voltage step down through capacitor

[neptune]

Hello,
i would like to know why dont we use capacitor to step down Mains voltage in battery charger.
like this i have shown, it dessipites 800mW of power in process.
but circuit is less bulky then Transformer one.

 

[ericgibbs]

Hello,
i would like to know why dont we use capacitor to step down Mains voltage in battery charger.
like this i have shown, it dessipites 800mW of power in process.
but circuit is less bulky then Transformer one.

Because the capacitor circuit is NOT isolated from the Mains supply, a transformer step down, is isolated

 

[neptune]

Because the capacitor circuit is NOT isolated from the Mains supply, a transformer step down, is isolated

what does that mean ?

 

[Nigel Goodwin]

what does that mean ?

It means the charger is live to the mains and dangerous.

There's also the problem when the capacitor goes S/C!.

 

[neptune]

It means the charger is live to the mains and dangerous.

There's also the problem when the capacitor goes S/C!.

so only accidental short circuit is the only problem, well we can provide a fuse at both input and output.

 

[Nigel Goodwin]

so only accidental short circuit is the only problem, well we can provide a fuse at both input and output.

NO - as I explained, the entire circuit (including the battery) is live to the mains - DANGEROUS!!!

 

[neptune]

NO - as I explained, the entire circuit (including the battery) is live to the mains - DANGEROUS!!!

can you explain to me in detail reasons for it being Dangerous !

 

[Nigel Goodwin]

can you explain to me in detail reasons for it being Dangerous !

Grab a live mains wire - you'll soon see why it's dangerous!

 

[colin55]

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.

 

[Reloadron]

Well written and informative Colin. Makes it easy to understand why power supplies of this type are unwise.

Ron

 

[neptune]

Forgive my naive reasoning but i am nebie to Electricity at high value.

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.

then what about the reactance , it increases with decreasing frequency. i started to decrease my output resistance from 100 to 0.1 ohm but current flowing through it remains to some extent same 1mA to 9mA but voltage wildely varies to 1mV to 1000mV , or is Spice can not simulate real life situation ?

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.

we can have fuse for that at both input and output of step down circuit

The neutral is connected to the chassis of your project and if you touch it, nothing will happen. It does not rise or fall.

there are 3 lines in supply live,neutral,earth & i think it is Earth is connected to body of appliance which we dont use in case of small charger. we only use live and neutral.

But suppose you connect the power leads around the wrong way.

if i connect it in opposite way this circuit behaves in absurd way by swinging voltage from 1000uV to 200mV but nothing serious to burn any component

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.

In LTspice when i cut output from the circuit the only thing that happens is voltage output falls, Current oscillation switches from capacitor to 50k resistance, I dont know how to make neutral supply line in LTspice line so i have grounded it.

Two things you have forgotten to mention are frequency and reactence, we can use capacitor that can withstand 500V.

 

[colin55]

Simulation software does not react (behave) anything like the "mains."
You have to take the above discussion and understand it in the context it is written.
Get a 0.47u capacitor. Connect it to the neutral and you will not feel anything.
Now connect it to the "active" and touch one lead and a water pipe. Feel the "tingle."

 

[Ratchit]

Nepture,

You would not use a simple resistor to reduce the line voltage, would you? Both a resistor or capacitor would reduce the current and voltage at the load, but neither will provide electrical isolation when connected the way you are contemplating. I hope you can see the folly of doing that.

Ratch

 

[neptune]

Nepture,

You would not use a simple resistor to reduce the line voltage, would you? Both a resistor or capacitor would reduce the current and voltage at the load, but neither will provide electrical isolation when connected the way you are contemplating. I hope you can see the folly of doing that.

Ratch

i still cant understand whats dangerous in this circuit , so i will ask in some other forums, google etc

 

[ericgibbs]

i still cant understand whats dangerous in this circuit , so i will ask in some other forums, google etc

Look at this Sim.!

 

[crutschow]

i still cant understand whats dangerous in this circuit , so i will ask in some other forums, google etc

The danger is simply that the capacitively coupled output is not isolated from the AC mains ground. Thus touching any live wire in such a circuit can generate a dangerous current to earth ground through your body and electrocute you.

A transformer isolated circuit has no path from the mains to earth ground so cannot electrocute you.

You can simulate that with LTspice. If you ground the mains input source to a transformer but not the output, you will see no current between an output resistor and ground. You will with a capacitively coupled circuit.

 

[Reloadron]

I have updated Eric's simulation to include Neptune in the picture. Note Neptune is grounded, note Neptune is toast or at least really having a bad day. Note why circuits like this are not a good design practice. There, done.

 

[ericgibbs]

I have updated Eric's simulation to include Neptune in the picture. Note Neptune is grounded, note Neptune is toast or at least really having a bad day. Note why circuits like this are not a good design practice. There, done.

hi Ron.
I did name the resistor in my Sim as 'neptune'!

Your 'flashing' image of neptune deserves an Oscar!..

Hope you have saved a mince pie for your morning coffee...

 

[Reloadron]

I caught the Neptune 10K along with the current.

Yes, enjoying my morning coffee while on holiday from work, no mince pie though, .

Ron

 

[4pyros]

You can't get that much power out of a cap power supply either.