Chokes
A choke is a special type of inductor designed to block or "choke" alternating current (AC), high-frequency signals or electrical noise while allowing direct current (DC) or lower-frequency current to pass. Chokes are widely used in power supplies, radio equipment, audio systems, industrial electronics and electromagnetic interference (EMI) filters. Depending on the application, chokes may be optimised for power filtering, radio-frequency suppression or common-mode noise reduction.
How a Choke Works
Like all inductors, a choke stores energy in a magnetic field. Its inductive reactance increases with frequency, allowing it to oppose rapidly changing currents while passing DC with only a small voltage drop caused by the winding resistance.
As the signal frequency increases, the choke presents a higher impedance, making it effective for filtering unwanted electrical noise.
Main Types of Chokes
| Type | Primary Purpose |
|---|---|
| Power Choke | Smooth DC current and reduce ripple. |
| RF Choke (RFC) | Block RF signals while passing DC. |
| Common-Mode Choke | Suppress common-mode electromagnetic interference. |
| Differential-Mode Choke | Reduce differential-mode noise. |
| Audio Choke | Filter unwanted frequencies in audio equipment. |
Power Chokes
Power chokes are designed to smooth current in power supplies. They are often used after rectifiers or within switch-mode power supplies to reduce ripple and improve voltage stability.
- High current capability.
- Low DC resistance.
- Usually use iron, ferrite or powdered-iron cores.
RF Chokes
RF chokes prevent radio-frequency signals from entering power lines or DC supply circuits while allowing DC current to flow normally.
| Typical Applications |
|---|
| Radio transmitters. |
| Radio receivers. |
| Oscillators. |
| RF amplifiers. |
| Antenna matching networks. |
Common-Mode Chokes
A common-mode choke contains two or more windings on the same magnetic core. It suppresses electrical noise that appears equally on multiple conductors, making it one of the most common EMI-filtering components.
| Typical Applications |
|---|
| Computer power supplies. |
| Laptop chargers. |
| Industrial motor drives. |
| Network equipment. |
| AC mains EMI filters. |
Construction
| Part | Purpose |
|---|---|
| Copper Winding | Produces the magnetic field. |
| Magnetic Core | Increases inductance. |
| Insulation | Prevents short circuits between turns. |
| Bobbin or Former | Supports the winding. |
| Leads or Pins | Electrical connection to the circuit. |
Typical Core Materials
| Core Material | Common Applications |
|---|---|
| Ferrite | RF chokes, SMPS and EMI filters. |
| Powdered Iron | Power filters. |
| Laminated Iron | Low-frequency power chokes. |
| Nanocrystalline | High-performance EMI filters. |
Advantages
- Excellent noise suppression.
- Reduce ripple in power supplies.
- Improve electromagnetic compatibility (EMC).
- Increase system reliability.
- Simple and highly reliable component.
Limitations
- Can saturate if overloaded.
- May generate heat at high current.
- Incorrect core selection reduces efficiency.
- Large power chokes can be physically bulky.
Testing Chokes
- Measure continuity with a multimeter.
- Measure inductance using an LCR meter.
- Inspect the core for cracks or damage.
- Check winding resistance.
- Look for signs of overheating or burnt insulation.
Common Faults
| Fault | Possible Cause |
|---|---|
| Open Winding | Broken wire. |
| Shorted Turns | Insulation failure. |
| Core Saturation | Excessive current. |
| Overheating | Overload or poor ventilation. |
| Cracked Core | Mechanical impact. |
Real-World Examples
| Equipment | Choke Function |
|---|---|
| ATX Computer Power Supply | Input EMI filtering and output ripple reduction. |
| Laptop Charger | Suppresses conducted EMI. |
| Radio Transmitter | Blocks RF from the DC supply. |
| LED Driver | Smooths output current. |
| Industrial Inverter | Reduces electrical noise. |
Choosing the Right Choke
| Consideration | Importance |
|---|---|
| Inductance | Must suit the filtering requirements. |
| Current Rating | Should exceed the maximum operating current. |
| Core Material | Must match the frequency range. |
| DC Resistance | Lower resistance improves efficiency. |
| Physical Size | Must fit the available space. |
Interesting Facts
- The term "choke" comes from its ability to choke or block unwanted electrical signals.
- Most AC mains filters contain one or more common-mode chokes.
- RF chokes allow DC current to pass while blocking radio-frequency signals.
- Power chokes are commonly found in high-current power supplies and industrial equipment.
- Many ferrite beads act as miniature RF chokes at very high frequencies.
Key Points
- A choke is an inductor designed primarily for filtering.
- Different choke types are used for power, RF and EMI suppression.
- Chokes improve power quality and reduce electrical noise.
- Core material and current rating are critical selection factors.
- Chokes are essential components in modern electronic equipment.