Inductor Package Styles
Inductors are passive devices used in electronic circuits to store energy in the form
of a magnetic field. They are the compliment of capacitors, which store energy in the form of an electric field. An
ideal inductor is the equivalent of a short circuit (0 ohms) for direct currents (DC),
and presents an opposing force (reactance) to alternating currents (AC) that depends
on the frequency of the current. The reactance (opposition to current flow) of an inductor
is proportional to the frequency of the current flowing through it. Inductors are sometimes
referred to as "coils" because most inductors are physically constructed of coiled sections
The property of inductance that opposes a change in current flow is exploited for
the purpose of preventing signals with a higher frequency component from passing while
allowing signals of lower frequency components to pass. This is why inductors are sometimes
referred to as "chokes," since they effectively choke off higher frequencies. A common
application of a choke is in a radio amplifier biasing circuit where the collector of
a transistor needs to be supplied with a DC voltage without allowing the RF (radio frequency)
signal from conducting back into the DC supply.
When used in
series (left drawing) or
parallel (right drawing) with its circuit
compliment, a capacitor, the inductor-capacitor combination forms a circuit that resonates
at a particular frequency that depends on the values of each component. In the series
circuit, the impedance to current flow at the resonant frequency is zero with ideal components.
In the parallel circuit (right), impedance to current flow is infinite with ideal components.
Real-world inductors made of physical components
exhibit more than just a pure inductance when present in an AC circuit. A common circuit
simulator model is shown to the left. It includes the actual ideal inductor with a parallel
resistive component that responds to alternating current. The DC resistive component
is in series with the ideal inductor, and a capacitor is connected across the entire
assembly and represents the capacitance present due to the proximity of the coil windings.
SPICE-type simulators use this or an even more sophisticated model to facilitate more
accurate calculations over a wide range of frequencies.
Related Pages on RF Cafe
- Inductors &
- Inductance Conversions
Standard Inductor Values
website has a very sophisticated calculator for coil inductance that allows you to en9ter
the conductor diameter.
Equations (formulas) for combining inductors in series and parallel are given below.
Additional equations are given for inductors of various configurations.
Total inductance of series-connected inductors is equal to the sum of the individual
inductances. Keep units constant.
Closely Wound Toroid
Coaxial Cable Inductance
Straight Wire Inductance
These equations apply for when
the length of the wire is much longer than the wire diameter (look
up wire diameter here). The ARRL Handbook
presents the equation for units of inches and µF:
For lower frequencies - up through about VHF, use this formula:
Above VHF, skin effect causes the ¾ in the top equation to approach unity (1), so
use this equation:
Straight Wire Parallel to Ground Plane w/One End Grounded
The ARRL Handbook presents this equation for a straight wire suspended above a ground
plane, with one end grounded to the plane:
a = wire radius,
l = wire length parallel to ground plane
h = height of wire above ground plane
to bottom of wire
Parallel Line Inductance
Multi-Layer Air-Core Inductance
Total inductance of parallel-connected inductors is equal to the reciprocal of the
sum of the reciprocals of the individual inductances. Keep units constant.
Inductance Formula Constants and Variables
The following physical constants and mechanical dimensional variables apply to equations
on this page. Units for equations are shown inside brackets at the end of equations;
e.g., means lengths are
in inches and inductance is in Henries. If no units are indicated, then any may be used
so long as they are consistent across all entities; i.e., all meters, all µH, etc.
C = Capacitance
L = Inductance
N = Number of turns
W = Energy
= Relative permittivity (dimensionless)
= 8.85 x 10-12 F/m (permittivity of free space)
= Relative permeability (dimensionless)
x 10-7 H/m (permeability of free space)
1 meter = 3.2808 feet <—> 1 foot = 0.3048 meters
1 mm =
0.03937 inches <—> 1 inch = 25.4 mm
Also, dots (not to be confused with decimal points)
are used to indicate multiplication in order to avoid ambiguity.
Inductive reactance (XL, in Ω) is proportional to the frequency (ω,
in radians/sec, or f, in Hz) and inductance (L, in Henries). Pure inductance has a phase
angle of 90° (voltage leads current with a phase angle of 90°).
Energy Stored in an Inductor
Energy (W, in Joules) stored in an inductor is half the product of the inductance
(L, in Henries) and the current (I, in amp) through the device.
Voltage Across an Inductor
The inductor's property of opposing a change in current flow causes a counter EMF
(voltage) to form across its terminals opposite in polarity to the applied voltage.
Quality Factor of Inductor
Quality factor is the dimensionless ratio of reactance to resistance in an inductor.
Single-Layer Round Coil Inductance
Formula for d >> a:
In general for a = wire radius:
Note: If lead lengths are significant, use the straight wire calculation to add that
Finding the Equivalent "RQ"
"Q" of an inductor is the ratio of the reactive component to the resistive component,
an equivalent circuit can be defined with a resistor in parallel with the inductor. This
equation is valid only a a single frequency, "f," and must be calculated for each frequency