Made some correction *impedence* ----- Original Message ----- From: John Chung To: Microcontroller discussion list - Public. Cc:=20 Sent: Monday, December 5, 2011 12:41 AM Subject: [EE] Question of bench PSU safety. Hi guys, Had an eventful night...... Tried to power a circuit "which" I thought was = a DC circuit ONLY to be greeted by a power trip and my bench power supply was smoking...... Needless to say I opened up the bench power supply to find that the transfo= rmer leads to the=20 banana jack had it's enamel coating melted... I found out later that the DC= ground was actually tied to the AC ground....... 145volts on the tested circuit and th= e bench PSU banana jack was hooked to AC ground too....... There was some reactance between the bench banana jack to AC ground which K= IND of save the PSU life...... The bench PSU still powers up and works*despite the transformer's enamel being = melted.* To NOT repeat history here are some questions: 1) Do I need to check ALL DC circuit to determine if it is tied to AC groun= d? 2) When the DC circuit*tied to AC ground* is tied to ground hooking up to t= he PSU which is also tied to ground would ruin it.... But IF the =A0=A0 PSU is NOT tied to ground, would it be safe to use. Since it cannot = be grounded since the PSU has not connection to AC ground. 3) Case scenario. The notebook DC ground has resistance of 1MOhm to AC grou= nd which is okay to hook up a scope probe BUT it was a impedence of 1Kohm =A0=A0=A0 it would be hazardous? Thanks, John --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .