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Laptops are often shipped with switching power supplies that among other have a phrase for use with information equipment only on their body.

Why can't I use the same power supply to power a motor or a set of LEDs or a lamp given it's output voltage and its wattage are suitable? I mean it outputs say 36 volts of direct current and can supply say 50 watts - okay, that would do for my motor/LEDs/lamp, why can't I use that supply?

What's so special in those power supplies that they bear the "IT equipment only" mark?

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It is part of the safety regulations. When deciding which specifications apply for testing, the product application is taken into account as the way of deciding which spec applies. So if a laptop manufacturer provides an external PSU, they will have it tested to the relevant specs, which includes the product category. There is no guarantee it will be suitable for other applications.

For example, if you drive a motor which has accessable metal parts, and that laptop PSU doesn't give you an earth connection back to the socket, unless you make a seperate connection to earth, there isn't naturally one back for you.

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    \$\begingroup\$ I'm also sure it has to do with safety regulations. Could be EMC, could be overload behavior, could be ambient temperatures. It would be great if anyone could list some examples where the tests and applicable standards vary. \$\endgroup\$ – zebonaut Jun 3 '11 at 14:01
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    \$\begingroup\$ ITE 60950, harmonized with IEC and UL. The standards are available for purchase - not cheap. There are other safety standard for "home" electronics, "video" electronics, and "industrial" electronics -- they have different set of rules. \$\endgroup\$ – Toybuilder Jun 3 '11 at 15:12
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Separate from the regulatory issue of marking the PSU as for ITE only (Martin's answer), you also have simple performance reasons why you can't just pick a power supply by volts and watts.

Driving a large brushed motor, for example, may require a PSU that will not properly handle start/stall conditions where the current draw far exceeds running current, and can withstand/suppress the spikes and electrical noise generated by it.

A LED illumination system may need a more stable/repeatable regulation, so that it doesn't dim when the input line voltage fluctuates (old refrigerators in an old house). Or causes visible flicker because the switch regulation frequency leads to beating.

Basically, When the powering requirements are "complicated", you need to make sure that the supply can handle that complication.

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Some laptop power supplies need to negotiate with the laptop to deliver full available current. I guess that is done to ensure you buy only their power supplies. Dell uses the DS2501 "unique ware" memory chip in its power supply to report to the laptop that it is an approved supply. However it seems that the Dell supply will still output full power if used for other purposes.

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Inductive loads, such as motors, generally require a more rugged but not necessarily a very stable power supply.

Although laptops and even mobile phones do have light motors in them (CD/DVD drive, vibration alert unit), heavier-duty motors require a different kind of power supply.

Emissions conformance is also a major factor in the classification of such power supplies.

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