Current Limiter
A current limiter is a circuit that restricts the amount of current flowing through a load to a safe value. The simplest current limiter uses a resistor connected in series with the load. Current-limiting resistors are widely used with LEDs, transistors, microcontrollers, sensors, relays and many other electronic components to prevent damage caused by excessive current.
What Is a Current Limiter?
A current limiter prevents excessive current from flowing through an electrical or electronic device.
The simplest implementation is a resistor placed in series with the load. As current increases, the voltage drop across the resistor increases, naturally limiting the current.
Basic Circuit
+V │ [R] │ LED / LOAD │ GND
The resistor absorbs part of the supply voltage, allowing only the desired current to flow through the load.
Calculating the Resistor Value
The resistor value is calculated using Ohm's Law:
R = (VS - VL) ÷ I
| Symbol | Meaning |
|---|---|
| R | Current-limiting resistor (Ω) |
| VS | Supply voltage (V) |
| VL | Load voltage (V) |
| I | Desired current (A) |
Worked Example 1 – LED Current Limiter
Design a resistor for a red LED.
- Supply voltage = 12 V
- LED forward voltage = 2 V
- LED current = 20 mA
R = (12 - 2) ÷ 0.02 R = 500 Ω
The nearest preferred resistor value is 510 Ω.
Worked Example 2
An LED operates from a 5 V supply.
- Supply voltage = 5 V
- LED voltage = 2 V
- Current = 15 mA
R = (5 - 2) ÷ 0.015 R = 200 Ω
A standard 220 Ω resistor provides a safe operating current.
Power Rating of the Resistor
The resistor must be able to dissipate the heat generated during operation.
P = I² × R
For the 510 Ω resistor carrying 20 mA:
P = 0.02² × 510 P = 0.204 W
A ½ W resistor is recommended because it provides a suitable safety margin over the calculated 0.204 W dissipation.
Common LED Resistor Values
| Supply | LED | Typical Resistor |
|---|---|---|
| 3.3 V | Red | 68 Ω |
| 5 V | Red | 220 Ω |
| 5 V | Blue | 100 Ω |
| 9 V | Red | 390 Ω |
| 12 V | Red | 510 Ω |
| 24 V | Red | 1.2 kΩ |
Applications
- LED indicators.
- Optocouplers.
- Transistor base current limiting.
- Microcontroller input/output protection.
- Relay driver circuits.
- Sensor interfaces.
- Battery chargers.
- Logic circuits.
- Communication interfaces.
- Display backlighting.
Advantages
- Very simple circuit.
- Low cost.
- Highly reliable.
- No active components required.
- Suitable for many low-power applications.
Limitations
- Current changes if the supply voltage changes.
- Power is wasted as heat.
- Not suitable for high-power LEDs.
- Efficiency is relatively low.
Common Mistakes
| Mistake | Consequence |
|---|---|
| Connecting an LED without a resistor. | The LED may be destroyed almost instantly. |
| Using the wrong supply voltage. | Incorrect current flows through the load. |
| Ignoring resistor power rating. | The resistor may overheat or fail. |
| Using the calculated value instead of the nearest standard value. | Preferred resistor values are easier to obtain and usually provide a safe margin. |
Resistor Limiter vs Constant Current Source
| Resistor Limiter | Constant Current Source |
|---|---|
| Very simple. | More complex. |
| Low cost. | Higher cost. |
| Current varies with supply voltage. | Current remains nearly constant. |
| Suitable for small loads. | Ideal for high-power LEDs and precision circuits. |
Key Points
- A current-limiting resistor protects components from excessive current.
- The resistor is connected in series with the load.
- Ohm's Law is used to calculate the resistor value.
- Always verify the resistor's power rating.
- Current-limiting resistors are essential in LED circuits and many electronic designs.