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'[OT]: Voltage Questions'
2001\05\20@215809 by Michael Cook

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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.

Just some questions to ponder, thx.

-Michael Cook

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2001\05\20@231404 by Josh Koffman

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I know this is really an answer, but I think a lot of what we take for
granted today was just decided by someone in the past, and then adopted
by others to maintain interoperability. I don't actually know why the
originals were chosen, but I know that if I want the equipment I design
to talk to the majority of the equipment out there, I better use the
same values it uses.

Just a thought.

Josh
.....joshyKILLspamspam@spam@mb.sympatico.ca

Michael Cook wrote:
{Quote hidden}

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2001\05\20@233933 by rottosen

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My guesses follow the questions.


-- Rich


Michael Cook wrote:
>
> 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?

Why not 12 volts? The resistive losses would be very high except for
small lamps and motors.

Why not 150 volts? I don't know the history of how motors and generators
were built at the time but I would guess that they somewhat determined
the voltage chosen. Maybe because it is the same as 10 12-volt
batteries?

Why 220 and 240 volts? I don't know those either.

You didn't ask but... why 25, 50 or 60 Hz for the frequency? I have seen
old motors and they have magnetic gaps that you can insert your fingers
into. The voltages and frequencies are probably related as well.

> 2. Why are most batteries 1.5 volts? Why not 1?

This one is easy. The battery voltage is determined by the chemistry
used in the battery.
Carbon-zinc and alkaline cells are about 1.5 volts. Lithium cells are
about 3 volts. Lead acid cells are about 2 volts.

Most chips I use need
> 5v, so why not 5? Or 2.5?


I think this is because DTL and the derived TTL circuits need a few
volts to work properly. At a much larger voltage they would waste power.
5 volts is a compromise.
Older ECL and RTL used -5.2 volts and 3.6 volts respectivley. (Yes, ECL
is older than TTL).

More and more, both analog and digital are starting to use lower
voltages such as 2.5 volts. Microprocessors use lower voltages to reduce
power and current requirements. They can now -- and must -- work at
these lower voltages because of the finer detail of their process
dimensions.

> 3. Why do computers use 12v and 5v instead of 15v and 5v, or some other
> combination.


They use 5 volts because computers are still based on TTL technology.
(This is changing.)
12 volts is because a lot of circuits originally used 12 volt batteries.
It is amazing how much inertia technology has.


I, as well as Michael, would be interested in others' opinions and
historical background on these questions.  :-)


>
> Just some questions to ponder, thx.
>
> -Michael Cook
>
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2001\05\21@001251 by Alan Beeber

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Michael Cook wrote:

> 2. Why are most batteries 1.5 volts? Why not 1? Most chips I use need
> 5v, so why not 5? Or 2.5?
>

Different battery chemistries produce different voltages. Managnese dioxide
(alkaline) produces a nominal 1.5 V potential per cell, lead-acid likes 2.0
V, NiMH & NiCd puts out 1.2 V, Li-Ion is about 3.6 V. Makes life
interesting when trying to design a range of battery products for a group
of devices.

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2001\05\21@081551 by Bob Ammerman

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----- Original Message -----
From: "Michael Cook" <KILLspamfoobarsoftKILLspamspamFOOBARSOFT.COM>
To: <RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU>
Sent: Sunday, May 20, 2001 5:06 PM
Subject: [OT]: Voltage Questions


> 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?

This is a tradeoff between safety (lower voltage) and efficiency (higher
voltage)

> 2. Why are most batteries 1.5 volts? Why not 1? Most chips I use need
> 5v, so why not 5? Or 2.5?

The voltage generated by a battery is determined by the specific chemical
reaction involved. Different battery chemistries have different voltages.
The old-fashioned Carbon-Zinc was 1.5V. Alkaline was also very close to
1.5V. NiCad and NiMH are actually about 1.2V. Some lithium based chemistries
are about 3V per cell. Lead-acid (like in your car) is a bit over 2V.

> 3. Why do computers use 12v and 5v instead of 15v and 5v, or some other
> combination.

5V because the traditional voltage for logic chips in the early days of
digital logic. 12V is a reasonable choice for driving the motors in disk
drives.

> Just some questions to ponder, thx.
>
> -Michael Cook
>
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>

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