LED with Current Limiting Resistor #

Difficulty: ⭐ Beginner Time: 5 minutes Components: 2 (Voltage Source, Resistor, Diode) Concepts: Ohm’s Law, LED forward voltage, current limiting

What You’ll Learn #

How to place and connect components in Breadpad
Why LEDs need current limiting resistors
How to calculate resistor values
Running your first simulation
Reading DC operating point results

Circuit Theory #

The Problem #

LEDs (Light Emitting Diodes) have very low resistance when forward-biased. Connecting an LED directly to a power supply will:

Draw excessive current
Destroy the LED instantly
Potentially damage the power supply

The Solution #

A current limiting resistor placed in series with the LED limits current to a safe level.

The Calculation #

I_LED = (V_SUPPLY - V_LED) / R

Rearranged:
R = (V_SUPPLY - V_LED) / I_LED

Example with red LED:
V_SUPPLY = 5V
V_LED = 2V (typical for red LED)
I_LED = 10mA = 0.01A (typical operating current)

R = (5V - 2V) / 0.01A = 300Ω

Use standard value: 330Ω (E12 series)

Circuit Diagram (Text) #

+5V (VCC)
  │
  R1 (330Ω)
  │
  │ Anode
 [LED] (Red, VF = 2V)
  │ Cathode
  │
 GND

Step-by-Step Build Instructions #

Step 1: Create New Circuit #

Open Breadpad
Tap + (New Circuit) button
Choose “New Circuit”
You’ll see an empty breadboard

Step 2: Place Voltage Source #

Tap Voltage Source in component toolbox (left side)
Tap on breadboard to place (any location works)
Double-tap the voltage source to edit
Set voltage to 5V
Ensure type is DC
Tap Done

Step 3: Place Resistor #

Tap Resistor in toolbox (or press R on keyboard)
Tap breadboard above the voltage source
Double-tap the resistor to edit
Set resistance to 330Ω (type “330”)
Tap Done

Tip: Use SI prefixes - “1k” for 1000Ω, “10k” for 10,000Ω, etc.

Step 4: Place LED (Diode) #

Tap Diode in toolbox (or press D)
Tap breadboard above the resistor
Important: Orient diode correctly
- Arrow/stripe points toward cathode (negative)
- Current flows FROM anode TO cathode
Double-tap diode to edit (optional)
Default parameters work for red LED

Note: Breadpad’s default diode model approximates a red LED (VF ≈ 1.8-2.0V).

Step 5: Connect Components with Wires #

Tap Wire in toolbox (or press W)
Connect positive terminal of voltage source to one end of resistor
Connect other end of resistor to anode of LED (non-stripe end)
Connect cathode of LED (stripe end) to negative terminal of voltage source

Your circuit should now form a complete loop: +5V → R → LED → GND

Step 6: Add Ground Connection #

Ensure the negative terminal of the voltage source is connected
This represents the ground (0V reference)

Step 7: Add Measurement Probes #

Tap on the connection between resistor and LED (place probe)
Tap on the negative voltage source connection (ground reference)

Free version: 2 probes maximum Premium: Unlimited probes

Running the Simulation #

Step 1: Open Oscilloscope #

Tap Oscilloscope button (bottom toolbar)
The oscilloscope/analysis window opens

Step 2: Select Analysis Type #

Choose Operating Point analysis
This shows DC voltages and currents at steady state

Step 3: Run Simulation #

Tap Run Simulation button
Wait a moment for SPICE to calculate
Results appear automatically

Expected Results #

Voltages #

V(VCC): 5.000V (supply voltage)
V(LED_anode): ~3.0V (5V - voltage drop across R)
V(LED_cathode) or V(GND): 0V (ground reference)
V(resistor): ~3.0V (VCC - V_LED)

Currents #

I(VCC): ~9.1mA (current supplied by source)
This should match calculated value!

Calculations to Verify #

V_R = V_VCC - V_LED_anode = 5V - 1.9V = 3.1V
I_LED = V_R / R = 3.1V / 330Ω = 9.4mA ✓

Close to our target of 10mA!

