An open circuit monitor (14) is disclosed which is configured to detect an undercurrent condition on the primary winding of a transformer (10) caused by an open circuit condition on the secondary winding. The monitor (14) is configured to mount directly upon terminals (12) of the transformer (10) thus avoiding the need for interconnecting cabling between the monitor (14) and the transformer (10). A main supply is directly connected to input terminals (23) of the monitor (14). Also disclosed is a current sensing monitor (106) which senses the undercurrent condition without the use of a resistor (50) connected in series with the primary winding. The current sensing monitor (106) preferably takes the form of a Hall effect sensor (108). The open circuit monitor is generally configured for use with a high tension transformer of a neon lighting display.