Teach lesson
Why does an LED need a resistor?
Compare two protected red-LED circuits at the same voltage and explain current limiting.
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Learning Outcomes
Compare currents calculated from measured resistor voltages with 1 kΩ and 10 kΩ.
Use evidence to explain why an LED needs current limiting.
Student activity preview
Activity Content
Preview only. In a class session, students can fill in responses and submit their work to the teacher.
The component that does not light up
5 min
A small indicator needs a resistor even though the resistor produces no useful light. Is that component simply wasting electricity, or doing an essential job? Compare two red-LED circuits at 5 V. Only the stated resistor value changes: 1 kΩ or 10 kΩ.
Which circuit do you predict will carry more current? Explain before measuring.
Compare two protected circuits
13 min
Read the circuit
The lab button opens a wired circuit on real equipment. A branch is one path containing a resistor and an LED. The LED has an anode (A) and cathode (K); their orientation is already set. Select the DC Power tab to read or change the positive supply setting; select Multimeter to read the voltage. Click Perform Measurement after every change and wait for the result and rack photographs to refresh. Changing a setting alone does not produce a new result. To adjust voltage with the keyboard, focus the voltage knob: Left/Right changes the step; Up/Down changes the value. Check the displayed voltage before measuring.
The multimeter reads the voltage across the resistor, called $V_R$. Record VDC in volts; divide an mV reading by 1,000. For example, 240 mV = 0.240 V. If the display shows uV (microvolts, µV), divide by 1,000,000: 300 uV = 0.000300 V. For a 1 kΩ resistor, $I\,\text{(mA)}=V_R\,\text{(V)}$; for 10 kΩ, divide the voltage in volts by 10 to obtain mA. This calculates current from measured voltage and the stated resistance.
Use the camera photographs for observations of the real lights. The drawing’s Estimated glow is an illustration of the last result, not a light measurement; turn it off during camera comparisons. A light hidden by glare is unclear, not necessarily off. The rack also contains status lights. If the circuit LED is unclear, keep the same circuit and voltage and press Perform measurement again. Wait for the result and camera photographs to refresh, then inspect the LED again. Briefly note the first and repeat readings and any change in visibility in your written answers; use the repeat result in the table. If the LED is still unclear, choose Unclear; do not judge it from another light or keep repeating until you obtain the answer you expected.
Read the equipment name shown under each result: successive measurements can use different physical Hives. Small differences may therefore reflect different components. A result labelled as a previous recording is not an independent new measurement.
Keep the circuit protected
Keep the supplied wiring and resistors in place. Use only the positive 0–5 V supply, between 1 and 5 V in this activity. Do not connect an LED directly to the supply. Leave the other supplies and function generator unchanged. The meter is already connected across a resistor: leave it in DC voltage mode, never current or resistance mode.
Read the voltage across the resistor to calculate the current through that branch. A and K identify the LED terminals.
Changing circuits
When you finish a circuit, use Leave now in Hive, then return to this activity before opening the next lab button. Each button loads its own circuit; do not reuse a previous circuit or rewire the meter.
Red LED with 1 kΩ
Red LED with 1 kΩ
Open this circuit, confirm 5 V, 1 kΩ and DC voltage mode. Make one measurement and fill its row before leaving Hive.
Available in a class session
Red LED with 10 kΩ
Red LED with 10 kΩ
Open this circuit, confirm 5 V, 10 kΩ and DC voltage mode. Make one measurement and fill its row before leaving Hive.
Available in a class session
Results
Complete two rows: one with 1 kΩ and one with 10 kΩ, both at 5 V. Enter V_R in volts. For 1 kΩ, I(mA)=V_R; for 10 kΩ, I(mA)=V_R/10. Choose a camera observation; leave extra rows blank.
| Resistor | Resistor voltage V | Calculated current mA | Camera observation |
|---|---|---|---|
Explain the resistor’s job
12 min
Use the reading for each resistor. Calculate current with 1 kΩ divided by current with 10 kΩ. Enter the ratio and show the two currents you used.
Which resistor limited the current more? Use your results to explain why removing the resistor would not be a sensible way to make this indicator brighter.
Did the camera make the electrical difference easy to see? Explain why the current measurements are stronger evidence than comparing the photographs alone.
For a low-current indicator, which of these two tested circuits would you choose? Justify the choice using the current and whether its light was distinguishable.
Submit
After all measurements, leave Hive and return here. Mark the lab practice as done, check every results row and required answer, then submit the activity.
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