A circuit for powering high-efficiency lighting devices from a thyristor-controlled dimmer includes a power converter for powering the high-efficiency lighting devices from input terminals of the circuit. The circuit also includes a control circuit that controls the input current drawn by the input terminals at least while the power converter transfers energy to the lighting devices. The circuit also includes a sensing circuit that determines or measures at least one attach current characteristic at the input terminals and stores an indication of the characteristic for subsequent operation of the control circuit.