Timing circuit for electromechanical switch - is used to minimise surce current when switching on transformer

摘要

The application is to circuits where the protection setting has to be near the full load value, as for example in train heating, and the surge current when switching on the transformer must therefore be limited. This is achieved by relating the switching instant to the charge on a capacitor. The closing relay coil of the switch is in series with an electronic switch and a contact. A small voltage is constantly applied to the coil via resistors and gives a measuring signal voltage comprising the drop across a coil and a ballast resistor. On closing the contact a capacitor is charged through resistors and the charge voltage measured on a divider.