Trimmers
A trimmer, also called a trimmer potentiometer or preset, is a small adjustable resistor used to set or calibrate a circuit to a desired value. Unlike a conventional potentiometer, a trimmer is normally adjusted during setup, calibration, or servicing rather than being operated continuously by the user.
What Is a Trimmer?
A trimmer is a variable resistance component with an adjustable wiper. Turning its adjustment screw changes the resistance or voltage in the circuit.
Trimmers are commonly found inside electronic equipment where a circuit requires a precise adjustment after manufacturing or during servicing.
- Small physical size
- Adjustable resistance
- Usually adjusted with a screwdriver
- Designed mainly for calibration
How Does a Trimmer Work?
A typical trimmer contains a resistive element and a movable contact called the wiper.
- The circuit is connected to the trimmer.
- The adjustment screw moves the wiper.
- The wiper changes the resistance ratio.
- The circuit operating point changes.
- The trimmer is left at the required setting.
Trimmer Terminals
A three-terminal trimmer is electrically similar to a potentiometer.
- Terminal 1 ā One end of the resistive element
- Terminal 2 ā Wiper
- Terminal 3 ā Other end of the resistive element
The trimmer can be used as a voltage divider or as a variable resistor, depending on how it is connected.
Trimmer vs Potentiometer
| Feature | Trimmer | Potentiometer |
|---|---|---|
| Adjustment | Usually screwdriver | Usually knob or slider |
| User operation | Normally occasional | Often continuous |
| Typical location | Inside equipment | Front panel or user interface |
| Main purpose | Calibration and setup | User control |
Common Types of Trimmers
- Single-turn trimmers ā adjusted through one rotation
- Multi-turn trimmers ā provide finer adjustment over several turns
- Through-hole trimmers ā mounted through PCB holes
- SMD trimmers ā designed for surface-mount PCBs
Common Resistance Values
Trimmers are available in many resistance values depending on the circuit. Common values include:
- 100 Ī©
- 500 Ī©
- 1 kĪ©
- 5 kĪ©
- 10 kĪ©
- 50 kĪ©
- 100 kĪ©
- 1 MĪ©
The correct value depends on the circuit design and the required adjustment range.
Applications
- Amplifier bias adjustment
- Power-supply voltage adjustment
- Sensor calibration
- Oscillator frequency adjustment
- Display calibration
- Audio circuit adjustment
- Radio and RF equipment
- Measurement equipment
Trimmers in Audio Equipment
Trimmers are frequently used inside audio equipment for adjustments that should not normally be changed by the user.
- Amplifier bias adjustment
- DC offset adjustment
- Gain calibration
- Level adjustment
The exact function depends on the circuit design and the manufacturer's service procedure.
Trimmers in Power Supplies
A trimmer may be used to adjust the output voltage of an adjustable power supply.
For example, a feedback circuit can use a trimmer to establish the desired output voltage.
The adjustment must be made carefully because a small change in resistance can sometimes produce a significant change in output voltage.
Single-Turn vs Multi-Turn
| Type | Adjustment | Typical Use |
|---|---|---|
| Single-turn | Fast adjustment | General calibration |
| Multi-turn | Fine adjustment | Precision circuits |
Multi-turn trimmers are useful when a circuit requires very fine adjustment because the resistance changes more gradually as the screw is rotated.
Advantages
- Small size
- Low cost
- Useful for precise calibration
- Can remain inside the equipment
- Available in many resistance values
Limitations
- Not normally intended for frequent adjustment
- Mechanical wear can occur
- Adjustment can be difficult in confined spaces
- Incorrect adjustment can affect circuit operation
Important Adjustment Precautions
- Use the correct adjustment tool when required.
- Make small adjustments rather than large changes.
- Record the original position before servicing when appropriate.
- Follow the equipment manufacturer's service procedure.
- Do not adjust a calibration trimmer without understanding its function.
Common Trimmer Faults
- Worn resistive track
- Dirty or unreliable wiper contact
- Broken adjustment mechanism
- Open circuit
- Unstable resistance
Testing a Trimmer
A multimeter can be used to check a trimmer's resistance.
- Measure between the two outer terminals to check total resistance.
- Measure between the wiper and one outer terminal.
- Adjust the trimmer slowly.
- The measured resistance should change smoothly.
An erratic reading, sudden open circuit, or failure to change may indicate a faulty trimmer.
Key Points
- A trimmer is a small adjustable resistor used mainly for calibration.
- It can operate as a variable resistor or voltage divider.
- Single-turn and multi-turn versions are common.
- Trimmers are widely used inside electronic equipment.
- They should not be adjusted unnecessarily.