Transformers by nature are wide band devices
regarding stray signal coupling. Where a transformer is
required to operate in electrically noisy environments,
a conducting shielding layer can be interposed between
the primary and secondary windings (or between
individual secondaries) to minimize the capacitance
between them. This can reduce (or even eliminate) some
types of common mode noise, but its effectiveness
depends on the noise characteristic as well as the
transformer's overall surface area.
This
type of shield is sometimes required to satisfy certain
safety regulations and circuit configurations.
Capacitive screens add layers and cost to a toroid's
build, because if they are required, a larger core may
have to be specified with a large enough inner diameter
to complete the windings. This type of shielding should
supplement and not replace the usual line filters and
suppressor networks that may be required for circuit
operation or EMC compliance. |