Why is that high voltage AC is more commonly found than High voltage DC? Example my battery powered fly swatter and fluorescent lamp both use high voltage AC. Why can't these devices increase the DC voltage from battery and directly use the High voltage DC?
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asked Jun 23, 2011 at 21:06
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\$\begingroup\$ What other example are you thinking of with high voltage AC being used over high voltage DC? \$\endgroup\$– KellenjbJun 23, 2011 at 21:22
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\$\begingroup\$ I think your use of "high voltage" isn't what most people think of it is. I'm thinking of higher than mains power, i.e. above 120/240 V AC. \$\endgroup\$– Brian CarltonJun 23, 2011 at 22:50
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2\$\begingroup\$ @Brian Carlton his two examples meet your requirement. The electric fly swatter is probably like 1500V, a Fluorescent lamp, depending on the size of the tube, can take several thousand volts to ignite it and likely more than the battery voltage to maintain it. \$\endgroup\$– MarkJun 23, 2011 at 23:47
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\$\begingroup\$ I wrote about a similar question in my blog about a few years ago. \$\endgroup\$– BlueRaja - Danny PflughoeftJun 24, 2011 at 6:34
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3 Answers
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All it takes to make high voltage AC is low voltage AC and a transformer.
To make high voltage DC, you have to chop it into (what else) AC, run it through a transformer, and then rectify it back to DC. Quite a bit more hardware is necessary.
So, with mass produced products, there's a strong economic bias to use AC high voltage, so that's what you'll see, unless there's a compelling reason that the high voltage needs to be DC.
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1\$\begingroup\$ In other words, it cuts out the... the opposite of the middle man. \$\endgroup\$– endolithJun 23, 2011 at 23:21
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\$\begingroup\$ I think his examples are both based on devices with a DC power source. Thus the real issue is that it doesn't make sense to go DC -> AC -> step up transformer -> DC. The last step isn't needed unless there is, as you said, a need for the final output to be DC. \$\endgroup\$– MarkJun 23, 2011 at 23:49
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\$\begingroup\$ You can also make high voltage DC with a motor-generator. However, it's a lot more inefficient, so no one really does that. \$\endgroup\$ Dec 16, 2012 at 9:44
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Another reason for high voltage AC has to do with arcing. If an arc is formed with DC, it's very difficult to extinguish it (you need to disconnect the power source until the air gap de-ionizes).
In the case of AC, the arc is extinguished in each cycle. Once your fly is fried, you are not left with a continuous arc.
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\$\begingroup\$ and that's the precise reason why there are no DC networks. A member of the network couldn't just disconnect, it would span very, very long arc. \$\endgroup\$ Jun 24, 2011 at 7:44
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\$\begingroup\$ I wonder how changes in technology would affect the relative desirability of using different frequencies for power transmission. My understanding is that there are substantial frequency-related losses which would be reduced if one used a lower frequency, but "simple" transformers work better at higher frequencies. Even if one couldn't use pure DC because of arcing issues, I wonder whether there might be benefits of converting 60Hz to a lower frequency before a long span, and then converting back to 60Hz at the other end. The conversion wouldn't be free... \$\endgroup\$– supercatDec 15, 2012 at 23:27
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1\$\begingroup\$ ...but if the LC-related losses over long distances are as significant as I understand them to be, the losses from such conversion might be smaller than the eliminated transmission losses. Any thoughts? \$\endgroup\$– supercatDec 15, 2012 at 23:29
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The changing current in AC makes it possible to step up and down voltages. DC to DC converters usually generate some form of AC to do the conversion most efficiently with a switching circuit of some type.
If the device can function on AC, then there is no reason to have the performance losses of converting back to DC after stepping up the voltage.
This is also why power distribution is via AC. The voltage can be stepped up to a very high voltage, which makes the current drop for the same power. This allows power to be supplied with less losses due to the resistance of the wire. Then it is progressively stepped down until it gets to the 220-240 that most homes are fed (to be used as both 110 and 220 in the US and usually 220 only else where.)
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3\$\begingroup\$ Depending on how generators are wound and made they can generate ac or dc. Power losses in transmission lines governed by the equation P = I^2*R. To minimize losses in transmission you need to reduce the current (as it has the most effect being squared). This is achieved by stepping up the voltage up to 500 kV near the source, transmitting it around the country and progressively stepping it down to 220/240 VAC for consumer us or three phase 425 VAC \$\endgroup\$ Jun 23, 2011 at 21:47