I was wondering this. I've noticed a lot of the newer alarm clocks, radios and other electronics frequently use the wall wart style transformers. Older ones usually had the transformer built into the unit itself. Is there a reason for this? Possibly to make the unit itself more compact and keep it from getting too warm inside from the transformer?

Also, why do a lot of electronics have an external transformer, but a separate cordset for the transformer? Such as our HP printer, has an external transformer but it has a detachable power cord that plugs into the transformer. My Wii and Playstations are the same way, but older video games tended to use the wall wart style.

Two reasons i can think of for question one both come down to cost. Reason one is that it's easier to put a barrel connector on the unit and have an external power supply than to design power supply to go into the unit which may have to be custom-designed to fit in that unit versus using an off-the-shelf wall wart.

Reson two mostly applies if the item is intended to be soil in multiple countries with varying mains voltages and uses a power supply that can only work with one (Ie, a transformer). I think it would be easier and cheaper to only have to swap out a peripheral power supply than it is to swap out a built-in supply.


As to question two, a few reasons i can think of are. One, to 'idiot-proof' the power supply and unit (Ie, someone can't plug the wrong power supply into the item and vice-versa). and it could also be used to make the power supply harder to replace. Two, for multi-output power supplies, If the item requires more than one input voltage to run (Ie 12v and 5v), then it would be easier (and cheaper) to use a power supply that can output the voltages necessary versus having a single voltage power supply and putting the necessary circuitry needed to step up/down the voltage to the other needed voltages.

@High Intensity the things you mentioned are 100% correct and things these day almost always use a switching power supply pretty much accept anywhere from 90-277VAC 50/60HZ so only a different plug type adapter is needed @ Cole D that is the reason for the removable cord on the brick type power supplies

It's true there's issues of weight, bulk, and heat, and not reinventing something that you can buy cheaply. But the main reason is there's all kinds of safety certifications to go through if you design something that plugs into the mains. Better to buy a wall wart or a line lump from China where they've already certified the power supply. You'll notice the power supply brick will say "UL" or "CE" but the alarm clock won't.

When using "wall wart" or external "brick", the device itself has not anymore dangerous mains inside, so its designers are freed up from all the safety problems and regulations linked to that. With internal supplies that becomes rather high part of the development cost.
Plus the energy efficiency regulations practically prohibit the use of the standard mains transformers (you can never meet the idle power consumption limit with a reasonable cost), which were sold as "all mains dnclosed, only SELV output on terminals" ready made and cheap supply block making the rest easier. SMPS have either open board design (so do not solve the safety) or are bloody expensive (the potted types; as a part for mass production).
Plus the safety things usually needs to be done to big extend for each market separately, because of the differences among the codes.

The external supply is the sold for many devices, so with way higher volume to dissolve the development cost (with many of them being so minor modifications among models, so the maker can use the "structural similarity" claim and so spend most of that cost just once for multiple models; like differing only in output voltage).

The voltage conversion is not any problem anymore, because the modern electronis needs many voltage domains anyway (even a "dumb" cell phone uses to have 4..6 different supplies; modern cheap pocket radio has already at least 3 domains), with many of them even not possible to wire for longer distances than few cm between the fast response accurate regulator and the load (processor core supply,...), so there will always be a ton of voltage conversions, so adapting everything to a single supply with wider tolerance is not making the thibg any more complex at all.

And the "dumb lroofing" now works mainly by designing everything at least a bit possible to run on "5V USB power" for lower power devices (then it uses one of the classic USB connectors) or "19V notebook power" when the power requirements are higher (then it uses some barrel type, but now this is moving towards USB-C. The first means anything in the 4.5..6V range, the later in the 12..24V range (the internal supply design does not differ if it has to run on any voltyge in that range, only the minimum voltage could become higher if it is supposed to be charging higher voltage batteries like in the notebooks - the charger circuits are usually not able to step up the voltage.

I should have thought about that with the brick power supplies, since usually works with multiple voltages so only the wall plug would need to be changed. And that does make sense external power supply is safer since only low voltage is present in device itself.

手机充电器使用277V电压绝对会爆炸！！！！！！

Only shady made one. And yes, there are lot of those on the market...

The 277V is just 15% above the 240V nominal voltage used on many places around the world, it leads to 390VDC behind the rectifier. Normally the converter should work properly with more than 400VDC without any damage (the weakest points for long term use are supposed to bet the 400VDC rated capacitors, but even these should handle more than a hour at 450VDC, so about 320VAC; most "bricks" using dedicated PFC circuit are designed to use 390..420V on the DC rail for normal operation).
The internal switch in the most common flyback uses to be 700V rated, with the common 120..150V reflected voltage on the primary it means still 100V margin.

Shady makers sometimes use just 500V switch elements with 120V reflected voltage transformer design, that leads to the margin becoming zero (so no margin even for leakage inductance overshoots) when the DC reaches the 380V mark, these then do explode.

The 500V components are designed to work in a halfbridge configuration without PFC where the voltage can never exceed the DC rail and where are even no leakage inductances, plus when the DC rail is normally in the 320..350V range and the 400V is exceeded only during surges.
For devices with PFC (so DC bus in the 390..420V range in normal operation, with surges up to 500V) the proper rating is 600V.
For topologies with single switch on primary the proper rating is 650V without PFC and reflected voltage below 100V, 700V for reflected voltages in the 150V ballpark and even 750V for converters used behind a PFC.
400V components (some shady makers use for halfbridge) are then intended for 120VAC only applications, today are practically extinct specie for low and medium power, where the wide "100..270VAC" range compatibility requirement is the name of the market game.