Trimmer Terminals

A typical three-terminal trimmer has two fixed terminals connected to the ends of its resistive element and one terminal connected to the movable wiper.

• Terminal 1 → One end of resistive element
• Terminal 2 → Wiper
• Terminal 3 → Other end of resistive element

The physical arrangement varies between manufacturers and packages, so always check the component datasheet when the terminal layout is uncertain.

1. Voltage Divider Connection

The most common three-terminal connection uses the trimmer as a voltage divider.

• One outer terminal → Supply voltage
• Wiper → Variable output
• Other outer terminal → Ground

Rotating the adjustment screw moves the wiper and changes the proportion of the supply voltage appearing at the output.

VCC
 |
[R]
 |
 +---- Wiper → VOUT
 |
[R]
 |
GND

For an ideal unloaded divider:

VOUT = VCC × R2 / (R1 + R2)

Example: Adjustable Voltage

Suppose a 10 kΩ trimmer is connected across a 5V supply. The wiper is used as the output.

• Total resistance = 10 kΩ
• Supply voltage = 5V
• Wiper position determines VOUT

Ideally, the output can be adjusted over most of the range from near 0V to near 5V, depending on the circuit connected to the wiper.

2. Variable Resistor Connection

A trimmer can also be used as a variable resistor by using the wiper and one end of the resistive element.

• Wiper → Circuit connection
• One outer terminal → Circuit connection
• Unused outer terminal → Normally left unused

As the adjustment screw is turned, the resistance between the selected outer terminal and the wiper changes.

Wiper and Outer Terminal Connection

For applications where reliability is important, the wiper can be connected to the selected outer terminal. This makes the effective connection less dependent on a single wiper contact.

Terminal 1 ───── Circuit
     |
     +──── Wiper
     
Terminal 3 ───── Not used

The exact connection should follow the circuit design and trimmer manufacturer's recommendations.

Reversing the Adjustment Direction

If the trimmer is used as a voltage divider and the output changes in the opposite direction from what is desired, the two outer terminals can be swapped.

• Swap the two outer terminals
• Keep the wiper connection unchanged
• The electrical range remains the same
• The direction of adjustment is reversed

Trimmer Adjustment

A trimmer is normally adjusted while monitoring the circuit parameter that needs calibration.

1. Identify the correct adjustment point.
2. Connect the required test equipment.
3. Power the circuit according to its normal test procedure.
4. Make a small adjustment.
5. Observe the circuit response.
6. Repeat until the specified value is reached.

Some circuits require a specific adjustment sequence. Follow the service manual or circuit documentation when available.

Multi-Turn Trimmer Wiring

A multi-turn trimmer is electrically connected in much the same way as a single-turn trimmer. The main difference is the mechanical adjustment mechanism.

• Outer terminals connect to the resistive element.
• The wiper provides the adjustable output.
• Several screw rotations may be required across the full adjustment range.

Common Wiring Applications

Testing the Wiring

A multimeter can help identify the terminals and verify the trimmer's operation.

• Measure between the two outer terminals.
• The reading should correspond approximately to the rated resistance.
• Measure between the wiper and one outer terminal.
• Turn the adjustment screw slowly.
• The resistance should change as the wiper moves.

When testing an installed circuit, measure voltage at the wiper while adjusting the trimmer if the circuit is designed as a voltage divider.

Common Wiring Mistakes

• Assuming terminal positions without checking the datasheet
• Confusing the wiper with an outer terminal
• Connecting the trimmer to a voltage higher than its rating
• Exceeding its power rating
• Adjusting a calibration trimmer without knowing its function

Important Safety and Service Notes

• Disconnect power before changing wiring.
• Use an insulated adjustment tool where appropriate.
• Be especially careful around mains-powered equipment.
• Do not adjust unknown calibration controls randomly.
• Check the circuit documentation before making critical adjustments.

Key Points

• A three-terminal trimmer can be used as a voltage divider.
• A trimmer can also be used as a variable resistor.
• The middle terminal is normally the wiper.
• Swapping the outer terminals reverses adjustment direction.
• Always verify the terminal arrangement from the datasheet when necessary.