Power control circuit 6 of a loop-mountable unit of a fire alarm system, comprises adjustable current source 12, inverting amplifier 13, and damper 14. The damper may have an input for connection to a loop and an output connected to the inverting input of the inverting amplifier such that the voltage at the output of the damper is smoothed with respect to the voltage at its input. The inverting amplifier provides a control signal, based on the loop voltage, to the adjustable current source. The variable current source adjusts the current from the loop through the power control circuit based on the control signal. The inverting amplifier may be arranged to provide the control signal with a voltage which is inversely proportional to the voltage at its inverting input. The power control circuit may further feature an output capacitor for supplying power to a beacon/flasher and a DC-DC switcher arranged to discharge a reservoir capacitor to charge the output capacitor.
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