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Mains power plugs and plug packs

  1. Oct 16, 2013 #1
    OK so I've always wanted to learn about how the mains power works in conjunction with things that you plug in ie. phone chargers, computer PSU's, TV's etc.

    Now I know here in Australia that in the home we get 240V AC at 50Hz with the standard 3-pin outlets rated at 10Amps. What do the 3 pins represent?

    Am I oversimplyfying things if I talk about V=IR? So hooking something directly to the mains and treating it as we would if a circuit was hooked up to a battery with a potential difference. So using the formula, V = 240, R = 24ohms and so the resulting current is 10 amps (too little resistance will allow too much current to flow and we will be left with excess?).

    Obviously things don't work like that. I would take a guess and say that when a phone charger is plugged in it converts the AC into DC so that the phone can charge. Does it use transformers? All of those plug packs have a Voltage and Current rating, again how does it know the current before an appliance is plugged into it? Always thought the resistance and voltage determined the current?

    Be as in-depth as you like as I have a degree in physics. Also if you have any links or something to point me in the right direction that would be helpful too.

    Cheers guys!
  2. jcsd
  3. Oct 16, 2013 #2


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    I believe they represent the live wire, the neutral wire, and the ground. Current should only be flowing between the live and neutral wires, not ground. It's just there in case there is a fault with your equipment.

    None of your devices have 24 ohms of resistance. That is WAY too small. The rating of 10 amps is what the entire circuit can handle before it trips a breaker/fuse. So you can have a bunch of small things plugged in and running at the same time, or a couple of big things like a microwave and vacuum cleaner.


    The device has a set resistance (or in reality an impedance) that determines the amount of current flow through it. The main circuit in the wall doesn't care what the resistance of a device is, it always delivers 240 volts.
  4. Oct 16, 2013 #3
    Yep got it!

    Not sure I understand. Are you following on from my oversimplification or are you talking about reality. What if the only thing I plugged in was my phone charger?

    I don't think you understood that last question. So you have your mains supply at 240V, then you plug in a phone charger rated 5V and 1amp. So the plug pack transforms 240V AC to 5V DC. And the 1 amp rating is just the maximum current that pack/phone can handle. So the makers of the phone would set the impedance high enough so that the current is below that limit?

    So if the phone is running at <1 amp what is the current running back through the mains. ie on the 240V side of the transformer. I guess it would be under 10 amps?
  5. Oct 16, 2013 #4


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    Then total current through the circuit would be equal to the current flowing through your charger. Typically in the milliamp range. Plug in a hundred phone chargers, all charging phones, and you'd blow the fuze/breaker.

    The rating is the output. So in this case your charger would output 5 volts and be able to supply UP TO 1 amp of current. The actual amount of current drawn would depend on the resistance of the device.
    Edit: The resistance can actually be less than what would draw 1 amp, but attempting to draw more than 1 amp of current would cause a voltage drop and the current would never exceed 1 amp. (Or your charger would implode at the speed of light, causing a supergalactic mega black hole to form. Err, I mean it would break.)

    Since we are using a transformer, I believe you'd have to convert to power and look at it that way. If your phone draws 1 amp at 5 volts, then that's 5 watts of power. To get 5 watts of power on the primary side of the transformer, IE the side the main circuit is on, you'd have .021 amps at 240v. (Power is V x I)
  6. Oct 16, 2013 #5
    Im still not sure what your trying to say. When you say plug in a hundred phone chargers... do you mean to the same outlet (in series or para?) or the whole house (ie. plugged into many outlets around the house)? Also wouldn't more chargers mean a higher resistance and therefore lower the current and not blow?

    Yep exactly what i was thinking.

    Ahh year 12 physics its all coming back to me hahaha
  7. Oct 16, 2013 #6
    well I understand drakkith wanted to keep it simple and i'm not super expert myself but , many many household devices use a transformer , both a smaller smps one for higher frequencies or a typical larger one which operates on the line frequency which in your case is as stated 50hz.

    Now in every device which has a transformer in the circuit the transformer is the limiting device.
    Transformer windings have low resistance , most of the time way less than those 24 ohms but don't be fooled by that because it is AC we are talking here and ac reverses direction so a term like impedance comes into play here which means that every AC cycle and you have 50 of them in each second the current reverses direction , and everytime the current does so it is being pushed against by the magnetic field which the previous cycle set up in the transformer core and that is the reason why you don't blow your fuses with such a small resistance because the wire is in a coil and has an iron core.
    If you would put DC in a transformer winding , first of all the transformer would not work , second of all you would probably melt the wires , blow fuses etc if the current would be high enough.

    Also the phone charger cannot draw more than the rating of it no matter how much the wall socket offers because it too has a transformer and transformers have core sizes and wire diameters and for a given core size and wire diameter you can draw only a given amount of current not more , so basically a transformer is also a limiter if you like it that way.

    By the way if you have only one or single phase AC to your house then all the things you put in your wall socket no matter is it a tv or a dish washer are parallel to each other , as there is only a single 240v phase and a neutral so two wires.
  8. Oct 16, 2013 #7


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    I mean in parallel. Everything you ever plug in is in parallel, otherwise you wouldn't have the 240v applied to each one.

    Since they are in parallel, each one draws current separate from the others. This means that as you add more chargers the equivalent resistance drops. So if you have 2 chargers, each with 1,000 ohms of impedance, the main circuit of the house they are on sees 500 ohms of resistance total. (By "sees", I mean that the total current through the circuit is equal to one device at 500 ohms of resistance)
  9. Oct 16, 2013 #8
    I get you now. So there is physically a limit to how many appliances you can have plugged into all your outlets depending on the current the impedance allows them to draw. 1/Rtotal = 1/R1 + 1/R2 and Itotal = I1 +I2. So current total must be less than 10 amps for the whole household?
  10. Oct 16, 2013 #9
    OK so impedance is more like the "resistance" the transformer imparts on its self due to the oscillations of currents and it has to fight against its own change.

