February 4

Class X/Class Y (Safety) Capacitor Parameters

“There are more capacitors in a distributor catalog, Horatio, than are dreamt of in your philosophy. “ – Shakespeare¬† (I think? Close enough)

Safety caps-Appliance Class ISpecifically, I am talking about Safety capacitors, aka X-capacitors, Y-capacitors, XY-capacitors, RFI/EMI suppression capacitors, line filtering capacitors, and no doubt other modes of reference.

When you have the occasion to mix hazardous household voltage with capacitors (perhaps to keep noise from leaking out of your circuit, or for surge protection), special care needs to be taken in selecting the capacitors used.

Normal ceramic capacitors have the distressing tenancy of failing short.¬† In the case of diagram to the right, such a capacitor in the “Cx” position would cause the mains to short through the capacitor, creating a risk of fire, (small) explosion, and a Bad Day.¬† Should the failing capacitor be in the “Cy” position, the mains could be shorted to earth ground (risking fire, etc) or, if the case is not connected to earth, could just directly connect the case to mains, creating the risk of arcing, electrocution, and a Bad (hair?) Day for someone. Continue reading