Trimmer Potentiometers (Trimpots)
A trimmer potentiometer, often called a trimpot or preset resistor, is a small adjustable resistor designed for calibration and fine adjustment of electronic circuits. Unlike ordinary potentiometers, trimmers are not intended for frequent user adjustment and are usually set once during manufacturing or servicing. They are commonly found on printed circuit boards (PCBs) in power supplies, audio equipment, instrumentation and industrial electronics.
What Is a Trimmer Potentiometer?
A trimmer potentiometer is a miniature three-terminal variable resistor that operates in the same way as a standard potentiometer.
Inside the device, a movable wiper travels across a resistive track as an adjustment screw is turned, changing the resistance or output voltage.
Because trimmers are intended for occasional adjustment, they are usually operated with a small screwdriver rather than a knob.
Construction
| Part | Function |
|---|---|
| Resistive Track | Provides the total resistance value. |
| Wiper | Moves across the track to vary resistance. |
| Adjustment Screw | Rotates the wiper mechanism. |
| Three Terminals | Provide electrical connections. |
| Plastic or Ceramic Housing | Protects the internal components. |
How It Works
Turning the adjustment screw moves the internal wiper along the resistive track.
The resistance between the centre terminal (wiper) and either outer terminal changes smoothly, allowing precise adjustment of voltage or current in a circuit.
Types of Trimmer Potentiometers
| Type | Description |
|---|---|
| Single-Turn | Full adjustment over approximately 270°. |
| Multi-Turn | Requires several turns for very fine adjustment. |
| Top Adjustment | Adjusted from the top of the component. |
| Side Adjustment | Adjusted from the side of the component. |
| Sealed | Protected against dust and moisture. |
| Open Frame | Economical design for general applications. |
Common Resistance Values
Popular Values
- 100 Ω
- 200 Ω
- 500 Ω
- 1 kΩ
- 2 kΩ
- 5 kΩ
- 10 kΩ
- 20 kΩ
- 50 kΩ
- 100 kΩ
- 500 kΩ
- 1 MΩ
Typical Power Rating
- 0.1 W
- 0.25 W
- 0.5 W
Applications
- Power supply voltage adjustment.
- Current limit adjustment.
- Audio amplifier bias adjustment.
- DC offset adjustment.
- Sensor calibration.
- Oscillator frequency adjustment.
- Operational amplifier offset trimming.
- Battery charger calibration.
- Industrial control equipment.
- Electronic test instruments.
Real-World Examples
| Equipment | Purpose |
|---|---|
| Bench Power Supply | Output voltage adjustment. |
| Battery Charger | Charging voltage calibration. |
| Class AB Amplifier | Bias current adjustment. |
| Operational Amplifier Circuit | Offset voltage adjustment. |
| Signal Generator | Frequency calibration. |
| Digital Multimeter | Factory calibration. |
Single-Turn vs Multi-Turn
| Single-Turn | Multi-Turn |
|---|---|
| Fast adjustment. | Very fine adjustment. |
| Lower cost. | Higher precision. |
| Less accurate. | Excellent repeatability. |
| General-purpose circuits. | Calibration and instrumentation. |
Testing a Trimmer
- Disconnect power from the circuit.
- Measure the resistance between the two outer terminals.
- Rotate the adjustment screw slowly.
- Measure between the centre terminal and each outer terminal.
- The resistance should change smoothly without sudden jumps.
- Replace the trimmer if the resistance is unstable or intermittent.
Common Faults
| Fault | Possible Cause |
|---|---|
| Open Circuit | Broken resistive track. |
| Intermittent Adjustment | Dirty or worn wiper. |
| Cracked Housing | Mechanical damage. |
| Stiff Adjustment | Corrosion or contamination. |
| No Resistance Change | Failed internal mechanism. |
Adjustment Tips
- Use a non-metallic trimming tool whenever possible.
- Make very small adjustments.
- Measure the circuit after each adjustment.
- Do not force the adjustment screw beyond its mechanical limits.
- Mark the original position before making changes if servicing equipment.
Key Points
- Trimmer potentiometers are designed for occasional adjustment and calibration.
- They provide precise control of voltage, current or resistance.
- Multi-turn trimmers offer finer adjustment than single-turn versions.
- They are widely used in power supplies, amplifiers and measuring instruments.
- Correct adjustment helps ensure accurate and reliable circuit performance.