Litmux Studio

LabPower and energy · 15 min

Lab: power and the resistor's rating

Measure a resistor's voltage, work out its power, then give it far more power than its rating and watch it fail - here it costs nothing. Then fix the design properly.

The circuit for: Lab: power and the resistor's rating
The circuit you start with

The steps

  1. Press RUN.RUN is on the toolbar (F5).
  2. Measure the voltage across R1 with the Multimeter on V⎓ (red to R1's top, black to its bottom). About 9 V, so P = V² / R = 81 / 1000 = 81 mW - a third of its 1/4 W rating.Across the part: one lead on each end.
  3. Now change R1 to 100 Ω. P = 9² / 100 = 0.8 W in a 0.25 W resistor - more than three times its rating. Watch what happens.Double-click R1 and type 100.
  4. It burnt out (and is now an open circuit - the meter shows the full 9 V across the gap). Fix the design: give R1 a power rating with a 2× margin - 2 W - then press Reset to put in a new part.Double-click R1, set Power rating to 2, then the Reset button.

What it shows