matt weber wrote:
On Mon, 29 May 2006 11:14:26 -0300, "jtaylor"
wrote:

When I was little(r), almost all small portable radios used 9-volt
batteries. There were some, of course, that used AA's (or even 1 AA; I had
an am portable with 1 AA cell that we snuck into school when
something-or-other REALLY important to schoolboys was going on), but now I
don't have & don't know of any radio that uses 9V batteries.

Why were they needed in the first place? The 9-volt package has a lot more
package and a lot less mWatthrs in it. Was it because of the voltage - did
the existing solid-state tech not work so well at 3V?

Mostly history and technology. The first transistors were germanium
point junction transistors. Germanium transistors has lower charge
carrier mobility than Silicon, and higher voltage losses across the
junction than Silicon. To get significant power, and gain, these
devices needed much higher voltages. 9 Volts also allowed
manufacturers to save some money. The speaker could be connected
directly to the output stage (no transformer), something that is hard
to do with 3 volts and a class B output stage. 9 volts is the norm for
these devices into the 1970's. So almost anything you see from the
1950's and 1960's uses 9 volts.

You are probably closest to the truth, still 9V radios lasted well
beyond the Ge age. I think it was mostly a matter of engineering. Often
when you design low voltage circuits, it takes more transistors to do
the same function as a higher voltage circuit. For instance, a resistor
might be suitable to bias a long tail pair from a 9V source, but at 4V
(the minimum of 4 secondary cells) you would probably have to go for a
transistor current source. The same goes for load transistors replacing
high value resistors. Once transistors became integrated, they became
the cheapest component on a per item basis. A bridged power amp is
cheaper than an transformer based solution.

Two AA cells occupy less volume than a 9V, but the capacity difference
is well in favor of the AA cells. I don't have the number handy, but
the difference is in multiples, not a few percent. An AA alkaline is
about 3AHr using 1.5 to 0.5V operation. Granted, that takes a DC/DC PS
to achieve the full capacity.
http://www.solarbotics.net/library/p...s_batcomp.html
If you trust this source, they have a 9V alkaline cell at 0.595Ahr and
a AA cell at 2.85Ahr. 9*0.595=5.335 versus 3*2.85=8.55, so the ratio is
almost 2:1. That is, 2 AA cells will last twice as long as one 9V cell.
And of course, the AA cells are really cheap.

The only place a 9V cell make much sense is in a circuit where the
operating current is so low that you nearly get the shelf life of the
battery. Maybe a smoke detector is close to that criteria.