> On Thursday 27 April 2006 20:32, Brendan Kehoe wrote:
> > We've got a tiny Ethernet
> > switch that wants 9V, but a 230->13.6V transformer, who has two contacts
> > (+13.6V, and a ground). A junction box feeds one device, a WRAP board,
> > that wants 12V. But the switch wants 9V.
>> It may want 9V but you may well find the 9V supply goes directly to an
> internal 7805 and the whole device runs off the 5V output from
> the 7805, in
> which case you can run it on anything from 7.5V to 28V. That's
> the case with
> an old 8 port hub I have here.
The 7805 might handle 28V according to the electrical spec, but you
also have to consider heat. If the device only has a small heatsink,
the maximum allowable input voltage may be much lower. There may
also be capacitors across the input lines that are rated for less
than 28V. If you go over their rated voltage, they'll blow. If
they are electrolytics, you'll get nice streams of plastic-coated
foil distributed around the inside of the case :-)
Play it safe and don't go too much higher than 9V.
Later,
Kenn
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