If it is more efficient to have high voltage, low current power lines, why aren't homes and appliances adapted to accept high voltage and low current (and remove the need for transformers)?
Efficiency of high-voltage power lines
currentpower supplyvoltage
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Let me give you a little background on myself... I've been working professionally in the audio industry for more than 14 years. I've designed circuits for most of the major pro-audio companies, one audiophile company, and several consumer audio companies. The point is, I've been around and know a lot about how audio is done!
SMPS can and are used for audio circuits! I've used them from sensitive microphone preamps to huge power amplifiers. In fact, for the larger power amplifiers they are mandatory. Once an amplifier gets over a couple of hundred watts then the power supply needs to be super efficient. Imagine the heat produced by a 1000 watt amp if it's power supply was only 50% efficient!
But even on a smaller scale, the efficiency of a SMPS often makes a lot of sense. If the analog circuitry is properly designed then the noise from the power supply gets rejected by the analog circuitry and doesn't impact the audio noise (very much).
For those super-noise-sensitive applications you can do a hybrid approach. Let's say that you have an ADC that requires +5v. You can use a SMPS to generate +6v, then a super-low-noise linear regulator to bring that down to +5v. You get most of the benefit of the SMPS, but the low-noise of the linear regulator. It is not as efficient as just a SMPS, but those are the trade-offs.
But one thing to keep in mind... A SMPS for audio applications needs to be designed with audio in mind. Of course you'll need better filtering on the output. But you will also need to keep other details in mind. For example, at very low current the SMPS might go into something called "burst mode" or "discontinuous mode". Normally a SMPS will switch at a fixed frequency, but in one of these modes the switching will become somewhat erratic. That erratic behavior might push the output noise into the audio frequency band where it becomes more difficult to filter out. Even if the SMPS is normally switching at 1 MHz, when in one of these modes you could get 10 KHz noise. Controlling how this happens depends on the design of the chip that the power supply uses. In some cases, you can't control it. In that case you have no choice but to use a different chip or use a hybrid approach.
Some people advocate using only linear power supplies for audio. While linear supplies are less noisy, they have lots of other issues. Heat, efficiency, and weight being the biggest ones. In my opinion, most of the people who preach linear supplies only are either misinformed or lazy. Misinformed because they don't know how to handle switching supplies or lazy because they don't care to learn how to design robust circuits. I've designed enough audio gear with SMPS to prove that it can be done without too much pain.
Those input power and voltages are rated input power and voltages.
For example you can drive electric motors over rated power but they will get too hot and eventually break. There are also electric motor duty classifications for industry. See this for short introduction: http://www.electricalengineering-book.com/duties-of-induction-motors.html
In practice this means that one same machine could be used for two different applications. For constant non-stop usage rated power can be e.g 100 W but then for cyclic usage where motor stops e.g for 5 minutes and then drives for 2 minutes the rated power can be e.g. 130 W.
This was just an illustrative example from industry machines but I have not checked how big difference in power output there actually is between these two types.
Back to this case: Peltier element's rated input power in this case is that around 96 W. You can also use it with lower power. For example you could attach a sensor system that measures temperature of cooled object and then the input power is adjusted by control circuit to adjust for example voltage given to the element. Since peltier is a semiconductor device it is likely more prone to break with over-rated power even for short times. I do not recommend trying that.
The rated numbers for that element can be based on theory and then it is tested properly to be sure that it works under that load for long time enough to be sold for consumers.
Also shortly about fundamental theory:
- Voltage (potential difference between two planes, nodes... etc.) produces electrical current -> resistance limits current -> power is consumed to that resistance to get over it. Refer to Kirchhoff 1st and 2nd and Joule's law.
In practice you can buy cheap multimeter, small battery and a small resistive lamp, couple of resistors and see with measurements when you change resistance in that circuit and see in practice in brightness of the lamp. This is brilliant way of getting started in practice.
Remember to stay safe while measuring and do not measure any high power device voltages or currents if YOU are not familiar with the theory of electric laws! Small 9 V alkaline batteries are safe enough but things get much more dangerous even with 12 V car batteries if you don't know what you are doing!
Related Topic
- High voltage power lines
- How is it possible to have high voltage and low current? It seems to contradict the relationship between current and voltage in E=IR
- Electronic – How to we have high voltage and low current (in transformers), if V = I * Impedance
- Electronic – Current flow in step-up and step-down transformers of a power system
- Electronic – Return path for current flow in transmission lines
- Electronic – Why are integrated circuits powered by low voltage and high current
Best Answer
In designing high tension lines, the safe distance between conductors is 11..17 feet for the common 500..900kV lines.
To have an 'appliance' that operated directly off high tension would first off require an outlet and plug that were more than 14 feet wide, plus an additional 14 feet clearance on both sides for the user's safety (totaling 42 foot wide x 28 feet high for single phase; 42 foot high for 3-phase). Regardless of the strength of dielectrics, how would you plug and unplug something without exposing the conductors to air?
Designing an appliance runs into the same problem. The load must be stretched out across the 14 feet to avoid arc-over. Be it a heating element or a motor. You would need bread 14 feet wide for such a toaster. Such a motor would necessarily be obscenely huge, maybe a 100 feet huge, because each winding must begin and end at least 14 feet apart. A 4 pole motor must have a circumference of at least 8*14 feet. The original generators at the power plant put out 28000 volts, and they cannot be made any smaller than the winding depth limits due to voltage generated in them. This is a good foot of depth to each coil. Minimally, there are 4 coils. This size would go up 30-fold with a 30-fold voltage increase to 600,000 vac.
In addition to the scale of things being out of whack, there is the fact of electron inertia. They don't weigh much, but 100 miles of high tension current cannot stop instantly. Break open a switch and they pour out as arc-over, jacob's ladder, and ball lightning. Without the isolating effects of a transformer, every kitchen worker would be at constant risk of such lethal surges. Many a good electrician (good but not excellent) has met his death to such surges.
And if all of that is not enough, there is the 4th state of matter: plasma. When electrons are forced (by their own inertia), to make their way without a conductor (metal); they get very irritated, let's say HOT. They emit a lot of photons, at about 30,000 degrees F. It can cut right through living tissue the thickness of your thigh in a millisecond, perhaps nanoseconds. Its called cauderizing -and is very disturbing to witness, less alone be the victim of.