A wide range of voltages, single current spec.

[zzwergel]

Hi Everyone,

I am a new forum member. There has been a thing that I have been wondering about for a while now.

I have a USB power adapter that says it can take any voltage from 100 to 240 Volts as an input. The rated output is 5 volts DC 1.0A. The only specified input current is 0.2 Amps. I would expect if it takes 0.2 amps at 100 volts, it would take 0.833 amps an 240 volts for the same output. Does this mean that the unit would be less efficient running on 240 volts as it would on 100 volts? Is that rating a worst case scenario if the adapter was connected to 100 volts and fully loaded?

[BobsURuncle]

" I would expect if it takes 0.2 amps at 100 volts, it would take 0.833 amps an 240 volts for the same output"

No that is not how it works, the current draw will be less at 240 than 100V.
You won't exceed the max input current to the adapter if the load draws one amp or less - regardless of whether the mains is 100 or 240V.

[uncle_bob]

Hi Everyone,

I am a new forum member. There has been a thing that I have been wondering about for a while now.

I have a USB power adapter that says it can take any voltage from 100 to 240 Volts as an input. The rated output is 5 volts DC 1.0A. The only specified input current is 0.2 Amps. I would expect if it takes 0.2 amps at 100 volts, it would take 0.833 amps an 240 volts for the same output. Does this mean that the unit would be less efficient running on 240 volts as it would on 100 volts? Is that rating a worst case scenario if the adapter was connected to 100 volts and fully loaded?

Hi

One of the interesting jobs the wife once had for a while was checking these magic numbers. Often the numbers make no sense when you look at them as written . Surprise !! The make no sense when you measure them. It turns out that there is no regulatory problem with saying a device pulls a "maximum current of 2A" when indeed the worst it will ever pull under any condition is 1A. Why in the world people would overstate the power used .... I have no idea. I've seen the data, people most certainly do overstate it.

Bob

[zzwergel]

I would think it would use half the current running on 240 volts as it would on 120 volts because P=IV where P is power in watts, I is current in Amperes, and V is electromotive force in Volts. If V decreases, I must increase.

[jitter]

I would think it would use half the current running on 240 volts as it would on 120 volts because P=IV where P is power in watts, I is current in Amperes, and V is electromotive force in Volts. If V decreases, I must increase.

Indeed, but that's not what you wrote originally:

I would expect if it takes 0.2 amps at 100 volts, it would take 0.833 amps an 240 volts for the same output.

For 0.2 A @ 120 V, you would expect 0.1A @ 240 V.

Most of the USB chargers I have indeed state one current, but I do have one that does more: the Nikon EH-69P has the following specs:
Input: 100V-240V~50/60Hz 0.068A-0.042A 6.8VA-10.08VA
Output: 5V=0.55A

My guess is that the mains current specified on the label is not the nominal current, but the inrush current, in other words: the current when plugging it into the mains, this will be higher but only for a short period of time.

To see if that assumption is true, I measured the nominal current from the mains at full output. I loaded the output with a DC-load set to 550 mA and measured the current from the mains.
At that load the current is 0.026 A @228.2 V. The apparant power drawn is 6.9 VA with a PF of 0.54 which equals to 3.8 W real power. So that's quite a bit lower than the spec would have you believe. That's why I think it's not nominal power but max power drawn only shortly as inrush current.

As an aside: efficiency of this AC/DC adapter is approx. 72%.

[jitter]

Just for fun, and for comparison, I tried a Huawei HW-050055E1W because it has the same input rating as your adapter, but also the same output rating as the Nikon adapter I tried in my previous post:
Input: 100-240V~50/60Hz 0.2A
Output: 5.0V=550mA

At the same 550 mA load from the output it wants almost the same from the mains as the Nikon: 0.025 A, 6.7 VA @227.9 V; PF 0.57; real power: 3.8 W.
Slightly better power factor, but in the end the same real power drawn and the same efficiency.

[zzwergel]

As I live in North America, I do not have easy access to a 240 volt circuit as we have a gas powered dryer and stove. Only electric stoves, large window/central air conditioners and dryers use 240 volt plugs and everything else uses 120 volts. Even the furnace (Gas) runs on 120 volts. I do not want to have to open the breaker panel/consumer unit and I do not have an ammeter. does anyone have easy access to both 120 volt and 240 volt circuits and an ammeter? It would be greatly appreciated if someone could test this for me.

Thank you,
Zach

[jeroen79]

One of the interesting jobs the wife once had for a while was checking these magic numbers. Often the numbers make no sense when you look at them as written . Surprise !! The make no sense when you measure them. It turns out that there is no regulatory problem with saying a device pulls a "maximum current of 2A" when indeed the worst it will ever pull under any condition is 1A. Why in the world people would overstate the power used .... I have no idea. I've seen the data, people most certainly do overstate it.

Because no-one will come back to complain when the product doesn't consume the promised 2 Amps.

[uncle_bob]

One of the interesting jobs the wife once had for a while was checking these magic numbers. Often the numbers make no sense when you look at them as written . Surprise !! The make no sense when you measure them. It turns out that there is no regulatory problem with saying a device pulls a "maximum current of 2A" when indeed the worst it will ever pull under any condition is 1A. Why in the world people would overstate the power used .... I have no idea. I've seen the data, people most certainly do overstate it.

Because no-one will come back to complain when the product doesn't consume the promised 2 Amps.

Hi

That's one theory. The other is that some people look for the "more powerful" device ... We never settled on which one was correct. Much beer consumed in the process though

Bob

[jitter]

This has nothing to do with specifying beyond actual specifications to draw customers (like e.g. those absurd PMPO specs for PC-speakers). I think there's a technically valid reason to spec it that way and, as I wrote before, it's inrush current.

No one cares one little bit about what chargers draw on the primary side, only what they deliver on the secondary (low voltage) side as that's the spec that determines if e.g. your tablet will take forever to charge or be done with in a couple of hours.

[zzwergel]

Doesn't it Matter how it will affect your power bill? Uses more power if fed a higher voltage or not?

[Paul Moir]

Does not take more power.  It's not a resistor but a switch mode power supply.  What it does internally is to first rectify the AC into highish voltage DC.  Then it's chopped back into AC at high frequency so it'll go through a transformer.  The low voltage side of the transformer then measures it's voltage, and commands the high voltage chopper to chop more or less to generate that 5v output.  If the load increases on the low voltage side, the voltage drops and so the chopper gets commanded to chop more.

So one of the great things about these switch mode power supplies is that it only "takes" as much current as it needs to keep the 5v up, irrespective of the input supply.

[jitter]

Agreed.
The decent switch mode AC/DC adapters will have some form of feedback to keep the output stable, and this also allows them to be operated off a wide range of mains voltages.

To revisit my remark about inrush current, let's look at the spec of the Nikon EH-69P again:
Input: 100V-240V~50/60Hz 0.068A-0.042A 6.8VA-10.08VA
Output: 5V=0.55A

The inrush current will depend on a few factors, one of which is the mains voltage. But once the inrush has completed, it should not matter whether the adapter is working off the lower end or the top end of the scale, the power drawn should be equal.

I have not yet explained the reason for specifying inrush current. The mains can supply that current without any problems, but imagine you're on a car or camper holiday and are trying to power the charger off a DC/AC inverter, one of those cheap ones. Those may not be able to start a load going that needs a high inrush current, but a lower nominal load. This probably won't be a problem for a small device like this AC/DC adapter, but something bigger might need quite a bit more than it nominally draws and then the inverter also needs to be able to supply that peak.