Agreed, more likely to be D1. Do you think a cap and TVS across the motor/diode would be sufficient? I have a 600V diode IN5406 which is rated at 420Vrms (600V DC). It's only rated to 3A continuous (200A for 8ms) but I'd have thought that should be sufficient for my low power testingThat bridge is rated at 600V (420VRMS), 10A ... I very much doubt that was the case.