Key takeaways
- A coil is a length of insulated wire wound into a shape that concentrates a magnetic field.
- The winding family decides the build, the heat path and the tooling, not just the appearance.
- Turns, wire, core and resistance are the four parameters that describe most coils.
- The application chooses the family: layer wound for control, wild wound for cost, toroidal for low stray field.
What a coil is
A coil is a length of insulated wire wound into a series of turns. When current flows, each turn produces a magnetic field, and the turns add together to produce a stronger field along the axis. When the field changes, the coil produces a voltage across its ends. That is the whole idea, and almost every electrical coil is a variation on it.
The wire is insulated so that the turns do not short to each other. The insulation is a thin enamel, sometimes with an additional layer for a higher temperature class or for mechanical protection.
Coils appear as relay and contactor coils, solenoid and valve coils, transformers, inductors, chokes, antennas and motor windings. The names describe the application, not a different physical principle.
The winding families
A bobbin wound coil is wound into a plastic former with cheeks at each end. The bobbin sets the winding width, holds the wire in place and carries the pins. It is the most common family for relay, contactor and small transformer coils.
A self-supporting coil is wound without a bobbin and held together by the wire's own bonding layer, by varnish or by tape. It packs more copper into a given space because there is no bobbin wall, which suits a coil where space is tight or where the winding has to sit close to a core. It is less resistant to handling damage.
A toroidal coil is wound around a ring shaped core. The turns follow the ring, so the magnetic field stays mostly inside the core and the stray field is low. Toroids are used in filters, current sensors and transformers where low radiated field matters. They are slower to wind and need a special machine.
Two more terms describe the traverse rather than the former. A layer wound coil places the turns in neat layers, which gives a predictable build and a controlled capacitance. A wild wound coil lets the wire cross itself at random, which fills a bobbin quickly and cheaply but gives a less predictable build.
DC and AC coils
A DC coil is designed around resistance. The current is set by the applied voltage divided by the resistance, and the resistance rises as the coil heats. The coil needs enough turns and enough current to produce the required ampere turns, and the wire has to carry the current without overheating.
An AC coil is designed around impedance. The alternating current sees the resistance and the inductance, and the inductance depends on the core and the air gap. An AC coil that is energised without its armature can draw a large current because the air gap is open and the inductance falls. That is why a contactor coil is rated with the armature in place and why a stuck armature can burn a coil.
Some coils are fed from a rectified supply, which is electrically DC but may carry a ripple. The ripple heats the core and the winding, and a coil that is fine on a smooth DC supply can run hot on a rectified one. The supply has to be part of the specification.
Cores and the magnetic circuit
An air core coil has no magnetic material in the middle. It is used where the field has to be linear, where the coil operates at high frequency or where a core would saturate. The field is weaker for the same current, so an air core coil needs more turns or more current for a given field.
A ferromagnetic core concentrates the field and increases the inductance for the same number of turns. The core material and the lamination or ferrite grade decide the loss at the operating frequency. A laminated steel core suits mains frequency, while a ferrite core suits a high frequency supply. A solid steel core is cheap but eddy currents make it unsuitable for AC.
The core also sets the mechanical behaviour. In a relay or a solenoid, the core and the armature form a magnetic circuit with an air gap. The force depends on the gap, the current and the geometry. A coil is therefore part of a magnetic system, and its performance cannot be described by its electrical parameters alone.
The parameters that describe a coil
Turns, wire diameter, resistance and inductance are the four numbers most coils are specified by. The turns set the ampere turns for a given current. The wire diameter sets the resistance and the current capacity. The resistance sets the current at a given voltage. The inductance sets the response to a changing current and, for an AC coil, the impedance.
Those numbers are linked. For a fixed winding space, more turns means thinner wire, which means higher resistance. A higher resistance reduces the current, which reduces the field. The design is a balance between the field, the heat and the space, and a change to one parameter moves the others.
The thermal class, the insulation resistance and the dielectric strength are the other calls. A coil that meets its electrical target and exceeds its temperature class will fail early. The class is an endurance statement, and the hot spot is what matters, not the average temperature.
| Family | Construction | Typical use | What to watch |
|---|---|---|---|
| Bobbin wound | Wire on a plastic former | Relays, contactors, small transformers | Wall thickness and creepage |
| Self-supporting | Bonded or varnished wire, no former | Tight spaces and close core fits | Handling damage and turn movement |
| Toroidal | Wire around a ring core | Filters, sensors, low stray field | Winding speed and lead routing |
| Layer wound | Neat layers with a controlled traverse | Predictable build and low capacitance | Build height and capacitance |
| Wild wound | Random traverse | Low cost bobbin fill | Build variation and resistance spread |
| Encapsulated | Winding potted or overmoulded | Harsh environments and high voltage | Voids and thermal expansion |
A coil can belong to more than one row, for example a layer wound coil on a bobbin that is later encapsulated.
Where each family is used
Relays and contactors use bobbin wound coils because the bobbin carries the pins and fits the magnetic assembly. Solenoid valves often use an encapsulated coil because the coil sits in a wet or dirty environment, and the encapsulation protects the winding and provides the creepage.
Current sensors and filters use toroidal coils because the closed core gives a low stray field and a stable inductance. Motor windings are a different family again, distributed in slots rather than wound on a former. The principles are the same, but the manufacturing equipment and the inspection are specific to the motor.
The application also sets the test. A relay coil is tested for resistance and dielectric strength. A transformer is tested for turns ratio, inductance and isolation. A sensor coil may be tested for inductance and quality factor. The test follows the function, and the drawing should say which parameter is controlled.
Choosing a winding for a new design
Start with the space and the field. If the coil has to fit a narrow housing, a self-supporting winding may give more copper in the same volume. If the coil has to be handled and assembled, a bobbin is easier.
Then check the thermal path. A bobbin adds a wall between the winding and the core, which is a thermal resistance. An encapsulated coil adds another layer, which can help or hurt depending on the fill. The winding family is a thermal decision as well as a mechanical one.
Finally, check the volume and the equipment. A wild wound coil is cheap at volume but has a wider resistance spread. A layer wound coil is more controllable but slower. A toroid is slow to wind and usually reserved for the applications that need it. The best coil is the one that meets the function at the volume the programme can support.
Frequently asked questions
What is a coil in simple terms?
A coil is insulated wire wound into turns. Current through the turns creates a magnetic field, and a changing magnetic field induces a voltage in the turns. Coils are used to create force, store energy, filter signals or transform voltage.
What is the difference between a bobbin coil and a self-supporting coil?
A bobbin coil is wound on a plastic former that holds the wire and carries the pins. A self-supporting coil has no former and is held together by a bonding layer, varnish or tape, which packs more copper into the same space.
Why does an AC coil overheat when the armature is stuck?
The open air gap lowers the inductance, so the coil draws more current. The extra current raises the temperature, and a coil that is rated with the armature closed can overheat quickly.
What is the difference between a layer wound and a wild wound coil?
A layer wound coil places the turns in neat layers, which gives a predictable build and controlled capacitance. A wild wound coil lets the wire cross at random, which is faster and cheaper but gives a less predictable build.
Sources and further reading
- IEC 60317 series, Specifications for particular types of winding wires
- IEC 61558 series, Safety of transformers, reactors, power supply units
- Qumek coil design and winding review notes
Qumek Engineering Desk
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