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[OM] Batteries/readylights, projector bulbs, meaning of life

Subject: [OM] Batteries/readylights, projector bulbs, meaning of life
From: *- DORIS FANG -* <sfsttj@xxxxxxxxxxxxxxxxxxx>
Date: Mon, 28 Jun 1999 10:45:14 -0400 (EDT)

     [Forwarded from an anonymous source]
                                     *= Doris Fang =*

BATTERIES

Until the early ?60s, the only commonly available single-use batteries were
carbon-zinc cells. They were cheap to manufacture and performed badly. Their
voltage began to decline from almost the moment you began using them, and they
were notorious for leaking acid.

Portable radios of that era (there was virtually no other kind of portable
electronic device) were designed to take into account the poor performance of
carbon-zinc cells. For example, a radio using four cells?which produce about 6
volts when the cells are new?was engineered to give reasonably good performance
when the voltage dropped to 4 volts. (One volt was, and still is, generally
considered the cutoff point for useful cell life.) In this way the radio would
work reasonably well over the life of the battery.

As for rechargeable batteries, the only consumer rechargeable (besides the
lead-acid battery in your car) was nickel-cadmium cells (invented by Thomas
Edison, by the way). They had their own share of problems. One of the worst,
oddly, was that unlike carbon-zinc cells, their voltage was very nearly
_constant_ throughout the discharge.

Why should that be a problem? Nicads are 1.25 volt cells, so you could directly
replace carbon-zinc cells with nicads and get good performance -- radios were
designed to work at only 1 volt per cell, and the nicads reliably delivered 1.25
volts.

The problem occurred because no two cells ever have exactly the same capacity.
As a group of rechargeable batteries ages, the differences in capacity among
them is only likely to get larger. And this causes problems.

Imagine a radio using six nicad cells. Freshly charged, they deliver 7.5 volts
to a device that can perform satisfactorily on only 6 volts. Now imagine that
one of the cells ages rapidly and has much lower capacity than the other cells.
For the first few minutes of operation it?s at 1.25 volts, but then it starts to
decline. Because nicads have a very flat discharge curve, followed by a very
abrupt decline, it might only be another few minutes before the cell is at 1
volt, then 0.6 volt, then 0.25 volt, then? zero.

The user doesn?t notice anything, because the total voltage is still 6.25 volts.
But the cell that?s hit zero is now being _reverse-charged_ by the other cells.
This is Not A Good Idea. It can permanently damage the cell. In fact, the cell
can explode the next time it?s recharged.

This tendency toward cell reversal is also exacerbated by another problem with
nicads -- "memory." If you use the battery for 15 minutes, then recharge it, it
will soon start to work only 15 minutes before it needs recharging. So it?s
desirable to fully discharge nicad batteries before recharging them -- except
that a deep discharge increases the chance of one or more cells "reversing." You
can?t win.

Nickel-cadmium cells have another advantage that can sometimes be a liability.
They have very low impedance -- that is, they can pump large amounts of current
for brief periods. That?s why nicads usually give faster recycling.

Unfortunately, many flashes (particularly cheap models) are designed to use the
higher resistance of alkaline cells to limit the current flowing into the
flash?s oscillator. This is why some flash makers warn against using nicads --
the oscillator might overheat and burn out.

There are also flashes -- like the notorious F280 -- that don?t work very well
even at 1.25 volts per cell. I don?t think I?ve ever gotten more than 10 flashes
out of a set of freshly charged nicads. I use alkalines. (You can get Toshiba AA
alkalines at Costco for about 25 cents each, in quantities of 40.) It also seems
likely that Ray-O-Vac Renewal cells won?t work well with the F280, as their
internal resistance supposedly rises rather rapidly with each recharge..

The nicad battery has only a few more years of commercial life. The NiMH cell
will almost certainly replace it, but how long that will take, no man can say.


READY LIGHTS

What does a ready light indicate? Well, that depends on how you define "ready."

Let?s say the flash's capacitor has to be charged to 300 volts for full output.
The earliest electronic flashes simply hung a neon lamp across the capacitor,
with a sufficiently large series resistor to keep the lamp from lighting until
300 volts was reached. (Like all gas-discharge lamps, neon lamps produce no
light below a certain voltage, then abruptly "arc over" and turn on.)

