Practical case: Current limiting in an LED

Current limiting in an LED prototype (Maker Style)

Level: Basic. Demonstrate how a resistor protects a sensitive component (LED) by limiting current flow according to Ohm’s Law.

Objective and use case

In this case, you will build a fundamental series circuit connecting a DC voltage source, a current-limiting resistor, and a Light Emitting Diode (LED).

Why it is useful:
* Component Protection: Prevents the LED from drawing excessive current and burning out instantly.
* Ohm’s Law Application: Visually demonstrates the relationship between Voltage, Current, and Resistance ($I = V/R$).
* Status Indication: Forms the basis for power indicators on almost every electronic device.
* Diagnostic Tooling: Simple LED circuits are often used to debug logic levels in complex systems.

Expected outcome:
* The LED lights up steadily without overheating.
* The current flowing through the circuit remains within the safe range (typically 10–20 mA).
* The voltage drop across the resistor corresponds to the supply voltage minus the LED forward voltage.

Target audience and level: Beginners and students starting with basic component analysis.

Materials

  • V1: 5 V DC supply
  • R1: 220 Ω resistor, function: current limiting
  • D1: Red LED, function: light emission
  • M1: Multimeter, function: current measurement (A)
  • M2: Multimeter, function: voltage measurement (V)

Wiring guide

This circuit uses a series topology. We define the nodes as VCC (5V Source), 0 (Ground), and NODE_A (Intermediate connection).

  • V1 (DC Source): Positive terminal connects to node VCC. Negative terminal connects to node 0.
  • R1 (Resistor): Connects between node VCC and node NODE_A.
  • D1 (LED): Anode connects to node NODE_A. Cathode connects to node 0.

Conceptual block diagram

Conceptual block diagram — Current Limiting
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

[ SOURCE ]                  [ CURRENT CONTROL ]              [ OUTPUT / LOAD ]

    [ V1: 5V DC ] --(VCC)--> [ R1: 220 Ohm ] --(Node A)--> [ D1: Red LED ] --(0)--> [ GND ]
Schematic (ASCII)

Measurements and tests

To validate Ohm’s Law and component safety:

  1. Calculate Expected Current:
    • Assume LED Forward Voltage ($V_f$) $\approx$ 2.0 V.
    • Voltage across R1: $V_{R1} = V_{source} – V_f = 5V – 2V = 3V$.
    • Expected Current: $I = V_{R1} / R1 = 3V / 220\Omega \approx 13.6 mA$.
  2. Voltage Measurement: Set multimeter M2 to DC Volts. Measure across R1 (leads on VCC and NODE_A). The reading should be approximately 3 V.
  3. Current Measurement: Break the circuit at node VCC or 0 and insert multimeter M1 in series (Amperemeter mode). The reading should be close to 13–14 mA.
  4. Visual Check: The LED should emit a steady, bright red light.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: Current limiting in an LED

* --- Power Supply ---
* V1: 5V DC Supply connected between VCC and 0 (GND)
V1 VCC 0 DC 5

* --- Components ---
* R1: 220 Ohm Resistor
* Function: Current limiting
* Connected between VCC and NODE_A
R1 VCC NODE_A 220

* D1: Red LED
* Function: Light emission
* Anode connected to NODE_A, Cathode connected to 0 (GND)
D1 NODE_A 0 DLED

* --- Models ---
* Model for D1 (Red LED)
* Parameters: IS (Saturation Current), N (Emission Coefficient), RS (Series Resistance)
* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

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* Practical case: Current limiting in an LED

* --- Power Supply ---
* V1: 5V DC Supply connected between VCC and 0 (GND)
V1 VCC 0 DC 5

* --- Components ---
* R1: 220 Ohm Resistor
* Function: Current limiting
* Connected between VCC and NODE_A
R1 VCC NODE_A 220

* D1: Red LED
* Function: Light emission
* Anode connected to NODE_A, Cathode connected to 0 (GND)
D1 NODE_A 0 DLED

* --- Models ---
* Model for D1 (Red LED)
* Parameters: IS (Saturation Current), N (Emission Coefficient), RS (Series Resistance)
* Tuned for approximately 1.8V - 2.0V forward voltage drop
.model DLED D (IS=1e-14 N=2.5 RS=5 BV=5 IBV=10u)

* --- Analysis Directives ---
* Calculate DC operating point
.op

* Transient analysis (Required for .print output generation)
* Step: 100us, Stop: 10ms
.tran 100u 10m

