Overload protection for supply transformer for electronic equipment - uses non-inductive additional winding on transformer as temp. sensor

摘要

The application is typically to the power supply transformers for audio amplifiers. An additional winding (5), of fine wire gauge, is mounted on the transformer, typically between the prim. and the sec. The coil (5) is bifilar wound (6,7) and is thus free from an induced a.c. voltage. The coil (5) is connected as the variable element in a dc bridge (1,2,6,7;3,4) whose diagonal is the input to a comparator (9). The comparator (9) is pref. an operational amplifier whose output controls switching transistor (15) in series with the operating coil (23) of a relay (16). If the dc bridge is unbalanced by a large increase in coil resistance, due to high temperature in the transformer, the amplifier output changes polarity. The transistor switches off and the relay contacts open to switch off the power supply to the transformer. The latter is then automatically reconnected when the winding have cooled to the point where the comparator again changes its output polarity.