# Inductors(Coils)

## FAQ

The following documents summarize Frequently Asked Questions regarding terms, specifications, characteristics and evaluation methods categorized by topic.

## Is an explanation of the basics of an inductor (a coil) available?

Please refer to "TDK Inductor's World", here. It provides easy-to-understand explanations of the functions, types, etc., of inductors using illustrations.

## Please explain how an inductor (a coil) functions.

An inductor has the functions of developing electromotive force in the direction that reduces fluctuation when a fluctuating current flows and storing electric energy as magnetic energy. It has the characteristic of allowing DC current to flow easily while presenting high resistance against the flow of high frequency currents.

Inductors have the following specific application purposes:
1. Dividing necessary signals and unnecessary signals (filtering)
2. Stabilizing voltage in a power circuit (smoothing)
3. Matching impedance in a high frequency circuit

## What is the Q value of an inductor (a coil)?

The Q value is a parameter that indicates the quality of an inductor.
When the Q value is higher, loss becomes lower and the inductor has superior characteristics.

## Why is inductance specified by numbers such as 1, 1.5, 2.2, 3.3, etc.?

This is because inductance is based on the preferred number series for passive components taking tolerances into consideration, as prescribed in JIS C 5063.
For inductors, E6 (tolerance: 20%), E12 (10%), E24 (5%) series are often used, and in the E6 series, for example, inductance is specified as 1, 1.5, 2.2, 3.3, 4.7, 6.8, 10, 15, etc.

## What is SRF (self-resonant frequency)?

An inductor has low distributed capacitance between terminal electrodes or the turns of a wire-wound conductor, and its inductance with such distributed capacitance resonates at a certain frequency.
This frequency is called the self-resonant frequency, and the inductor does not function as an inductor beyond the self-resonant frequency.

## Please explain the types of inductors (coils) and their differences.

Inductors are largely divided into inductors for power circuits, inductors for signal circuits, and inductors for high frequency circuits according to their application purposes. Furthermore, their construction methods give them different characteristics.

For power circuits For signal circuits For high frequency circuits
Wire-wound
inductors
These inductors feature low DC resistance (Rdc), since thick wires can be used, and are suitable for large currents.
A wire-wound power inductor for power circuits has a structure wherein a copper wire is wound around a drum core made with a magnetic material (ferrite or a metallic magnetic material). By using a material with high permeability and low loss as the core material, even a small-sized inductor can achieve high inductance.
In addition, a magnetic material with high permeability and low loss requires fewer turns of the wire to achieve the same inductance, creating the DC resistance (Rdc) of the wire and contributing to the reduction of battery power consumption.
These inductors feature low DC resistance (Rdc), since the core can be wound with a thick wire, and support large currents. In addition, they feature high accuracy and narrow tolerance.
Multilayer
inductors
Multilayer inductors can be made compact, and are therefore suitable for small mobile equipment including cellular phones.
In addition, because of their structure in which the coil is covered by ferrite, they feature a superior magnetic shielding effect by the ferrite, and are suitable for high-density packaging on circuit boards.
A multilayer structure allows for greater size reduction and a detailed product lineup, compared to a wire-wound structure.
In addition, due to their structure in which the coil is covered by ferrite, these inductors feature a magnetic shielding effect by the ferrite, and are suitable for high-density packaging on circuit boards.
A multilayer structure allows for size reduction and a wider product lineup. They support high frequency applications by using dielectric ceramic as the base material.

## Would it be safe to use an inductor in which a current exceeding the rated current flows momentarily?

It depends on the magnitude of the current and how long the current flows.
If you can provide us with the usage conditions within your company (current value and duration), we will be able to provide a guideline as to whether or not inductors can be used; please consult with the nearest point of contact.

However, please perform a final confirmation under the conditions used in your company.

## Would it be a problem to use inductors in parallel?

It is possible to use inductors in parallel.
When inductors are used in parallel, a greater amount of current can flow.
However, there is the disadvantage that inductance will decrease, as it is a synthetic inductance.
The synthetic inductance Lt can be obtained using the following formula:

## Please provide a basic explanation of the process for choosing an inductor.

1. Specify the dimensions of the inductor.
Specify the dimensions of the inductor, including the height.
2. Specify the required inductance.
3. Specify the rated current.
Specify an inductor with a rated current greater than the current that flows through the circuit to be used.

Generally, larger inductors have better characteristics. However, when choosing an inductor, consider a balance among the above 3 points.

## There are direction identification markings on some products. What are those markings for?

An inductor has a winding direction (polarity), and the marking is placed so that the polarity can be confirmed from its external appearance.
Depending on usage conditions, the polarity of the inductor can affect the inductor's characteristics.

## Where can I see the inductance value for the frequency at which I would like to use the inductor?

You can see the inductance value in the characteristics graph displayed on the bottom of the detailed product data page.

## Do the inductor products of TDK support lead-free soldering?

Please check the description in the product catalog on our website.

↓↓Product catalog example

## Was the ferrite that is used as the core material of inductors invented by TDK?

Ferrite is a magnetic material invented by Drs. Yogoro Kato and Takeshi Takei of the Tokyo Institute of the Technology in 1930.
Tokyo Denki Kagaku Kogyo K.K., the predecessor of TDK, was established to commercialize this breakthrough magnetic material, "ferrite", born in Japan.
In 2009, "Development of Ferrite Materials and Their Applications" by the Tokyo Institute of Technology and TDK became the 89th IEEE Milestone worldwide and the 10th in Japan.

## What is the FIT (Failures in Time) of your inductors?

FIT is calculated by each product.
The following data is required for the calculation of FIT; please provide the nearest point of contact with this data.

1. Product number
2. Application purpose　Ex: cellular phone, industrial camera, automobile engine unit, etc.
3. Average operating temperature
4. Average operating duration throughout a day

## I would like to obtain a sample.

You can check distributor inventory data by clicking the "Buy Now" button displayed in the upper right of the search result page or the detailed product data page.
By clicking the "Add to Cart" button displayed there, you can buy the product with ease (some distributors may require user registration).

## Please provide alternative product information for a discontinued product (including products to be discontinued).

"Status", which is displayed at the top of the detailed product data page shows the current supply system. For non-recommended products (discontinued products, etc.), the product numbers of the alternative products are displayed.
If the product itself is not displayed in the search result or no product numbers of alternative products are shown, please consult with the nearest point of contact.