> Hi Everyone,
> Thanks for the responses. Well, it didn't melt,
> so I guess I am OK there. It was hot to the touch,
> where holding for more than a few seconds would not be
> possible. As for the filter cap what value would you
> recommend ?

Woa, any wal wart that you CAN NOT hold in your hands for any time you want, is
very hot and it is asking for a firetruck to come by to cool it off.

That is totally out of control, probably the wire insulation is melting away,
what means a possible short circuit pretty soon. Now you know how SOME home
fires could starts.

Obviously that a 20V @ 800mA unit could generate some good heat if your circuit
is consuming the 16Watts the unit can deliver, but you are talking about 10V @
1000mA, thinking about your step down PWM working at 75%, it means 10W / 0.75 =
13.3W power consume, so it would be running at 83% of maximum power delivery,
what should run warm to warm-hot, but not hot enough to burn your hand after 10
seconds.

Of course the step-down requires a nice filter capacitor to have enough power to
be delivered to the PWM circuit.  The function of such capacitor is exactly
"convert" the 20V @ AnyCurrent to the PWM conversion. The PWM circuit will
extract as much current it needs from the CAPACITOR.  If the capacitor is not
present, the PWM circuit will try to extract the current it needs from the power
transformer, and THAT will generate heat, not even talking that the step down
conversion productivity will go south to less than 50% what means an enourmous
current hunger from the PWM.

The capacitor formula calculations is

C = 2.4 i/vr

where i = current in milliamperes and vr is the accepted ripple voltage.

Lets imagine the accepted ripple would be 500mV and the current 1000mA, so
capacitor value should be  2.4 x 1000 / 0.500 = 4800uF.

Probably the power transformer is working at its current limit, core magnetic
saturated, what reduces more and more its capacity to transport energy, heating
yet more.  That is an auto-destruction cycle.

Wagner Lipnharski
http://www.ustr.net
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