All dc chokes use low-permeability cores, either powdered
iron or ferrite cores with a substantial air gap.
A low permeability is essential to prevent the core
saturating with a large dc current.
Inductance is a measure of the slope of the B-H curve. A
core with high permeability can have a slope or inductance value with a
The low permeability of cores with an air gap or made of
powdered iron causes these cores to exhibit an inductance that can be
specified within very tight limits.
Cores for low dc bias currents (<400mA)
Cores for high dc bias currents (>400mA)
| Measure Inductance
with specified dc current in winding.
Accept wide limits on range of inductance values but result must
be greater than a certain minimum value.
| Measure inductance without dc
Set limits as tight as possible e.g. 5% as means to verify gap.
It is essential during design testing to confirm that a
dc choke exhibits the proper inductance at rated dc current.
In production testing, however, some dc chokes can be
tested by checking the inductance without dc bias, but
Specifying tight limits will verify that the core has the
correct turns and, therefore, the correct slope to give the required
inductance at the specified dc current.