The answer to this is obvious. 1. The transformer has no shield between the primary and secondary windings= .. 2. One side of the output is grounded to earth. This produces a situation where there will be capacitive coupling between t= he transformer windings which while of high impedance at mains frequency is= still low enough to allow a small current to flow. This is then rectified = by the diodes on the transformer secondary and charges the capacitor. The way around this is to have an earthed shield between the primary and se= condary windings. A simple form of this is a single layer winding with only= one end brought out for connection to ground. A properly done shield is a = layer of copper tape with the two ends insulated from each other so it does= n't form a shorted turn, and then a wire is soldered onto it to bring the g= rounding connection out. The shield always has to be grounded to be effecti= ve, if it is not then the original scenario described below will exist. -----Original Message----- From: piclist-bounces@mit.edu On Behalf Of Manu A= braham Sent: 21 October 2018 11:03 To: Microcontroller discussion list - Public. Subject: [EE] Voltage accumulation Hi, I did not forget to put the sq brackets this time, Bob . :-) Not a query, but tripped on this one, wondered for a while, later on alone realization dawned. Thought about sharing this one on the list. Have a 230V/12-0-12 CT transformer, of which 0 &12 connected to a Bridge rectifier, the output of which is smoothed out by a 680uF/200V capacitor, providing about 16VDC (12 x 1.4142). The power supply worked well and good enough. The transformer was left connected to the mains for a while, in absent minded mode of operation. A few days later, just before wanting to test a small board measured the voltage on the power supply, saw it was about 80V. Sat in thoughts for about an hour what was wrong, thought maybe the transformer windings had sprung a leak. Thought about it for a moment, then thought probably the capacitor was getting charged due to no load. Discharged the capacitor with a huge spark. Things pretty much settled down= .. At least thoughts burned up a few hours, where that 80Vdc was coming from. After that, sat back and laughed at myself. Cheers, Manu --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .