On Wed, Nov 22, 2006 at 11:29:01PM -0500, Michelle Dupuis wrote: > 48VDC is a long time telco standard - and has become the Power over Ethernet > standard. > > Keep in mind that 'electricity' isn't the measure - it's power. Power is > not synonymous with voltage.
More to the point, there is a tradeoff. For a given power, the higher the voltage, the lower the current. The lower the current, the thinner the wire you can get away with. Power over ethernet uses very thin wire, so you want high voltage and low current. Power transmission lines use very high voltage because they need (comparatively) low current through the wires. The higher the voltage, the more power you can put through the same wire. To a point. As voltage gets higher, it also gets more dangerous, and needs a bit more insulation. It's very hard to hurt somebody with 12 volts. And 48 volts, while not quite as safe, is still pretty safe. It's been chosen as a voltage that mixes the right combination of safety and power. The higher the voltage, the more heat you can generate if you have the current behind it. (If you are current limited or fuse/breaker protected you are just as safe from fire if things are calibrated right.) In the past, we often drove things with batteries, or wanted to sometimes. Getting 48v with batteries takes a lot of cells with most technologies. Phone central offices had big banks of batteries -- no problem. Today, with advanced switched-mode power supply technology, we can turn just about any voltage into any voltage. So we don't care as much about being able to run on batteries as low voltage, though it's still nice in portable tech. And of course the chips all run on very low voltages today (TTL was 5 volts and it's getting rarer) and they want to be low power. Most of the PoE phones that take 48 volts are converting it down to lower voltages to use. But 48 is a good voltage to be sending on the wires. The USA uses 120v for house current. That's enough to hurt you and can kill you if you touch it wrong, though I've touched it a few times. A lot of the world uses 220. This causes enough of a spark that they require all receptacles to have a switch on them so you don't plug things in live. On the other hand, 220 can deliver twice the power in the same current. Kettles in the 220 world are _really_ fast. Your dryer and oven run on 220 even in the 110 world, only way to get enough power. Same with electric car chargers. _______________________________________________ --Bandwidth and Colocation provided by Easynews.com -- asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
