Let's explain what we mean by a CAPACITOR POWER SUPPLY.
A Capacitor Power Supply uses a capacitor to interface between a “high voltage supply” and a low voltage – called THE POWER SUPPLY.
In other words a capacitor is placed between a “high voltage supply” we call THE MAINS (between 110v and 240v) and a low voltage of 9v to 12v.
Even though a capacitor consists of two plates that do not touch each other, a Capacitor Power Supply is a very dangerous project, for two reasons.
You may not think electricity can pass though a capacitor because it consists of plates that do not touch each other.
But a capacitor works in a slightly different way. A capacitor connected to the mains works like this:
Consider a magnet on one side of a door. On the other side we have a sheet of metal. As you slide the magnet up the door, the sheet of metal rises too.
The same with a capacitor. As the voltage on one side of the capacitor rises, the voltage on the other side is “pulled out of the ground” - and it rises too.
If you stand on the ground and hold one lead of the capacitor and connect the other to the active side of the “mains,” the capacitor will “pull” 120v or 240v “out of the ground” and you will get a shock.
Don’t ask “how” or “why.” This is just the simplest way to describe how you get a shock via a capacitor that consists of two plates.
How does this refer back to your power supply question?
If the capacitor “shorts” between the two plates, the 120v or 240v will be delivered to your power supply and create damage.
Secondly, if any of the components in your power supply become open-circuit, the voltage on the power supply will increase.
But the most dangerous feature of this type of power supply is reversal of the mains leads.
The circuit is designed so that the neutral lead goes to the earth of your power supply.
This means the active is connected to the capacitor.
Now, the way the active works is this:
The active lead rises 120x 1.4 = 180v in the positive direction and then drops to 180v in the opposite direction. In other words it is 180v higher than the neutral line then 180v lower than the neutral.
For 240v mains, this is 325v higher then 325v lower.
The neutral is connected to the chassis of your project and if you touch it, nothing will happen. It does not rise or fall.
But suppose you connect the power leads around the wrong way.
The active is now connected to the chassis and if you touch the chassis and a water pipe, you will get a 180v or 345v shock.
That’s why a CAPACITOR-FED power supply must be totally isolated.
Now we come to the question: How does a capacitor produce a 12v power supply?
When a capacitor is connected to the mains, one lead is rising and falling.
Depending on the size of the capacitor, it will allow current to flow into and out of the other lead.
If the capacitor is a large value, a high current will flow into and out of the lead. In addition, a high voltage will allow a higher current to flow.
This current is “taken out of the ground” and “flows back into the ground.”
It does not come from the mains. The mains only: “influences” the flow of current.
Thus we have a flow of current into and out of the capacitor.
If you put a resistor between the capacitor and “ground,” the amount of current that will flow, depends on 3 things, the amplitude of the voltage, the size of the capacitor and the speed of the rise and fall.
When current flows through a resistor, a voltage develops across the resistor and if we select the correct value of resistance, we will get a 12v power supply.