    Analog meters must first be calibrated before they can give an accurate resistance measurement. To calibrate an analog meter, touch the two meter leads together. Then, adjust the meter’s calibration knob until the meter indicates 0 resistance.

  4. Touch the meter leads to the two points in the circuit for which you wish to measure resistance.

    For example, to measure the resistance of the resistor, touch the meter leads to the two leads of the resistor. The result should be in the vicinity of 470 Ω.

Here are a few additional thoughts about measuring resistances:

  • When you measure the resistance of an individual resistor or of circuits consisting of nothing other than resistors, it doesn’t matter what direction the current flows through the resistor. Thus, you can reverse the multimeter leads and you’ll still get the same result.

  • Some components such as diodes pass current better in one direction than in the other. In that case, the direction of the current does matter.

  • Resistors aren’t perfect. Thus, a 470 Ω resistor rarely provides exactly 470 Ω of resistance. The usual tolerance for resistors is 5%, which means that a 470 Ω resistor should have somewhere between 446.5 and 495.5 Ω of resistance.

    For most circuits, this amount of imprecision doesn’t matter. But in circuits where it does, you can use the ohmmeter function of your multimeter to determine the exact value of a particular resistor. Then, you can adjust the rest of your circuit accordingly.