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I am a bit confused about some supposedly basic electronics concepts :/ (I kind of have an answer but just want to confirm)

So let's say I have got this setup:

Question: If I plug the 2 chargers and fully utilize them (connect all ports to charging iPads), am I overloading the socket?

The spec says it can deliver 2.4A per port, or 12A overall. Simple addition of the amperage is:

12A + 12A = 24A > 13A, so I am overloading

However, the wattage is just 60W so maybe the correct way to calculate it is:

Power = Current * Voltage
Current = 60 / 220 = 0.27A

So the input current is actually just 0.27A so I can actually safely connect as many as 13 / 0.27 = 48 of those chargers to the same mains without problems. The 12A is provided by the transformer inside the charger and doesn't concern the mains circuit.

Which one is the correct interpretation?

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You're comparing apples to oranges. You're using the power output, to try and determine the input. Without knowing about the transformer, and other circuitry, that's going to be quite difficult. According to the specification listed by the manufacturer

enter image description here

The unit will draw about 1.4 amperes at 100-240 volts AC (VAC), which I'm assuming is the value when the unit is fully loaded.

The 12 ampere value you're using, is the output current which is at 5 volts DC (VDC).

This unit takes in AC power, steps it down and converts it to DC power. The devices that are connected to it, then draw DC power to charge. The unit seems to be able to supply 12 amperes at 5 VDC, though it's not likely all the connected devices will take advantage of the full available current.

The 60 watts, is the amount of power available on the output side of the device.

12 amperes * 5 VDC = 60 watts

  • Assume 220V input voltage. 1.4A * 220V = 308W that means the transformer loses 1 - 60W / 308W = 80% of the energy (to heat, likely). Is that correct? – kizzx2 Sep 30 '15 at 1:14
  • Perhaps that bad, but probably not quite. Note that the input voltage is given a a range from 100-240V, but the current is simply given as a single (probably maximum) value. It likely draws 1.4A at 100 V when fully loaded, and less than that at higher voltages. But that's still less than 50% efficient at 140W in and 60W out. If it is dumping that much power as heat, it will get quite hot to the touch. It's also hard on the power bill. – Ecnerwal Sep 30 '15 at 2:15
  • @Ecnerwal plenty of these types of transformers get plenty hot. There have been recalls of certain laptop computer power cord transformers because they'd occasionally get so hot they'd melt and/or catch on fire, or ignite whatever they were sitting on. – Craig Sep 30 '15 at 5:55
  • I have that same brand of charger in a 4 port model and it remains quite cool to the touch (just slightly warm) even when charging tablets in all 4 ports so I think the current rating is just being conservative or maybe some short term peak value... I'm certain its not wasting half the power as heat. – Johnny Sep 30 '15 at 15:58
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The important thing is what the charger is pulling, not what it is delivering, because it is a transformer.

60W is either 12A at 5V, or 0.27A at 220V. So it is only pulling 0.27A, nowhere near overloading anything, as long as it's a perfect conversion. In reality there will be some losses, so it will pull more than 0.27A (but still nowhere near 12A).


Note: As long as the 13A powerstrip contains a fuse, that fuse will be the first thing to blow if you do overload the powerstrip. This is why powerstrips should be used rather than a 2-in-1 plug adaptor, as these don't have fuses and can hence overheat if overloaded.

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If I plug the 2 chargers and fully utilize them (connect all ports to charging iPads), am I overloading the socket?

No.

Which one is the correct interpretation?

Neither.

The step down converter trades off voltage for current but it's not perfectly efficient and it's input won't have a unity power factor. Furthermore under some loading conditions the actual output current could be higher than the rated output current.

So assuming the load is drawing a constant current.

(RMS input current) = (DC output current) * (DC output voltage) * (efficiency) * (RMS input voltage) * (input power factor)

Efficiency is the ratio of input power to output power. Simple enough.

Power factor is about the relationship between voltage and current waveforms. If voltage and current waveforms are the same shape and in phase then average power is equal to RMS voltage times RMS current. We call this a power factor of 1. If the voltage and current waveforms are of different shapes and/or are not precisely in phase then less power will be transffered..

Cheap DC power suppliers often have a low power factor. It's possible to engineer DC power supplies with a power factor very close to 1 but it costs more.

The EU reconised electronic loads with low power factors as a problem and introduced regulations to try and force power supply vendors to improve power factor. However these regulations generally only apply to devices that draw more than 75W in normal optionation so your charger probablly sneaks in under the limit. Furthermore even where the regulations do apply they are frequenly ignored.

Tester101 found the specification listed by the manufacturer for your power supply.

enter image description here

This states an input current of 1.4A. The current will be based on the low end of the input voltage range and possiblly other pessimistic assumptions. Actual current on a 240V supply is likely to be considerablly less but exactly how much less is difficult to say.

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