The Flowtron Replacement Lamp Ballast/Transformer is an essential replacement part for the UV Bulb and zapping grid to keep with you at all times. The UV Bulb and zapping grid are essentially the heart of the Bug Zapper as they are the main method of attracting and killing mosquitoes and other insects. This Flowtron Replacement Lamp Ballast/Transformer is compatible with Flowtron BK-40D.