    So the mains power "sees" the charger with a certain impedance and allows the appropriate current flow. Can the impedance of the charger change, lets say if the phone itself accidentally draws too much current than what the charger is rated at?

    EDIT: BTW what do you mean by single phase. could you please elaborate??
    EDIT EDIT: I guess it means you can only run one appliance in parallel as opposed to two if you have double phase? Also why is it called phase? would double phase mean two sine waves out of phase travelling down the mains?
  11. Oct 16, 2013 #10
    well the impedance is not only a characteristic for a transformer , it happens everywhere where you have some coil and a current that periodically changes direction , the iron core just escalates this phenomenon to higher levels as without it.
    For example an electric motor has impedance too.

    Impedance is not the only thing which limits the inrush current , the iron core size of a typical transformer also limits how much the secondary windings can draw current as their current is induced by the magnetic field from the core and a given core size at a given frequency can only have a limited given magnetic field strength.

    Well you can make a gigantic transformer with a core size that of a chair and then you would go beyond the limit of your wall outlet , but typical household transformer used in various stuff usually doesn't come close to that limit.

    A water heater comes close or beyond that limit , but it has no transformer in it , and works on (most of the cases) pure conductor resistance or a special metal alloy wire which has high resistance and most of the current through it transforms into heat which heats the water.
  12. Oct 16, 2013 #11


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    Usually it's just 10 amps for whatever leg of the household circuit you are on. IE the living room could all be on the same circuit while your bedroom is on the other, and both will have their own breakers. You can draw as much current through each as the breakers can handle, not exceeding the mains breaker. (I don't remember, but I think the main breaker in my house was more than 10 amps)
  13. Oct 16, 2013 #12
    it' s 230v 16A for a single phase household supply where I live.Maybe in night hours you could draw more from the line than those 16 amps but the circuit breaker will not let me, the older ones probably did.
  14. Oct 16, 2013 #13


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    That seems small by standards here. In the U.S. I grew up in a house with 80 amps, 240 volts at the service entrance. A trip to Google indicates that 100 or 200 amps is common for single-family detached dwelling.
  15. Oct 16, 2013 #14
    whoa , 100 to 200 amps @ 230v ? that sounds pretty crazy for a single household.

    Well I live in a apartment block , a small one in a countryside town but still an apartment block , the biggest devices in terms of power consumption for me is a water heater , a vacuum cleaner maybe, I dunno even hard to name because I don't have anything that would be worth 100 amps of current not even 80, I cant even use my 16 at full power , well maybe if I would light up all the lights and everything I have and some electric heaters (don't need them as i use the heat from the city) + stick some fingers in a socket :D

    Maybe it's because I live in the european part of the former USSR , we did things kinda differently here if you know what I mean :d
  16. Oct 16, 2013 #15


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    Yes, American per capita electrical consumption is just a tad above world averages :smile:

    However, almost all American residential branch circuits operate at 115v not 230 (the service to the house is 230v but the neutral is central-tapped, so you get 230v between the two hot legs and 115v between either hot leg and neutral; 230v is only used for the big appliances such as water heaters, air conditioning units, stoves, and the like) so the kilowatts consumed is not quite as amazing as it seems.
  17. Oct 16, 2013 #16
    Ok yes I am informed that the US has 110 v system but tell me, ok even with 110 volts if you had like say 100 amps on 230 you would have about 50 amps when you split the 230 in half and say use it in your living room or kitchen or wherever.

    Now what exactly in modern days uses that much electricity ? I'm not talking about some special auto repair garage equipment or electrical saws or whatnot , I'm speaking about the things one would normally use in a living house or an apartment , ok aside from my electronics hobby , I know of nothing that uses that much electricity even if you are kinda rich and have alot of stuff, like many pc's , LCD or Led tv's etc etc.
  18. Oct 16, 2013 #17


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    Heh, I have 2 breakers just for the oven that are rated at 50 amps each, so I assume I can pull a LOT more than that total in my apartment. Of course, I'm at 120v, so I don't know how much of a difference that makes.
  19. Oct 16, 2013 #18


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    FWIW In a fairly small house in the UK, my total power supply rating is 100A at 230V. That is two separate 30A breakers to different groups of wall outlets, another 30A for an electric cooker, and 10A for the lighting circuits.

    Of course it's unlikely I would use the full 100A at any one time, but 30A at 230V is about 7kW, and an electric kettle plus two 2kW fan heaters all plugged into the same group of outlets would be getting close to that.
  20. Oct 16, 2013 #19
    Ok yes I forgot the electric oven , that may very well be the most power consuming electrical thing in a household.

    Here in europe and also in the times of the USSR , we have basically only three phase supplies everywhere , so in an apartment like mine we have that a three phase supply goes into the building and every phase is divided to a different apartment so that the load is divided equally , and if someone has an electric oven he just instals a three phase cable from the supply box and uses the cable to power the oven.

    Seems like in US you have made the single phase string enough so that you could hang literally everything on it without serious sagging of line voltage issues is that right?
  21. Oct 16, 2013 #20


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    If you split the 230 in half, you double the amps. So 100 becomes 200 and 200 becomes 400.

    100 amps from hot to neutral plus 100 amps from neutral to hot (180 degrees out of phase) is a total of 200 amps across your 110 volt household circuits but still only only 100 amps across the 230 volts from hot to hot at your service entrance.

    That said, the big power consumers are likely things like electric ovens, stoves, clothes dryers, air conditioners, heat pumps and backup resistance heating systems that already run on 240 volts hot to hot.
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