# Trimmer Capacitors FAQ

Characteristics

What is the principle,feature and application of Trimmer Capacitors?

Basic knowledge of Trimmer Capacitors.

What is Trimer Capacitor

Trimmer capacitors are the parts that it's capacitance can be changed freely.
Similarly, as the parts can be changed impedance freely, trimmer potentiometer are very known too.So trimmer capacitors are the parts for capacitance adjustment. Trimmer capacitors are used in some applications that usually almost have no need to adjust again after first time adjustment.

Appearance of Trimmer Capacitors

Applications of Trimmer Capacitors

Example1: Frequency adjustment in oscillating circuit

Adjustment for external capacitance around crystal oscillator.
For the adjustment of oscillator element, trimmer capacitors are used.

Oscillation Circuit
Applications
• Wireless equipment
• Remote key - less entry
• Watch
Example2: Antenna matching

Inductance will change if there is any distortion, winding change on antenna in physically.
With trimmer capacitors, frequency can be adjusted to desired value by adjusting capacitance.

Application
• Cell - phone base station
• RF - ID reading device

Principle of Trimmer Capacitors

The principle how trimmer capacitors keep capacitance is same with fixed capacitors. And there is a rotation structure for capacitance adjustment.

Principle of fixed capacitors

With the structure have two parallel conductive plates and a dielectric in middle (Like a sandwich) , fix capacitors can store charge.
According to formula, capacitance is direct proportion to conductive plate area and inverse proportion to distance between plates.

Basic Structure of Capacitor

C: Capacitance [F]
ε: Dielectric constnat [F/m]
S: Conductive plate area [m2]
D: Distance between conductive plates [m]

Principle of Trimmer capacitors

Same with fixed capacitors, there are two parallel conductive plates clamp a dielectric in middle.
For adjustment, one side electrode is movable (rotation) .
Capacitance can be changed by adjusting the opposite area between movable and fixed electrode.
According below formula, capacitance will become higher if opposite area become bigger.
Depending on the rotation angle of movable electrode, capacitance value will change. According the below picture, there must be a 180° rotation to change the capacitance value from Min. to Max. (One cycle is 360°)

Basic structure of Trimmer Capacitor

Change in electrostatic capacity of Trimmer capacitor

Depending on the rotation angle of movable electrode, capacitance value will change like below graph.
The capacitance value changes continuously because alomost trimmer capacitors don't have a stopper.

Relationship between movable electrode and capacitance

Feature of Trimmer Capacitors

Because of the proximity design of print pattern in circuit board, there is unintended capacitance emerge. Therefore we can use trimmer capacitors for total capacitance adjustment.

With pre - set trimmer capacitors, it is possible to realize nonadjustable shipment.

The customer product must be scrapped, if the characteristic can not be gratified by fixed capacitor.
If scrappage rate is high, we recommend pre - set trimmer capacitors instead of fixed capacitors.

• Capacitance of pre-set trimmer capacitors will be set to a value at customer's request in advance before shipment. It is customized product. (In standard product, the capacitance is Max. value.)
• With pre - set trimmer capacitors, most of products don't need adjustment before shipment.
Although for same products need adjustment before shipment, the scrappage rate will be zero after adjustment with trimmer capacitors.

By using trimmer capacitors, it is no problem to set surrounding circuit constant roughly.

Would you please use trimmer capacitors if it is difficult to decided on a capacitance value in your circuit.
You can design your circuit roughly and quickly as the result.

Differences between Trimmer Potentiometers

Trimmer capacitors Trimmer potentiometers
Adjustment unit Capacitance (C[F]) Impedance (R[Ω])
Terminal number 2 3
Main material Dielectric, ceramic, air etc.. Carbon or cermet
resistive element
Sliding contact point mode Surface contact (*Left figure) Slider point contact (*Right figure)

Trimmer capacitors sliding mode

Trimmer potentiometers sliding mode

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