need some basic circuit assistance - capacitor / transformer stuff 1

[USAeng]

If you have a high volage transformer 120vac to 2000vac then a 1/2 wave rectifier (diode) is in series to change to dc and then a large nonpolar capacitor that is in parallel with the item that it discharges to... is it ok to have the return to the other side of the capacitor? See attached sketch in the file.


Can someone explain to me what happens inside the capacitor to make this work without a connection to neutral or ground? Maybe the capacitor can reuse the leftovers? or does it connect to neutral somehow through the transformer... I'm trying to learn more about circuits so I appreciate any help.

 

[Skogsgurra]

There is really no need to get Neutral or Ground involved.

Think of a torch battery. There is voltage between the terminals and you needn't connect to ground to make a lamp light.

Same thing with a transformer. Voltage (AC in this case) is induced in the secondary winding and that voltage is rectified in the diode and reaches the capacitor, which is charged. The resistor just loads the capacitor and there is a current flowing in it which discharges the capacitor when AC is removed from transformer primary.

Don't confuse yourself by using terms like 'leftovers' - no use.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[zappedagain]

This is called an isolated power supply, as there is no connection back to the mains (line, neutral, ground) on the left side of the transformer.

 

[USAeng]

is there any danger here if on the right side the current is going out through a wire and coming back through a bare metal rod? wouldnt a ground be needed for safety?

What if you connected a the bottom right terminal of the transformer to ground?

Thank you for your answers so far... very helpful

 

[Noway2]

Your last post is taking this series of questions to a whole new level, that of safe practices. These are typically governed by local regulations and the NEC and then UL (and other agencies) make a business out of establishing and testing to safety standards.

"Ground" is an arbitrary reference, there is nothing special about it. It is used because it is both convenient and operators are usually in contact with it making it a good choice for safety purposes. Whether or not the circuit shown in the diagram is safe will depend on a lot of factors that are not part of the diagram, especially those related to whether or not someone can come into contact with 'live' conductors, the supply voltage, fault current capacity, the operating conditions, and the mechanical design. By using a transformer, the secondary is no longer referenced to the same point as the primary and you are free to choose a new (and arbitrary) reference.

Often times when you have a form of "mains" circuit on the secondary of an isolated transformer, a ground is required and is established. If for example, this circuit is the power supply of an enclosed device, this is not done.

 

[Warpspeed]

Yes, "ground" is entirely relative.

Grounding something in your car, or even in an aircraft !! still applies.
It just means connecting one side of something to the main local mass of wherever the equipment is located, usually for both convenience and safety.

But something operating from a small battery is often completely isolated and not grounded to anything.

 

[zappedagain]

Except for the capacitive coupling to ground that lets the touch screen work!

John D

 

[VEBill]

"Except for the capacitive coupling to ground that lets the touch screen work!"

???

An iPod Touch (an example that uses a capacitive touch screen) works on airplanes at 35,000 feet. And it should still function even if you were floating in the intersteller void.

 

[zappedagain]

Okay, capacitive coupling to the objects nearby in the environment. And the new ones don't need stray capacitance to function; they sense the change in dielectric and work even better. My point was to make sure you don't forget about the parasitic elements.

Z

 

[zzzjim]

To understand circuits , practical hands on experience is usefull . THIS circuit has a lethal voltage , so don't try
anything practical !


A circuit is basically a loop , by normal convention power
arrives from the left ( but not always )
the diode allows half of the AC cycle to get through
to the capacitor and charge it up (More than 2000V)


A circuit is a simplified view of the real world ,
insulation may fail , isolated becomes connected !

and stray capacitances connect every conductive object,
more noticable at high voltage-or-frequency

 

[Syncopator]

I am convinced that the word "ground", while used by many, is understood by few.

Take a look at this

http://www.davidbridgen.com/earth.htm

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Equal opportunities for equal abilities.