LED Forward Voltage #

The LED voltage drop (~2V for red) is an intrinsic property. You can’t change it significantly - it’s determined by the semiconductor material:

Red LED: 1.8-2.2V
Yellow LED: 2.0-2.2V
Green LED: 2.0-3.0V
Blue/White LED: 3.0-3.6V

Understanding the Results #

Why isn’t the LED voltage exactly 2V? #

LED forward voltage varies with:

Current level (higher current → slightly higher VF)
Temperature
Manufacturing tolerance
SPICE model parameters

Why use 330Ω instead of calculated 300Ω? #

Standard values: Resistors come in E12 (10% tolerance) or E24 (5% tolerance) series
330Ω is the nearest standard value to calculated 300Ω
The small difference (10% higher) reduces current slightly, which is safer

Power Dissipation #

P_R = I² × R = (0.0094)² × 330 = 0.029W = 29mW
P_LED = V_LED × I = 2.0V × 0.0094A = 0.019W = 19mW

Both well below typical 1/4W (250mW) resistor rating ✓

Experiments to Try #

Experiment 1: Change Resistor Value #

Double-tap resistor
Try: 100Ω, 220Ω, 470Ω, 1kΩ, 10kΩ
Run simulation for each
Observe how current changes
Question: What happens with 10kΩ? Is LED still bright enough?

Expected:

Lower R → Higher current → Brighter LED
Higher R → Lower current → Dimmer LED

Experiment 2: Change Supply Voltage #

Double-tap voltage source
Try: 3V, 5V, 9V, 12V
Important: Recalculate resistor for each voltage!
Formula: R = (V_SUPPLY - 2V) / 0.01A

For 9V supply:

R = (9V - 2V) / 0.01A = 700Ω → Use 680Ω or 750Ω

Experiment 3: Multiple LEDs in Series #

Add another LED in series (LED → LED → GND)
Recalculate: R = (5V - 2V - 2V) / 0.01A = 100Ω → Use 100Ω
Observe voltage distribution

Note: Total forward voltage must be less than supply voltage!

Experiment 4: Wrong LED Orientation #

Rotate the LED (place backwards)
Run simulation
Observe: No current flows, LED off
Voltage source voltage appears across LED
Fix it: Rotate back to correct orientation

Common Mistakes #

Mistake: No Current Flows #

Symptom: All voltages show 0V or supply voltage Cause: LED backwards, circuit not complete, or wire missing Fix: Check LED orientation (arrow points to cathode), verify all connections

Mistake: “No DC path to ground” Error #

Symptom: Simulation won’t run Cause: Ground connection missing Fix: Ensure voltage source negative terminal is connected and circuit is complete

Mistake: Calculated Current Doesn’t Match #

Symptom: Simulation shows different current than calculated Cause: LED forward voltage assumption incorrect Fix: Use voltage shown in simulation for LED, recalculate

Mistake: Wrong Resistor Unit Prefix #

Symptom: Extremely high or low current Cause: Typed “330m” (0.33Ω) instead of “330” (330Ω) Fix: Check units! m = milli (÷1000), k = kilo (×1000)

Real-World Considerations #

Component Selection #

Resistors:

Standard value from E12 or E24 series
1/4W (250mW) power rating sufficient for most indicator LEDs
5% tolerance acceptable

LEDs:

Choose color based on application
Typical indicator LED: 20mA max, 10-15mA nominal
High-efficiency LEDs work well at 2-5mA

Why This Matters #

This simple circuit is used everywhere:

Indicator lights on electronics
Status LEDs on circuit boards
Power indicators
Custom lighting projects

Every electronic device with an LED uses this exact circuit!

Next Steps #

Build This Circuit Physically #

Once you understand it in Breadpad:

Get: 330Ω resistor, red LED, 5V power supply, breadboard
Build the physical circuit
Measure voltages with multimeter
Compare with Breadpad simulation
Verify your understanding

Expand Your Knowledge #

Try these related circuits, available as examples in the app:

Voltage Divider — learn resistor networks
A transistor switch — control an LED with a logic signal
Multiple LEDs in parallel (each needs its own resistor!)

Quiz Yourself #

Why can’t we connect LED directly to 5V?
If supply is 12V and LED VF = 2V, what resistor for 10mA?
What happens if LED is backwards?
Why use 330Ω instead of calculated 300Ω?
Can we use 1/8W resistor instead of 1/4W?

Reference SPICE Netlist #

* LED with Current Limiting Resistor
* Supply: 5V, LED: Red, Target current: 10mA

VCC vcc 0 DC 5
R1 vcc led_anode 330
D1 led_anode 0 LED_MODEL

.MODEL LED_MODEL D (IS=1e-12 N=1.8)

.OP
.PRINT DC V(vcc) V(led_anode) I(VCC)
.END