A coil usually is a round-wound insulated copper wire on a carrier material.

The measure of inductance value is Henry (H).

Common applications

A coil can be used as an electromagnet or a (low pass) filter. Spikes on a power supply can be filtered with a coil in series, or an audio signal can be filtered, decreasing frequency (spectral) components in amplitude. Another example can be found in transformers (containing 2 coils), where an AC voltage is transformed to a higher or lower voltage. Electricity is converted to magnetic power, and converted back to electricity.

Coils can be used as electromagnets. By supplying a voltage, a current will run through the coil. Adding a ferrous (magnetic metal) core bundles the magnetic field of the electromagnet, which translates to a larger number or weight of metal parts you can attract. Coils can be made with thin or thick wire. The thicker the wire, the lower the resistance of the coil, increasing the current carrying capability of the coil.

Basic Theory

A coil resists changes in current, therefore, it can filter out noise and spikes in a power supply.

To analyze the frequency behaviour of a coil, the formula below can be used:
$X_L = 2 \pi f L$
where:
$X_L$ = reactance or resistance at a certain frequency, measured in Ω
$\pi$ = 3.1415…
$f$ = frequency (Hz)
$L$ = inductance in (H = Henry)

From the equation you can see that for low frequencies (f -> 0) the resistance is 0 Ω whereas for very high frequencies the resistance is very large. In short: the coil resists fast changes.

Another important equation related to the behaviour of a coil when the current is changing is:
$V = L \frac{dI}{dt}$
where:
$V$ = voltage difference across the coil (V)
$dI$ = change in current during small time interval dt (A)
$dt$ = time interval (s)

What you can see from this equation is what happens when you switch off the current through a coil. In that case dI is negative (current I goes from a certain value to 0) and the switching time dt is very small. The result is a high voltage V in the opposed direction compared to the direction of the original current. This high voltage can damage other parts of your circuit. That is the reason why you always see so-called flyback or freewheel (Dutch: “vrijloop”) diodes in circuits using coils.

Remember: every time you use coils or components containing coils: Use a fly back diode.

Parts containing coils: tag Relay, tag Solenoid, tag various motors.