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I purchased a device labelled "AC Adapter" that labels the input to be 110 to 240 VAC, 50/60 Hz, 1.5A max and the output is labelled as 12, 15, 16, 18, or 19 volts, DC, 4.5A max or 20 or 24 volts, DC, 4A max. Physical dimensions of the plastic enclosure: 2 x 4 x 1.5 inches. The unit is surprisingly/very light.

The price of this new unit was 11 USD in a surplus store.

There is a slide switch for output voltage selection. This was purchased as a potential replacement for a power supply to a gaming console that requires 12 VDC, 3.5A. I will glue the switch at 12V to prevent misapplication. Given the range in possible outputs, will this unit be operating inefficiently at this extreme (low) setting?

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Unfortunately it's difficult if not impossible to answer whether the unit will be efficient at any setting based solely on its description. It's probably a switching supply rather than linear, which means it lacks a heavy transformer typical of those units. In theory, because it is a switching supply, it will be fairly efficient at any setting, but there's only one way to determine this conclusively: measure it.

Though these type of power supplies have come down in price considerably, and are so ubiquitous that they are generally taken for granted, remember that "you get what you pay for."

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  • \$\begingroup\$ The remarkably low price is the reason I am curious about its efficiency. \$\endgroup\$ – H2ONaCl Apr 28 '14 at 15:59
  • \$\begingroup\$ You can get an idea of the worst-case efficiency by looking for a circled Roman numeral indicating conformance to the Energy Star standard, as explained by this calculator. \$\endgroup\$ – MBer Jun 27 '16 at 23:01
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I use these cheap universal SMPS all the time and reliability is not great, but considering you are only using 50% of rated power, the temperature rise ought to be reasonable, so good luck.

Always connect DC before plugging and unplugging AC will reduce the DC surge current stress on the output side and hopefully it has soft-start AC built in with an inrush current limiting, ( ICL) a cheap metal-oxide part ... Or not.

Efficiency in a light weight SMPS must be good enough to prevent overheating from 12 to 19V and generally is in the >80% range. It gets worse when lightly loaded and at maximum Voltage, which is why the current limit is reduced to 4A for >=20V

So at 50% load, I would guestimate the efficiency is good and 70% or more but may have poor p.f. and THD, which will not concern you.

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  • \$\begingroup\$ Any comments on OP's question of efficiency? Your answer is focused on reliability IMHO... \$\endgroup\$ – Adam Lawrence Apr 28 '14 at 16:53

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