Nuts & Volts - June 2007

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.