WIND YOUR OWN TRANSFORMER
AND BUILD A DC-TO-DC
BY JIM STEWART
With switch-mode projects, there’s
always the problem of where to
obtain inductors and/or transformers
with the necessary specifications.
Parts can be hard to find and
expensive. So why not “roll your
own?” In this project, we will design
and wind a transformer and use it to
get +12V and -12V from a 9V battery
with the DC-to-DC converter shown
in Figure 1.
Many hobbyists who look into designing transformers
go away thinking it's too hard. There are two
reasons for that. First, magnetic material is non-linear,
which leads to some arcane equations. Second, there's
a profusion of magnetic terminology. There's one set
of magnetic parameters, but two sets of units to
describe them: cgs (centimeter-gram-second) and SI
(Standard International). A third set is English units — is
obsolete. We use SI units, but a conversion table is
The switching circuit in this project is bare-bones.
■ FIGURE 2
■ FIGURE 1
There's no feedback between output voltage and the
control IC. We kept it simple because its main purpose
is to allow the builder to experiment with winding
Magnetics: MMF and Flux
Once it was thought that magnetism and electricity
were separate things. They're not. Magnetism is caused
by quantum electron-spin and by the relativistic effects of
current in a conductor. But let's keep it simple.
We start with magneto-motive-force (mmf) and
magnetic flux (Φ). Wrap a coil of wire around a steel core
and put current through it (Figure 2). You get mmf = N x I
where N is the number of turns of wire and I is the
current. The mmf causes flux to flow through the core.
We can write Ohm's Law for magnetic circuits:
Φ = mmf/ℜ where ℜ is the reluctance.
Resistance depends on three things: the length of the
resistive material (l), the area of its cross-section (A), and
its resistivity (ρ): R = ρ(l/A). Length and area can change,
but the resistivity of a material is constant.
Reluctance has a similar equation: ℜ = ν(lF/Ac) where
lF is the flux path length, Ac is the core's cross section
area, and ν is the material's reluctivity. The inverse of
reluctance is more commonly used: 1/ℜ = μ (Ac/lF) where
μ = 1/ν is permeability.