The capacitor-fed power supply is sometimes referred to as a TRANSFORMERLESS POWER SUPPLY.
This article is going to shock you because it is going to tell you all the reasons why you should NOT design, produce or use a CAPACITOR-FED POWER SUPPLY.
The circuit shows a typical design for a capacitor-fed supply.
It consists of 4 diodes in a bridge, a capacitor and a LOAD. Sometimes a current-limiting resistor is included and sometimes a resistor across the capacitor, but this is the basics:
After reading this discussion, you will see how dangerous it can be.
Its danger has never been described before.
Firstly you have to understand the action of the MAINS.
The Mains is an alternating voltage and enters all premises via two wires. One wire is called ACTIVE and the other is NEUTRAL.
The Neutral has NO VOLTAGE on it. It is “harmless” and you can touch it. BUT the other wire has a voltage that rises to 340 volts in the positive direction and then falls to minus 340v (in the negative direction).
Forget about 240 volts, that’s only a voltage used by engineers to work out the heating effect of a radiator or the output of a motor.
There is NEVER 240 volts on the mains, it always passes 240v on the way to 340 volts at the rate of 50 times a second (actually 100 times per second).
In the diagram you can see a man touching the “Active.”
He is detecting a voltage ranging from +340v to -340v.
You can “feel” a voltage as low as 50v AC and a wire with 80v AC must be dropped.
As the voltage gets over 120v AC, your hand is thrown off the wire and above 200v AC you can be thrown across a room. Imagine what can happen with 340v AC.
That’s exactly the risk you are taking when building a capacitor-fed power supply.
The AC mains consists of one “safe” wire and one DANGEROUS wire.
You may have a power-lead (commonly called an extension-lead) with the wires reversed and the “safe” wire will become the dangerous wire. Someone may have connected one of the plugs at the end of the lead to the wrong wires.
This can very easily happen.
In the diagram above, the input to the capacitor-fed power supply is connected around the wrong way and BOTH LEADS of the output of the supply are either +340v or -340v.
Look at the diagram carefully, to see this.
Either a positive or negative voltage can kill you instantly.
The mains voltage will go through the resistor R (and one of the diodes) and KILL YOU. The bottom rail of the power supply will be -340v. This bottom-rail is called the CHASSIS and it is always considered safe to touch. But not with a capacitor-fed power supply.
Let’s take the second instance of DANGER.
In the first diagram, the LOAD resistor allows the output voltage to be say 12v and the capacitor-fed power supply is working perfectly.
But what happens if the LOAD “breaks” or “goes open” or “burns-out?”
The lead going to the load immediately has 340v on it.
The “safe” 12v power supply now has 340v !! Not 240v but 340v.
Some engineers will say the capacitor will only pass a small current and you will not be killed. But that is not so.
A 1 microfarad capacitor will pass more than 30mA during the time when the voltage rises to 340v and anything above 5mA to 15 mA will KILL YOU.
I have never built, used or recommended a capacitor-fed power supply.
They are TOO DANGEROUS to have in the work-room or connected to ANY PROJECT.
It is not worth the risk of having a “ticking time-bomb” connected to anything you will be touching.
All projects and appliances MUST have a correctly-rated ISOLATING TRANSFORMER – commonly called a MAINS TRANSFORMER.
Only use an ISOLATED POWER SUPPLY.
