Nifty wall wart trick

[OperaHouse]

I was working on my UNO water heater project where  the power point of the panels was upped to 51V.  The supply for the controller was just a resistor and a zener.  That was now dumping over 3W and was digging around to find a resistor value that would lower that.   Wondered if an electronic wall wart would work at these voltages.  The lower stated voltage input is often 85V.  A few years back I was playing around with the TNY series of chips that are often found in these.  They will work at under 30V, even down to 12 with a little trick.  I thought that a 12V version ought to at least give me 9-10V unregulated at 50V input, enough to power a UNO with some limited drive.  It was right on the money at 12V running the UNO.  Tried four more units and they all worked too.  If you have a 48V system this is an easy way to provide  an isolated supply for a high side driver.   

[SparWeb]

Nice trick
Any evidence that the current rating still holds?

[madlabs]

Along the same lines, I used this trick as Opera suggested with my Solyndra 100V solar PV tube. I hooked to a cell phone charger and while the voltage drooped to around 60V, it was charging the phone. So at least it was doing some work. I didn't measure current or voltage out though.

Thanks BTW, Opera. Very handy trick.

Jonathan

[OperaHouse]

Never did any actual testing.  I figure I can get 100ma out of a 1A unit.  It looked like my laptop would charge but the unit was getting warm.  If I remember the TNY chips had a FET on resistance of 35 ohms so pushing them hard would result in overheating.  Had some power boards out of old DVD players that that were more sophisticated and they didn't work.   Now that I know the transformer ratios are workable I will play around with fooling the low voltage detect circuit.