Gareth Magennis wrote:
Hmm, I keep hearing such things, but so far I haven't come across a
SMPS that will not sit there quite happily supplying voltage, without
a dummy load in sight.

I have one that delivers +5 V, +12 V, and a couple of other things at a
couple hundred watts total - along the lines of a PC power supply but
not the same shape. It needs a load on the +5 V to start up; I use a
12 V automotive lamp. It probably dates from the early 1990s.

I recall talking to someone online who had a PC power supply that had
1/4 watt resistors across the outputs, each sized to draw a few mA from
the output. This was too much power for one of them and it had burned,
but I think there were also some other failures too.

I also have a couple of 5 V, 0.5 A wall-wart cell phone chargers, and
they at least don't draw much current from the AC line/mains with no
load, according to my Kill-a-watt meter. If I unplug/replug the charger
several times, I can sometimes catch a 1 W reading on the meter for a
second, but it then goes back to 0 W.

For the cell phone chargers, I suspect they are deliberately designed to
"gulp" a little power when first plugged in, charge a cap, and then look
for a load every so many milliseconds. If the cap gets low, it takes
another "gulp", but it doesn't have to do that very often. The goal is
that you can leave them plugged in to the wall without wasting a lot of
energy.

Matt Roberds