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.