How to control the capacity error of film capacitors

How are film capacitors with small capacitance error produced?

Generally, after film capacitors are produced, their actual capacity will be somewhat larger or smaller. The produced capacitors will finally pass through a sorting machine, which can automatically identify the capacity error of the film capacitors. If you want film capacitors with smaller capacity error, you can use the sorting machine to automatically select the capacitors that meet the requirements. In this way, the manufacturer's cost will increase, and the price of the capacitors will naturally become more expensive.

Is it better for film capacitors to have smaller capacitance errors?

Capacitance error is a very important parameter when selecting film capacitors. Capacitance error refers to the deviation between the actual capacitance value of a film capacitor and its nominal capacitance value. It is usually expressed as a percentage, such as 1% or 5%. Generally speaking, the smaller the capacitance error, the better, as it improves circuit accuracy and stability.

However, smaller capacitance tolerances aren't always preferable. In some applications, excessively small capacitance tolerances can cause problems. For example, when using multiple film capacitors to form a circuit, if the capacitance tolerances of each film capacitor are very small, their actual capacitance values may be very close, which can lead to problems such as resonance in the circuit.

Therefore, when selecting film capacitors, it is necessary to consider the actual application scenario comprehensively. Generally, for applications that require high precision and stability, such as precision measuring instruments and audio amplifiers, the capacitance error should be as small as possible. However, for applications that require multiple film capacitors to form a circuit, the capacitance error cannot be too small, otherwise it may cause problems such as resonance in the circuit.

There are four common errors in film capacitors

1. During the winding process, regularly measure the capacity and height of the core to control the discreteness of the capacity. If the product is tested shortly after packaging, the capacity of the product will change after a period of storage, resulting in excess capacity. Polyester film capacitors are more obvious, usually shifting in the positive direction.

2. Track the volume distribution of finished products and provide immediate feedback if any deviation is found to adjust the volume adjustment value.

3. Spot-measure the core capacity after flattening and shaping, and adjust the winding center value in time if deviation is found.

4. In order to accurately determine the center value of the winding capacity (also called the correction value), the capacity change rate during hot (cold) pressing, heat treatment and packaging must be included in the core capacity correction value. The adjustment value will vary for different models and specifications, different specifications and models, and even different winding machines.

Solution to the error of film capacitor bias

According to the changing law, the deviation value is controlled during the test process. For example, the polyester film capacitor is clamped in the positive deviation and the negative deviation is relaxed.

Process factors that cause capacitance variation during manufacturing.

The packaged product was subjected to another heat treatment.

After encapsulation, the product was stored naturally for one week before testing.

The capacitance C of a wound film capacitor is 0.177 ε s/d ε, the dielectric constant S of the medium, the effective area D of the plate, and the thickness of the medium. The capacitance is proportional to ε and S, and inversely proportional to d.

Although the document has made precise regulations, these three parameters will change during the process, resulting in capacity deviation. The focus of the process is to reduce these deviations and improve the capacity hit rate.

Winding process: Film layer width, thickness, and thickness can vary. Changes in film tension from large to small coils and tension errors on each winder can occur. Roller pressure can be too low. Directional deviation and misalignment can occur throughout the winding process. High humidity can lead to an oversized core volume.

Hot pressing process: Core thickness errors result in uneven stress distribution, leading to uneven core tightness and dispersed capacity. Uneven hot pressing platens. Temperature deviations. Errors in heat treatment time or temperature, impregnation, vacuum level, time, and curing temperature.