A trip unit for a circuit interrupter includes a power supply, a plurality of input terminals and a plurality of output terminals. A heater conductor is electrically connected between each pair of the input and output terminals. The heater conductor is adapted to generate heat responsive to current flowing through the conductor. A diode is disposed proximate each of the conductors for heating thereby. The diodes are electrically interconnected with the power supply and are adapted to provide a forward voltage representative of the temperature of the corresponding conductor. Another diode is electrically interconnected with the power supply and is adapted to provide a forward voltage representative of ambient temperature. A thermal trip circuit generates a trip signal for the circuit interrupter responsive to the forward voltage of the diodes.