You're seeing an effect called "bounce back". This commonly occurs
with high voltage caps.
Putting a 1M resistor across the cap will help with the safety here.
Lots of info in the answers to the stackexchange question:
https://electronics.stackexchange.com/questions/282972/capacitor-gaining-vo=
ltage-over-time

Take care.
On Sun, Oct 21, 2018 at 6:04 AM Manu Abraham <abraham.manu@gmail.com> wrote=
:
>
> 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 do=
wn.
>
> At least thoughts burned up a few hours, where that 80Vdc was coming from=
..
> After that, sat back and laughed at myself.
>
> Cheers,
> Manu
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