* --- Output / Measurements ---
* Simulating M2 (Multimeter - Voltage): Probing NODE_A (Voltage across LED)
* Simulating M1 (Multimeter - Current): Probing I(V1) (Total circuit current)
* Note: I(V1) will be negative as current flows out of the voltage source.
.print tran V(VCC) V(NODE_A) I(V1)

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Show raw data table (108 rows) 
Index   time            v(vcc)          v(node_a)       v1#branch
0	0.000000e+00	5.000000e+00	1.880179e+00	-1.41810e-02
1	1.000000e-06	5.000000e+00	1.880178e+00	-1.41810e-02
2	2.000000e-06	5.000000e+00	1.880178e+00	-1.41810e-02
3	4.000000e-06	5.000000e+00	1.880178e+00	-1.41810e-02
4	8.000000e-06	5.000000e+00	1.880178e+00	-1.41810e-02
5	1.600000e-05	5.000000e+00	1.880178e+00	-1.41810e-02
6	3.200000e-05	5.000000e+00	1.880178e+00	-1.41810e-02
7	6.400000e-05	5.000000e+00	1.880178e+00	-1.41810e-02
8	1.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
9	2.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
10	3.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
11	4.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
12	5.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
13	6.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
14	7.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
15	8.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
16	9.280000e-04	5.000000e+00	1.880178e+00	-1.41810e-02
17	1.028000e-03	5.000000e+00	1.880178e+00	-1.41810e-02
18	1.128000e-03	5.000000e+00	1.880178e+00	-1.41810e-02
19	1.228000e-03	5.000000e+00	1.880178e+00	-1.41810e-02
20	1.328000e-03	5.000000e+00	1.880178e+00	-1.41810e-02
21	1.428000e-03	5.000000e+00	1.880178e+00	-1.41810e-02
22	1.528000e-03	5.000000e+00	1.880178e+00	-1.41810e-02
23	1.628000e-03	5.000000e+00	1.880178e+00	-1.41810e-02
... (84 more rows) ...

Common mistakes and how to avoid them

  1. Reversed LED Polarity: Connecting the LED cathode to positive. Solution: Ensure the longer leg (Anode) faces the positive voltage side (towards R1).
  2. Omitting the Resistor: Connecting the LED directly to 5V. Solution: Always verify the resistor is in series before applying power to prevent destroying the LED.
  3. Measuring Current in Parallel: Attempting to measure current by probing across the LED like a voltmeter. Solution: Always break the circuit path and place the meter in series for current measurements.

Troubleshooting

  • Symptom: LED does not light up.
    • Cause: LED connected backwards or broken circuit.
    • Fix: Check orientation (Anode/Cathode) and ensure all breadboard connections are tight.
  • Symptom: LED flashes once and dies.
    • Cause: No current limiting resistor used (LED burned out).
    • Fix: Replace the LED and ensure R1 (220 Ω) is correctly installed.
  • Symptom: LED is very dim.
    • Cause: Resistance value is too high (e.g., using 10 kΩ instead of 220 Ω).
    • Fix: Verify the resistor color bands or measure R1 with a multimeter.
  • Symptom: Multimeter reads 0 A.
    • Cause: Blown fuse in the multimeter or improper mode selection.
    • Fix: Check probe connections (Com/mA) and ensure the meter dial is set to DC Current.

Possible improvements and extensions

  1. Variable Brightness: Replace R1 with a 1 kΩ potentiometer in series with a 100 Ω safety resistor to manually adjust the brightness.
  2. Multiple Colors: Swap the Red LED for Blue or Green and measure the change in current (different colors have different forward voltages, affecting the calculation).

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: What is the primary function of the resistor (R1) in this circuit?




Question 2: Which fundamental law is visually demonstrated by this circuit?




Question 3: What is the expected outcome regarding the LED if the circuit is built correctly?




Question 4: What is the typical safe current range for the LED mentioned in the text?




Question 5: What is the voltage of the DC supply (V1) used in this circuit?




Question 6: Which topology is used to wire this circuit?




Question 7: What determines the voltage drop across the resistor in this circuit?




Question 8: What is a common use case for this type of simple LED circuit mentioned in the text?




Question 9: Why is this circuit useful for component protection?




Question 10: What is the resistance value of R1 specified in the materials list?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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