> Hi guys. Just a few interesting questions for you here. Hopefully some > of you could tell me the answers! > > 1. Why are wall sockets (in the USA at least) 120 volts? Why not 12? Or > 150? > 2. Why are most batteries 1.5 volts? Why not 1? Most chips I use need > 5v, so why not 5? Or 2.5? > 3. Why do computers use 12v and 5v instead of 15v and 5v, or some other > combination. All this (apart from battery chemistry) is somewhat "IMHO" but should give a guide - Partially the answer is "standards". Someone has to decide to do something a specific way and then most need to agree that's a valid way and get on with life. Modern standards are often enough set by interest groups (often SUPPLIER rather than CONSUMER interest groups at that). But, part of the answer is 'Physics" 2. The terminal voltage of the most common battery chemistry is ABOUT 1.5 volts when new. This is the carbon zinc cell. It's also approximately true of its derivatives the "Alkaline" cell and the rechargeable alkaline. The electropotential between the metals used is such that the MINIMUM battery (a 1 cell battery) you can build has this voltage (some will argue that a battery must contain 2 or more cells but we know better, don't we :-) ). Other chemistries produce different voltages. Lead Acid 2v Lithium Thionyl Chloride (say what?() 3 volts Mercury 1.4 Silver Oxide 1,5v (again) Zinc Air 1,4 Nickel Cadmium 1.2 ish 1. Household voltage needs to be high enough to be able to be distributed efficiently and low enough to not cause too many problems. People dying and heavy insulation are deemed problematical. Some countries use as little as ?90 v, 100 is not unknown, 110, 120 common enough. Above that they tend to jump to double at 200/220/230/240. Power loss in wiring is I^2R so if you double the I you get 4 x the power losses and need 4 times as heavy conductors to take it any distance for the same percentage losses. More V makes sense to a distribution engineer.. Much under 100 and the currents get largish for real world tasks. Power = V x I. For a 2 kW heater you need 20 odd amps at 100V. Wiring is getting heavy and expensive. Fewer people die at 110 than 230V (but some do). Power is usually distributed in 3 phases so the low voltage you see is one leg of three at 120 electrical degrees apart. The phase to phase voltage is also used industrially and commercially and this is 1.414 (square root of 2) higher. 150/root2 - 106 and 110 is next roundest figure so this may have been an incentive. As far as I know 240 VAC is the highest voltage used for home distribution anywhere. 3. 12V is historical and MAY have come from the fact that the lead-acid cell is 2v per cell and 6 cells (std car battery) is 12 volts. Interestingly in ye mid to later olde valve days filaments were 6.3v and less often 12.6v. Dunno why. 5 volts was chosen semi-arbitrarily for the TTL logic families (preceded by RTL & DTL which didn't AFAI dimly R) and this has dominated logic design ever since. Then it was a tradeoff between various design parameters but nowadays many of the then relevant constraints are gone. The 12v was a reasonable bottom level for an op-amp power supply (often +/-12 or +/- 15v spec) and this was picked up as a "better" voltage than 5 volts for motors etc. (eg floppy disk, tape drives etc). For early highish power motors 5 volts was just too low at the currents used. It's about as easy to make a 5v or 12v regulator and the 12v one can handle 2.4 times as much power - current tends to be the limiting factor in low voltage power design. (Some power devices won't handle 12v but they are usually VERY specialist). -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu