TRANSFORMER LESS POWER SUPPLY

The above design uses 4 diodes in a bridge to produce a fixed voltage power supply capable of supplying 35mA. All diodes (every type of diode) are zener diodes. They all break down at a particular voltage. The fact is, a power diode breaks down at 100v or 400v and its zener characteristic is not useful.

The current is limited by the value of the X2 capacitors and this is 7mA for each 100n when in full-wave (as per this

circuit). We have 1u capacitance. Theoretically the circuit will supply 70mA but we found it will only deliver 35mA before

the output drops. The capacitors should comply with X1 or X2 class. The 10R is a safety-fuse resistor.

The problem with this power supply is the "live" nature of the negative rail. When the power supply is connected as

shown, the negative rail is 0.7v above neutral. If the mains is reversed, the negative rail is 340v (peak) above neutral

and this will kill you as the current will flow through the diode and be lethal. You need to touch the negative rail (or the

positive rail) and any earthed device such as a toaster to get killed. The only solution is the project being powered must

be totally enclosed in a box with no outputs. 

A TRANSFORMER-LESS POWER SUPPLY is also called a CAPACITOR FED POWER SUPPLY.
It is very dangerous.
Here's why:
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 that may be 9v to 12v.

Safety precautions 

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.
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.

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