Abstract: A power supply circuit for a neon tube includes a step-up power transformer having a primary winding connectable to a power source, a secondary winding to which the neon tube is connected, and an auxiliary winding which is used to sense if the secondary winding is loaded or not and develops a control voltage proportional to current flowing through the neon tube, the control voltage maintaining a relay operated so that the primary winding of the transformer is connected to the power source in the absence of load fault conditions, a load fault condition causing the control voltage to decrease below the level required to maintain the relay operated, so that the primary winding of the transformer is disconnected from the power source, interrupting high voltage in the secondary winding of the transformer.