AC power-strips with USB -how does the USB work?

[analogix]

I'm considering getting a 19" rack mounted power strip (with an on/off switch at the front) which comes in two varieties:
8 AC outlets
or
8 AC outlets + 2 USB charging sockets



Since the price difference is negligible and USB might come in handy at some stage I'm curious as to how USB power (+5V as far as I know) is generated from AC voltages and if this draws power/generates heat (and thereby a potential fire hazard) continously (when the power strip's power switch is turned on) -unlike a plain power strip without any USB ?
I've noticed that USB charging ports are getting common in AC wall sockets as well these days, and I assume the technique (and potential dangers) for getting +5V is generally the same.

[fcb]

They will have a small isolated SMPSU inside.  Will always draw a small amount power. The fire-risk will statistically increase over the non-USB version.

[info]

or if you are very unlucky and they skimped on safety etc etc , you get a capacitive dropper which means your USB devices are all floating on mains live

[NiHaoMike]

or if you are very unlucky and they skimped on safety etc etc , you get a capacitive dropper which means your USB devices are all floating on mains live

UL/ETL will not allow that, so make sure what you get is UL/ETL listed. Anything from a reputable brand will be.

[mariush]

In majority of cases, it's a small switching power supply, like the one in phone chargers.

Expect around 80% efficiency, and maybe less than half a watt wasted when nothing is plugged in ... so heat wise not much extra added.

BUT... the quality of switching power supply inside can vary A LOT... some can have really poor isolation between primary and secondary traces, or poorly wound transformer (with primary and secondary windings not well separated by tape as the transformer was made, or other issues.)

[analogix]

or if you are very unlucky and they skimped on safety etc etc , you get a capacitive dropper which means your USB devices are all floating on mains live

UL/ETL will not allow that, so make sure what you get is UL/ETL listed. Anything from a reputable brand will be.

It's from a German brand called Adam Hall which I believe is reputable and well known withing the stage/lighting/music industry.
But since I don't actually need USB power sockets in that location I think I'll go for the non-USB power strip instead to eliminate any risks.

I have another power strip on my desk (also with an on/off switch) which I use for charging (phone, iPad etc.) -I have several chargers plugged into it, and instead of plugging/unplugging them all the time I just switch on/off the power strip's switch: will this incur a lot of unnecessary power usage (i.e. when one/several of the chargers don't have any phone or other device plugged into it)? I've heard of chargers which don't use any power unless a device is actually drawing power off it (a phone or whatever) but don't know if this is an urban myth or true.

[mariush]

They're kinda expensive.

You could buy some aluminum tube or profile (plastic would also work) and buy the sockets separately and wire them yourself to a standard IEC connector and then buy a regular computer cable (or two) separately

Example.. ~ 2$ per socket : https://www.tme.eu/en/details/10584-0s/mains-connectors/pce/

and 10-16A IEC connectors : long tme.eu link

[NiHaoMike]

I have another power strip on my desk (also with an on/off switch) which I use for charging (phone, iPad etc.) -I have several chargers plugged into it, and instead of plugging/unplugging them all the time I just switch on/off the power strip's switch: will this incur a lot of unnecessary power usage (i.e. when one/several of the chargers don't have any phone or other device plugged into it)? I've heard of chargers which don't use any power unless a device is actually drawing power off it (a phone or whatever) but don't know if this is an urban myth or true.

Modern wall warts use negligible standby power. You'll get more savings adding a timer switch (the mechanical wind up type is a good bet for zero standby power) to end charging early, generally target 80-90% since above that is where battery aging really accelerates.

[atmfjstc]

I wouldn't rely on any USB charging ports built into sockets or extension cords, for a number of reasons:

- As mentioned by other posters, the quality and safety of the SMPS could vary widely. Given that these are consumer products built to the lowest cost, and that space inside them is very restricted, and that USB charging is not the primary function of the product but rather a bonus... the odds are not looking good for it being a high quality, safe SMPS. Sure, wall wart chargers can be crap too, but you can easily replace those, as opposed to a socket or an extension cord which is a more long-term investment.

- I've noticed that with USB ports, the old adage applies, that as technology expands, the requirements of devices also expand so as to fill it. About 5 years ago, most of my devices could be reasonably operated and charged with a 1 Amp capable USB port. Now I have a tablet that needs at least 2 Amps and the phone needs 3 for reasonably fast charging. Point is, any specific port installation becomes obsolete faster than you think, and again, it's easier to upgrade a wall charger than the whole socket or cord.

[Gyro]

or if you are very unlucky and they skimped on safety etc etc , you get a capacitive dropper which means your USB devices are all floating on mains live

It wouldn't be a capacitive dropper. The capacitor would need to be so large to get any significant current, that it physically wouldn't fit, and would be impossibly expensive when compared to a small SMPS. -  In this case, the cheapest solution is the proper one.

[analogix]

or if you are very unlucky and they skimped on safety etc etc , you get a capacitive dropper which means your USB devices are all floating on mains live

It wouldn't be a capacitive dropper. The capacitor would need to be so large to get any significant current, that it physically wouldn't fit, and would be impossibly expensive when compared to a small SMPS. -  In this case, the cheapest solution is the proper one.

Those are very good points indeed, the last one being something I hadn't thought about but indeed something to keep in mind.
I'll go for the non-USB power strip.

Talking about USB chargers (sorry, a bit off-topic): for several years now, and using many times a week we've had a nice and cheap 3.4A USB charger with 3 ports (max 2.4 per port) from.... Ikea, believe it! And according to this test it comes out both safe and with good performance. Not all cheap stuff is rubbish 

[tooki]

or if you are very unlucky and they skimped on safety etc etc , you get a capacitive dropper which means your USB devices are all floating on mains live

It wouldn't be a capacitive dropper. The capacitor would need to be so large to get any significant current, that it physically wouldn't fit, and would be impossibly expensive when compared to a small SMPS. -  In this case, the cheapest solution is the proper one.

Ummm... Nope. Bigclive has shown Chinese camping lanterns using capacitive dropper USB supplies. Neither huge nor expensive.

[Gyro]

Talking about USB chargers (sorry, a bit off-topic): for several years now, and using many times a week we've had a nice and cheap 3.4A USB charger with 3 ports (max 2.4 per port) from.... Ikea, believe it! And according to this test it comes out both safe and with good performance. Not all cheap stuff is rubbish 

Yes. Ikea and even places like Aldi and Lidl simply can't afford to cut any corners on safety and approvals, the damage to their reputation wouldn't allow it. You are pretty much guaranteed a safe product.