Transformers

[5614]

Many household appliances get power from the mains via a transformer. My question is that when a transformer is plugged in and the application turned off is there still current flowing through the primary?

 

Yes: then there'd be an induced current in the secondary, where does that current go if the app is off? And it'd be a big waste of electricity.

 

No: then the switch needs to go all the way from the appliance to the primary of the transformer (which for some appliances there doesn't seem to be a wire doing this).

[YT2095]

yes there is , and yes it is (a waste of elec).

the primary afterall is Still a resistance across the mains.

[5614]

hmm, I thought so and that explains the lack of switch wires and the heat given by a transformer, but that's stupid and total waste of energy, hasn't anything ever been done about this?

[Thomas Kirby]

Transformers are very inefficient and do use power while plugged in. There is an issue of both safety and expense. In a wall-mounted transformer, it is physically difficult to arrange to cut off the AC power. No amount of quality control can prevent the odd bad part or bad hookup from slipping through the net, then someone's house catches fire. They also get beat up. Eliminate the switch, tie all the loose ends with wire nuts or other secure fasteners, you eliminate that safety issue. Switches that cut off the high side are only suitable for sturdy, highly fire resistant cases. They are not suitable for small plastic cases. Most, but unfortunately not all transformers that are used that way are completely enclosed in metal, making them "Class II."

 

Some devices need a continuous trickle of power, like clocks and anything run by remote control. That's almost everything but the electric can opener.

 

Each watt consumed by a transformer takes about a month and a half to run up a kilowatthour on the meter. It isn't worth the risk to try to save the energy by wiring a switch into a wall transformer. The best you can do is just unplug the device or attach it to a power strip that has a switch on it.

[insane_alien]

its not difficult to cut power to the transformer. its a mysteriour device called a "switch" they are usually found on the wall sockets already! you just push the top part and the electricity ceases to flow. Amazing isn't it.

[YT2095]

american and canadian mains outlets don`t often have switches on them.

[Douglas]

yes there is ' date=' and yes it is (a waste of elec).

the primary afterall is Still a resistance across the mains.[/quote']I agree.

But most appliances like toasters, frying pans etc wouldn't use a transformer.

[YT2095]

well obviously not

 

but I tell what CAN drain the power on some of these items, the Filter Caps!

many of these devices (esp the ones that use motors) have a filter system, these use capacitors across the mains, and the more heavy duty the motor the larger the cap, and the larger caps have a a leak resistor across them (usualy a Meg or so) by law, as there were a few out there that didn`t and gave customers electric shocks when touching the plug pins after it was unplugged for a while.

basicly any inductive load will (should) have one.

now depending if it`s placed Before or After the switch, depends on whether it`ll draw power or not, some have the switch After the mains conditioner, and thus will draw power (not much, but some, and constantly).

[5614]

OK, so my question is this:

 

If there is a current flowing through the primary then surely there'd be an induced current in the secondary, but it's not a complete circuit so the current can't flow. So what happens?

 

One sec, I think I remember a thread, maybe about this... is the answer that a current is not induced, and then YT asked how does it know if its a closed or open circuit so it knows whether to induce a current or not, something like that!

[YT2095]

actualy my question was along the lines of "how does the Primary "Know" what`s happening at the secondary" (or something like that).

 

but back to your question, basicly you`ve just got a simple AC electromagnet, there`s no Current induced in the secondary, only Potential (if we ignore resonance and eddys).

[5614]

actualy my question was along the lines of "how does the Primary "Know" what`s happening at the secondary" (or something like that).

Yeah, I remember it was quite long and confusing, was their a simple answer you learnt from it that you could pass on, as opposed to me going through the whole thread?!?

[YT2095]

I remembered some key Words and found the thread we were discussing.

 

here: http://www.scienceforums.net/forums/showthread.php?t=6600

 

to answer your question, No, I`m non the wiser in all honesty

 

I`ve conducted (excuse the pun) a few "thought experiments" since then and have an idea what might be happening, but I`de rather NOT post it here, as it`s not very a Scientific thing to do, and I can`t put it into words easily.

[5614]

Not that long, I see why you say you're none the wiser, I like the comparison the the "how to know if it's an open/closed circuit" but thinking, in that situation theres an electric potential supplied, if it's closed then current will flow due to the potential, easy, whereas I can't see an 'easy' for the transformer version!

 

So maybe we're looking at the fact that a magnetic field is produced and it's to do with whether the magnetic field is being 'used' to induce a potential/current (which one?) into the secondary.

[YT2095]

I already understood that bit, but not the WHY, or the means of this process.

as I said, I have an "Idea" why, but I can`t word it

 

I suspect the answer will involve lots of formula and Maths and other stuff too, that`ll leave me just as "dead in the water" as I am now

[insane_alien]

why the hell would you not have a switch on the socket?? seems a bit silly considering that the switch has saved my life numerous times.

[YT2095]

no idea at all????

but their plugs (unlike ours) will pull out the socket very easily, the cable comes out horizontaly, whereas ours comes out 90 degrees to the vertical and needs to be pulled out, not only that but we have Britisg Standards as marked by BS1363, I`m not sure they have the same or equiv.

Oh yeah, and many appliances are only 2 pin plugs unlike our 3 pin.

[5614]

As well as the situation YT suggested there's the fact the main transformer for me that is plugged in and not used would be the one for my plasma lamp, which is plugged into a splitter thingy, turning that off would turn off my computer and clock etc which I don't want! (The wire is not long enough to reach anything other than this splitter) So turning off the plug isn't always the best option.

[5614]

Oh yeah, and many appliances are only 2 pin plugs unlike our 3 pin.

And many of our 3 pin plugs only use 2 of the pins due to double insulation being used, so the 3rd pin isn't really essential to Britain (other than in old appliances).

[insane_alien]

the third pin has also saved mylife a few times like when i tried to fix the toaster when it was on(what a stupid mistake that was!)

[5614]

It might be cool to play around with electronics but you seem to be heading towards stupid (meant in a funny way)

[Thomas Kirby]

Insane Alien, are you talking about a GFCI outlet? I'm not sure it's a good idea to use the "test" switch just to turn off the outlet, if that is the case.