AC Current

[sparky100100]

Hi everyone, I'm new to this forum and although the name suggests so, im not a sparky.

AC Current - from what I remember from school, this stands for alternating current - i.e. the current alternated between the live and neutral? Is this right?

So basically the live becomes neutral and the neutral becomes live according to the frequency? Is this correct or am I completely off course?

 

[ericmark]

Definitions comes into this question and to start with neutral and line are both classed as live.

So what we would say in line alternates between positive and negative when compared to neutral.

 

[sparky100100]

Right, so neutral stays neutral, but live alternates between live and neutral? How does that actually work in real world terms?

As in if I have a light connected to an AC circuit, when switched on, how does the light stay on and not flicker on and off when the live current alternates between live and neutral?

 

[ericmark]

A LED or florescent lamps does go on and off 100 times a second the tungsten lamp will not cool down that quick which is why for rotating machines we don't use florescent lamps because of the stroboscopic effect.

Line says as line and live stays as live with neutral staying neutral it the positive and negative which swap.

 

[Spark123]

The live voltage oscillates between around 340v+ and 340v- 100 times per second, the neutral sits in the middle at 0v.
Current will follow the voltage in resistive circuits such as filament lamps, heaters etc.
In a filament lamp where the wire is glowing hot it doesn't extinguish as it doesn't have time to do so.
In other types of lamp such as fluorescent there is a flicker but is too fast for the eye to see.

 

[sparky100100]

Line says as line and live stays as live with neutral staying neutral it the positive and negative which swap.

Didn't understand that last bit?

Is that the same with TV's and appliances like that too? They're actually switching on and off but just too fast for anyone to notice?

 

[Taylortwocities]

The measurement of the voltages takes place between the neutral and the line conductors.
The neutral is the reference. Zero volts if you like.

The voltage on the line conductor starts at zero, goes to a maximum positive voltage of around 340 volts goes back to zero and then to a maximum negative voltage of around 340 volts and then back to zero again.

This is done in a smooth change called a sine wave. In the UK this happens 50 times a second (frequency) so your eyes don't see any flickering.

I suggest you Google "AC Theory" to get more detailed information.

PS I do hope this is a serious question and not a wet afternoon troll!.
PPS Suggest you change your 'handle' from Sparky until you understand this very basic stuff!

 

[riveralt]

 

[sparky100100]

my name is steve Parkinson - sparky for short. That's why I made it clear im not a sparky at the beginning of the post.

Is a serious post. Trying to understand the theory behind electrics having read some of the posts in the UK Electrics forum.

Will google it now......

 

[ban-all-sheds]

my name is steve Parkinson - sparky for short. That's why I made it clear im not a sparky at the beginning of the post.

Well, then unless you have a magic piano please follow TTC's advice and change your username.

 

[ericmark]

Now come on Bossy Boots your only his girl friend

 

[sparky100100]

my name is steve Parkinson - sparky for short. That's why I made it clear im not a sparky at the beginning of the post.

Well, then unless you have a magic piano please follow TTC's advice and change your username.

Magic piano? I didn't know there were rules on having a specific username on this forum?

 

[ban-all-sheds]

Sparky had a magic piano.

There are no "rules". Just sensible behaviour about whether you should call yourself "sparky" on an electrical forum.

 

[SimonH2]

Actually, I remember in physics lessons at school hooking up a photo detector to an oscilloscope and showing that a tungsten light does in fact flicker. It doesn't drop a huge amount in brightness during the null points, but there is a very detectable 100Hz flicker.

As pointed out, magnetic ballast flouros go out almost* completely 100 times a second. As do many other types of discharge lighting. That's why it can be dangerous in a workshop with moving equipment where the strobe effect can make equipment appear stationary even when running. Splitting the lighting over 3 phases helps a lot.
* I think the phosphor actually has a bit of delay to it.

Now, to TVs and such. These all have a power supply that includes some energy storage. Most equipment these days simply rectifies the mains and stores the resulting DC in a capacitor to get around 340V DC. A switch then chops this up into pulses (at high frequency so the transformer is small), puts it through a transformer (for isolation) which steps it down to a lower voltage where it's again rectified and stored in a smoothing capacitor. That's a gross simplification - amongst other things, such a simple design is not legal any more as it puts a lot of "harmonics" onto the supply current.
This is what's called a "switch mode power supply" or SMPS. "Electronic transformers" for low voltage lighting, and "electronic ballasts" for flouros work much like this.

Older equipment would have had a 50Hz transformer to drop the voltage, then the lower voltage would be rectified and stored in a smoothing capacitor.

 

[viewer]

This might help Sparky:
http://www.bbemg.ulg.ac.be/en/general-information/electric-magnetic-fields/electrical-concepts.html

It is the best explanation I've seen in simple language (written by Belgians!). Many explanations talk of voltage changes but fail to explain how this translates into the release of energy.