Wall socket with integrated USB - Power drain when off.

[Nick]

I bought this double plug wall socket with two additional integrated USB sockets.

.

The on/off switches do not control the USB sockets. So I started wondering about the USB transformer eating current when no USB devices are connected.

So I wired it up to a standard plug and tested it with a standard plug power meter. Probably not the most accurate test.

With nothing plugged in it reads 0 Watts but 2 VA.

With a couple of things plugged in it reads 12 Watts and 21 VA.

Not being an electrician I'm not sure about this VA stuff. Is it the Watts or the VA that will appear on my Electricity Bill.

[Andy Burns]

For a domestic supply, it's the watts.

[Dave Plowman (News)]

I'd not trust that sort of device to be anywhere near accurate at a very low reading. You need to measure the current being drawn with a decent DVM.

Does the device have a written spec giving the accuracy?

[harry]

These no load readings are technically unlikely and therefore incorrect. The indication is that it is a pure inductor or capacitor. The first doesn't exist and the second is unlikely.

Your meter is not up to the task. However the power consumption must be very small indeed.

[Mike Tomlinson]

En el artículo , Nick escribió:

I got the same one. It worked for a while but then my iPhone started acting all weird when plugged into it, as if a phantom finger was pressing random points on the screen. It was fine with the Apple charger.

Took it back and got a refund.

[ARW]

The meter cannot be that bad.

On load have at most a 3A load at 5V (we have no idea what was plugged in or it's charging rate), that's not far off the 12W showing on the meter.

When not on load the meter is showing the expected sweet FA apart from a bit of capacitance.

[Brian Gaff]

Having said all of that, I'd suspect between 50 and 100 Milliamps off load at least. My main concern about these devices are. What fuse protection is there?None probably. also such small transformers even if switch mode will be struggling to put out the max current for 2 USB devices at the same time and.. 3 Assuming switch mode, the interference will be terrible as hardly any room for nice suppression of waveforms generated. Brian

[Brian Gaff]

Yep that is the dirty nasty rfi generating crap in these devices alright. Brian

[Chris Green]

I agree about low readings being inaccurate. However reading the current won't help much as you'll just get the VA not the watts and, as someone else said, you're paying for the watts.

[Nick]

OK I put a Maplin N56FU multimeter on it in series.

I'm reading 0.40 milliamps when off.

I'm getting 127 millamps on the N56FU when on.

When on the plug power socket reading about 10% less amps and 14 watts.

That looks to me like it really is not taking much power when off.

[Nick]

Seems to be 0.4 Milliamps when off.

What fuse protection would a normal usb plug in transformer have. Why should I worry?

I don't understand this comment. AIUI It is one transformer rated at 3.1 Amps. A samsung tab and a smart phone do appear to put it at full load (15 watts I assume 5v*3a)

Should I care about radio interference?

[Nick]

I've had one for a month or so. I just bought a couple more as I liked it so much.

Hopefully you just had bad luck.

[Scott]

I thought the MK ones used no current at all when not in use.though I cannot find authority for this.

[Bill Wright]

Not with such small amounts of power. Best thing you can do is go out and buy another 99 sockets, wire all 100 up to the same supply, make your measurement, and divide by 100.

Bill

[John Rumm]

Which is what you would expect for a switched mode PSU.

[John Rumm]

The second is not only likely, its exactly what you would expect the see. Most switching supplies of that type will rectify the mains and charge a capacitor. That will in turn feed the rest of the supply. The current draw will tend to only be near the peaks of the voltage waveform, as the cap is only "topped up" by the mains peaks when under load. The result being nearly zero power factor when under no load.

This is likely true. You would probably need to measure the voltage drop across a calibrated shunt resistor on a scope to capture the current pulse waveform. You could then integrate the capture to calculate the actual power drawn.

Indeed

[John Rumm]

This may help:

wiki.diyfaq.org.uk/index.php/Power_factor

[tabbypurr]

as good as zero, if that's ac current. Most meters only measure dc current.

Someing in the vague region of 1A

fire & shock risk

completely trivial

if you care about radio reception.

The real reason not to buy these things is that 5v USB is most likely a short term standard that will be replaced by negotiable voltage. Ie their usability will fizzle out. Plus they're overpriced and often unsafe.

NT

[harry]

It must be consuming something. Even if very tiny.

[harry]

It's possible that these devices are only switched "on" when the plug is inserted. That would account for the ultra low readings. It would also make them a lot safer. Without a switch I would say they are a significant extra fire hazard.

You need to check the load with a plug inserted (but not connected to anything) to check this out. Or take one to bits