A system is disclosed for setting an electrical circuit parameter at a
predetermined value. The system comprises a first electrical component
having a first electrical parameter associated therewith. Sensing means
generate a control signal indicative of the value of the first electrical
parameter. A second electrical component has a second electrical
parameter associated therewith, the value of which has a predetermined
relation to the value of the first electrical parameter. Adjustment means
receive the control signal generated by the sensing means; and, in
response to the control signal being indicative that the electrical
circuit parameter is not at the predetermined value, selectively connects
or disconnects at least one further electrical component to or from the
second electrical component thereby to provide said predetermined value.