A constant output circuit is disclosed that includes a power supply circuit, a LM-FOT control circuit, a switching circuit and a transformer. Output ends of the power supply circuit are respectively connected with a first input end of the LM-FOT control circuit and an end of a primary winding of the transformer. A controlling end of the LM-FOT control circuit is connected with a controlled end of the switching circuit. A second input end of the LM-FOT control circuit is connected with an output end of the switching circuit. An input end of the switching circuit is connected with the other end of the primary winding of the transformer. An end of a secondary line winding of the transformer is configured for outputting constant voltage signals. Another end of the secondary line winding of the transformer is grounded.