<p>A disclosed power supply circuit includes a transformer including a primary coil and a secondary coil; a primary circuit connected to the primary coil; a secondary circuit connected to the secondary coil; a photocoupler including a light-receiving element and a light-emitting element, the photocoupler coupling the primary circuit and the secondary circuit; and a breakdown element connected in parallel with the light-receiving element or the light-emitting element. The breakdown element is configured to break down at or above a first voltage that is lower than a second voltage at or above which the light-receiving element or the light-emitting element breaks down and thereby to cause a short-circuit between terminals of the light-receiving element or the light-emitting element.</p>
