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Re: [Phys-L] AC power calculations

Actually Bernie, there are a lot of cheapie power supplies out there using capacitive droppers to avoid the transformers in wall warts. Many LED lamps use these.

If you buy a super cheap (eg quite lethal when wet) $2 cell phone charger that’s for sure a capacitive dropper. The Apple charger costs 10X as much, but is much safer (real HV isolation).

Dan M

On Feb 22, 2021, at 21:58, bernard cleyet via Phys-l <> wrote:

On 2021/Feb/22, at 12:02, John Denker via Phys-l < <>> wrote:

For example, if you connect an AC signal to a capacitor,

Do not do this! (If it's domestic power.)

⟨ΔV⟩ will be
nonzero, ⟨I⟩ will be nonzero, but [P] will be zero. Energy will flow
into and out of the capacitor, but the average [P] is zero. If we
integrate P over a cycle, we find the net energy transfer is zero.

Power companies know all about this, and they have a name for it:
PF ≡ *power factor*

Later JD uses the word “ideal” in reference to reactive “load”, in which case, OK.

Even an air dielectric not ideal? Try a noble gas?

I stupidly connected a resistor in series W/a cap. Fortunately, I switched off before it melted.

A BTW: In the same lab an exercise is to connect, IIRC, a parallel LC to an AC source, wow much higher EMF [1], than the source, results.
The L is adjusted by moving the iron core.

My medium tech. “kill a watt” meter includes PF in addition to watt, V, freg., etc.

And finally:

"Grid operators sometimes install eeeenormous capacitors in an effort
to improve the power factor. Customers who are being charged by the
kVA may install their own capacitors."

In CA they’re intermittently on power poles. At least I’ve always assumed that’s what they are. I should ask someone in the PG&E yard.

bc … wonders how much loss due to the caps.

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