Abstract
This paper introduces a failure case of PhotoMOS relay applied to signal control. Its output is long-term or intermittent conduction when no input signal added as failure phenomenon. After eliminating the failure modes such as electronic over stress, ion pollution and structure aging etc., this paper reveals the failure mode and cause of PhotoMOS relay in the case, judges that the failure is a lot quality problem of products. Three prevention methods are studied on this kind of failure, and one method is suggested for the failure prevention of this PhotoMOS relay.