On 01/12/2015 00:30, Danny D. wrote:
Danny D. wrote, on Tue, 01 Dec 2015 00:20:57 +0000:

The voltage and current fluctuate so, even if you pick the
RMS voltage and current of the one AC signal, if you try to
add it to another AC signal with a different phase, then
you have to factor in the power factor.

This article says the AC power factor plays a role whenever the
AC current and the AC voltage are not in phase:
https://en.wikipedia.org/wiki/Power_factor

I think the whole power factor thing is a red herring because
we're talking output power, which is DC, which has both the
current and voltage in phase.

Only if the 40W is for the input power would the AC power factor
matter, and, if that 40W was for the input, it wouldn't be called
Watts - it would be called VA (volt amperes) anyway.

So, I don't think the AC power factor applies in this question.


It could be the input power.

VA is if you measure the RMS input voltage and then separately measure
the RMS input current and multiply them.

But depending on what is inside the charger the voltage and current may
not be in phase so the power, in watts, drawn from the mains might be
less than the VA value.

--

Brian Gregory (in the UK).
To email me please remove all the letter vee from my email address.