Knowing What We Don't Know Is the Last Step Before Knowing

As someone once said, there are things we don't know we don't know.

Part 15 dot US is in the business of at least helping us realize what we don't know so we can try to learn more about it.

When I came here I had no clue about the importance of grounding for RF signal propagation. To me "grounding" was an electrical safety issue, and all the homes and apartments I've lived in had old two-wire wiring with no "3rd blade" for ground, yet no one died.

Now I realkize that "ground radials" are the other half of the AM antenna.

More recently, in the Alpmic Project I've been messing with since 1990, I gained a huge new insight into the numerous uses of capacitors in electronic circuits and am much closer to some day building a time machine.

But there's still more that I am not able to compute.

Here's the down low:

Capacitors block DC, ok that's easy to comprehend.

Capacitors pass AC, and since analog audio is generated as an Alternating Current we know that much.

So, why can't capacitors be used, say, in series with a lamp cord to turn on a lamp with 110 VAC? Of course the voltage rating of the cap would need to be taken into account, but capacitor voltage ratings are given in terms of their DC handling capacity, so what about that?

The fact that there's no reason to put capacitors in series with a lamp cord is not the point. My point is that, given everything else, it should work.

Mark's picture

As long as the cap is the correct value to pass the 60hz it would work and since no DC is present the DC rating isn't important.

But why use this instead of a switch?



Carl Blare's picture

Mark asked:  "But why use this instead of a switch?"

There is no practical reason that I know of for placing capacitors in line with AC power, but because theoretically it should pass juice, I am asking the question academically and never intend actually doing it.

I have this plan... if I keep asking questions about the very outside limits of common sense, some day I might figure a few things out.

We don't always know if our questions are worth asking, but we should not avoid asking them.

Carl Blare

MRAM's picture

Some capcitors show an RMS current rating which is important regarding power supply ripple current.

Basically, this relates to how much AC current the capacitor can pass without heating up. Usually considered for power supply filters.


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radio8z's picture

One application for using capacitors in series wih the AC line is line powered LED lamps.  The lamps are generally made from groups of LED chips wired in series which require DC voltages on the order of 20 to 30 volts.  Rather than using a transformer or switching supply to drop the AC line voltage, the right value of capacitor can be used wtih the capactive reactance at 60 Hz selected to drop the voltage at the operating current.  This is similar to using a resistor for this purpose except the reactance does not dissipate power and get hot as would a resistor.

Such capacitors must be designed for the purpose and they have an AC maximum voltage rating.  One disadvantage is that the cicuit has a very bad power factor and the use of a switching supply for this purpose is preferred.  Another problem is high frequency spikes on the line are passed since the reactance is lower as the frequency is raised and this can cause damage to the circuits.

Here's a link to a video of the teardown of a LED bulb with an explanation and schematic.