Unfortunately, the voltage at which a neon bulb fires is not consistent. Even if
the circuit were carefully calibrated when the flash left the factory, neon
bulbs age and their threshold gradually changes. (I think it drops, but I don?t
remember.) Even bright light falling on the bulb reduces its threshold.

If your flash uses nothing more than a neon lamp, the best way to tell if full
charge has been reached is to listen to the pitch of the oscillator. When it
stops rising, you?re at (or near) full charge.

A much better system is the use of a monitor circuit. When the capacitor voltage
reaches 300, the monitor cuts off the oscillator. Because the monitor uses a
solid-state voltage comparator that?s much more stable than a neon lamp, the
capacitor will be consistently charged to the desired voltage. Battery life is
also extended, because the oscillator isn?t running all the time.

If you own such a flash, you can hear the monitor working. On my Vitovar 292,
there?s a repeated "peep -- peep -- peep" as the monitor turns the oscillator on
and off.

So? what constitutes "ready"? Do you always need to wait for a "full" recycle
before you can take the next shot.

If you?re on manual (or Super FP), the answer is "yes" -- the capacitor _has_ to
be fully charged for correct exposure. The instruction manual should tell you
how to know when this level has been reached.

Things are different on automatic. If you?re shooting close at a wide aperture,
you can often pop off a half-dozen shots in quick order before the capacitor
discharges to the point where the flash won?t fire and you have to wait for it
to recycle. * But at far distances and narrow apertures, you'll often need to
wait for a full recycle.

To put it another way, the slower the film/smaller the lens opening/greater the
distance, the more time it takes for the oscillator to pump enough charge back
onto the capacitor for the next shot. If you fire too soon, the picture will be
underexposed, even though you?re shooting on automatic.

Some flashes (like the Sunpak 622 Super) have two lights. The "Ready" light
comes on when the capacitor is 800r 90harged. On automatic, you can almost
always fire at this point and get correct exposure. On manual, you _must_ wait
until the "Full" light comes on.

* The first automatic flashes (such as the Honeywell Auto-Strobonar 660) placed
a second flash tube across the capacitor. It was larger than the regular tube
and had a very low impedance when triggered. It effectively shorted out the
capacitor and cut off the main tube.

Unfortunately, the capacitor then had to be fully recharged. The introduction of
thyristor circuitry made it possible to actually _disconnect_ the flash tube
from the capacitor, retaining the unused charge.


SLIDE PROJECTORS AND FANS

The purpose of the fan in a slide projector is to keep the bulb cool enough that
it won?t melt or prematurely burn out. This is particularly necessary for
quartz-halogen bulbs, which run at a higher temperature. However, any lamp can
burn out quickly when confined to such a small space. The fan provides the
needed ventilation.

Quartz-halogen bulbs run at a very high temperature to produce more and whiter
light, and so that a chemical recycling can occur in which tungsten that
evaporates from the filament is redeposited on the filament, rather than on the
glass. This also keeps the bulb clear until nearly the end of the lamp?s life.
The bulb itself is made of fused quartz, to withstand the very high
temperatures. Ordinary glass would melt.

For over 25 years, Kodak has recommended running the fan briefly to cool off the
lamp, then letting the projector cool without moving it. The purpose is to
protect the condenser lenses.

The lamp is made of thin glass and cools off quickly. Its filament is a
relatively small volume of metal and it drops to a "safe" temperature fairly
quickly, too. Unfortunately, the condenser lenses are thick and store a lot of
heat. If you blow cool air over them, the outside will cool much faster than the
inside. This sets up stresses that can contribute to the condenser breaking when
the projector is jarred, or even moved.

By the way, all incandescent lamps have tungsten filaments.


THE RAISON D?ETRE OF THE 40/2 LENS

The OM-1 was Olympus?s attempt to produce an SLR equivalent of the Leica.
Several Leica lenses are collapsible. When shoved into the  camera body, the
camera becomes thin enough to fit into a coat or jacket pocket.

A collapsible lens wouldn?t work very well with an SLR -- you?d smash the
mirror. So Olympus created the 40/2, a very shallow lens.
                       ----------------------


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