Ca APPLE 24v 1.875a AC/DC Adapter, Power Supply CAD$43.93

Question Related APPLE 24v 1.875a laptop ac Adapter ( From internet ):

When a computer power supply says voltage range 100-240, does it actually mean that it supports any voltage between 100v ~ 240v or does it mean either 120V or 240V? I only have 208V outlets available?

Yes, 100–240V means any voltage input within this range is acceptable. In most cases, it would not matter whether it is 50 Hz or 60 Hz supply.

Internally the computer power supply (called SMPS) converts incoming AC into DC supply. This DC voltage is then switched at high frequency, passed through a step down transformer and rectified again to produce 3.3V, 5V, 12V and other similar voltages.

The switcher needs only sufficient DC voltage and that is available in this AC range. Some low cost low- power wall adapters even work for “Universal range” i.e. 90V to 270V AC.

Why does Europe have a voltage of 220V, Japan has 100V, and North America has 110V? Is 220V better, more dangerous, or more efficient? Why doesn't the world converge into a more unified voltage?

Back in the early 20th century Edison experimented extensively to find the ‘sweet spot’ voltage for incandescent lamps. That is, most lumen’s vs filament life. He settled on 110 volts as the best compromise given the state of the art on filament design. 60 cycle AC was determined by Tesla as an appropriate speed for induction motors. Also, in 1890 it was decided that arc lights operated better using 60 cycles instead of lower frequencies. In the early 1970’s the electrical industry in the US raised the voltage to 120/240 volts, from 110/220 volts in the US. This was perhaps to accommodate expanding consumer use of electricity.

Higher voltages allow smaller wire sizes for the same power (watts) since watts is equal to volts times amps, one can double the voltage as in Europe, and draw half the amperage to get the same power. This allows smaller wire sizes.

The heating of wires is due to the numbers of factors, such as:

1. The type of wire, such as copper or aluminum.
2. The size of the wire, such as no. 12 or no. 14.
3. The number of amperes flowing through the wire.
4. The insulation covering the wire.
5. Airflow around the wire.
6. Ambient temperature.

Counter-intuitively the larger wire size is no. 12 rather than 14 in this example. No. 10 is larger then no. 12.

Higher voltages do not directly cause heating, higher amperage's do, with everything else being equal.

Therefore, by using higher voltages, one can reduce the wire size for the same amount of power (watts) being transmitted.

Another example is when transmitting large amounts of power over long distances as the power companies do, the voltages are extremely high, as in hundreds of thousands of volts which allows the power companies to use smaller cables. Since heat losses in conductors is mostly dependant on AMPERAGE, higher voltages reduce line losses. There are other factors, to consider, such as corona (ionization of the air) losses when using extremely high voltages. The air actually starts conducting electricity.

Europe, having watched electrical development in the US doubled the 110/220 volts to 220/240 to reduce wire size and thus wiring costs. Europe having observed the pros and cons of 60 cycles and 110/240 V current decided that 50 cycles and 220 V volts was more cost-effective than the US system. They also selected 50 cycles vs 60 cycles because generating 50 cycles current allowed slower prime mover speeds, such as diesel engine speeds, turbine speeds etc which would reduce wear on the machinery.

From a usage standpoint, there is little difference between 50 cycles vs 60 cycles. Six of one a half dozen of the other. However, lower frequencies require slightly more transformer iron so slightly heavier transformer sizes were required which apparently was offset by reduce wiring costs. Also, smaller wire sizes lower installation costs as smaller wire is easier to install.

Another factor when transmitting power over long distances as in the power grid, is that higher frequencies cause larger line losses due to increased effects of inductance and capacitance. At higher frequencies, there is the skin affect to consider. At 50 to 60 cycles skin affect has little effect on conductors.

Once one gets up to radio frequencies in the kilocycles range, skin affect begins to matter. Skin effect is due to Eddy currents in the cable which are caused by alternating current inductance, ‘pushing’ current flow toward the surface of the conductor, which effectively reduces the cross-sectional area of the conductor, which increases resistance.

Commercial airplanes use 400 cycles because the savings in weight on planes is important and smaller on board transformers and electrical equipment save weight. The size of transformers is inversely proportional to the frequency used. Therefore, higher frequency systems can use smaller, lighter transformers.

And, as others have said, once the infrastructure is in place configured for specific voltages and frequencies, the costs of changing from one standard to another is unnecessary and cost prohibitive.

240 volts is riskier to work with than 120 volts, because for a given resistance, such as the resistance of a human body, the amperage passing through ones body will be higher. The body is sensitive to current (amps) passing through it, especially across the heart. A sustained current of about 30 to 50 milliamps will kill most people, due to electrical interference of the hearts rhythm. People ask which is more dangerous AC or DC? AC interferes with the hearts rhythm and can causes the heart to go into ventricular fibrillation which if not immediately treated will cause death. High voltage warning signs imply that high voltage is what kills. Actually, amperage kills but voltage pushes the amps, so the high-voltage warnings really mean that the voltage is high enough to push enough amps to kill you. It doesn’t take much amperage across the chest to cause death 10 mA is often enough.

DC tends to cause muscles to seize onto the conductor so the victim cannot let go. The heating effects due to the resistance of the body is a large factor in DC shocks.

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