BY TOM NAPIER
is a straight line with unit slope.
The ratio B/H is the relative
permeability of the core. It may be
as low as five in some dust cores
and as much as 5,000 in ferrite
cores. The curve is now much
steeper; the oscilloscope’s Y gain
must be readjusted to create a
reasonable display.
Typical core materials show a
straight line which curves towards
the horizontal at both ends (see
Figure 1). The curved tips show
the core’s permeability falling at
high currents.
■ FIGURE 2. This noise
filter toroid won’t make a
high-Q inductor.
old keyboard.
The B-H
Instrument
Using the Curves
In many DC-DC converters, a
constant voltage is applied to
an inductor. Initially, the current
increases linearly with time, storing
energy in the inductor. When the
current reaches the saturation level,
the inductance falls and the rate of
current increase rises. This increased
current stores little additional
energy in the inductor but may
damage the switching
transistor. In DC-DC
converters, inductors
nearly always operate below their
saturation level so
this level is essential
design information.
Low-loss core
materials have a B-H
curve in which
the positive- and
negative-going curves
almost match. A loop
with a gap between
the two traces, as in
Figure 2, reveals both
core losses and residual magnetization. I’d
wound this inductor
on a large ring used
as an interference
suppressor in an
For clarity, I’ve split
the tester schematic
into the ramp current
generator (Figure 3)
and the core analyzer
(Figure 4). The part
numbers are sequential across both
figures.
The current ramp drives the
inductor under test with equal positive
and negative peak currents. These are
adjustable up to 0.5 amps with switch
SW1. A one ohm resistor (R18) in
series with the inductor measures the
actual current flowing and supplies
the horizontal (X) sweep signal for the
oscilloscope.
It isn’t practical to directly sense
the flux in the core so it’s calculated
from the coil voltage. Since the rate
of current change is constant, this
voltage is proportional to the coil’s
inductance. With a pure inductor, it
would be a square wave with positive
and negative values proportional to
the inductance times the rate
of change of the current. If the
permeability changes, then so does
the inductance and the voltage
changes accordingly.
For the oscilloscope’s vertical (Y)
input, we want a voltage that rises
■ FIGURE 3. The ramp
current generator.
June 2007 47