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Towards
an improved flash
by
Gil Green
Reproduced
from in focus 57 (Sept..
1996)
The
worst thing that can happen to an underwater flash unit, apart
from gross mechanical damage, is an inleakage of water. If
the inleakage is of fresh water there is some hope of recovery
after careful drying. If salt water is involved the problem
is much more serious. Salt water is an electrolyte, ie it
can conduct electricity by virtue of chlorine (-) and sodium
(+) ions and when dissimilar metals are present an electric
current will flow resulting in an electroplating process which
will wreck a printed circuit board and any components that
are not watertight. The presence of batteries in the circuit
will greatly accelerate the process of destruction. Above
all electronic flash units must not leak.
Where
and how do leaks occur? Almost invariably at demountable joints
and rotating shaft seals, both of which rely upon '0' rings
to exclude water. Demountable joints are regularly broken
and re-made to allow batteries to be changed or removed for
charging and it is after this operation, not always carried
out under ideal conditions, that leaks are most likely to
occur. Rotary shaft seals allow internal switches (ON/ OFF,
TTL, etc) to be operated often under water and there is always
the possibility of failure due to deterioration of '0' - rings
or the lubricant necessary for smooth rotation.
The
writer has modified an Oceanic 2000 flash unit (rebuilt after
flooding but now using a Cobra CM 20 flash circuit) in an
attempt to eliminate these causes of in-leakage. This has
been done by sealing Ni/Cd rechargeable batteries permanently
into the unit.
Charging
the batteries is accomplished by connecting an external charger
to two 1.5 mm diameter stainless steel electrodes sealed through
the body of the unit and connected to the batteries via a
diode which allows charging current to flow but prevents discharge
through the water when submerged. In order to permit external
monitoring of the batteries the diode can be bypassed by an
internally mounted reed switch actuated by a small hand-held
magnet (Note It). ON/ OFF switching is accomplished by a second
internally mounted reed switch actuated by a sliding bar magnet
mounted on the outer casing (see photograph).
It
is important to know something about reed switches. In spite
of their proven reliability when operated within the manufacturer's
specification they do not have a very good reputation in underwater
equipment and the writer believes this may be due to lack
of understanding by designers, as to what takes place during
the first few milliseconds following switching on flash units
(inductive loads) or even torches (resistive loads, Note 2*).
A
reed switch consists of two metal "reeds" of rhodium
plated ferromagnetic material mounted in a cylindrical glass
envelope containing a inert gas (argon) or a reducing gas
(hydrogen) or a mixture of the two. In the presence of a magnetic
field from, say, a small bar magnet the reeds are mutually
attracted thus closing the switch. Compared to mechanical
switches of comparable size, reed switches are poor performers
when it comes to switching large currents. The initial current
surge on an electronic flash is quite likely to exceed five
amps, it needs an oscilloscope to observe it! Because the
load is inductive breaking the circuit can give rise to a
similar problem.
In
the case of the Oceanic the problem was overcome using the
circuit illustrated. The two NPN transistors constitute a
"Darlington pair" and are available on a single
chip (price about 80p). The emitter current of the first transistor
is the base current of thesecond transistor, if the current
gain of each transistor is, say, 100 then the current gain
of the pair will be 100 x 100 = 10,000. The current through
the reed switch is limited by the 2.2 k resistor.
*Note
1 During discharge the onload voltage of a fully charge Ni/Cd
cell fails approximately linearly from 1.25 v to 1.20 v (discharge
at the standard one hour rate) before decreasing rapidly.
For a battery of four cells this represents a drop of 0.2
v in 5 v. It is easy to measure this fall accurately, with
a sensitive voltmeter by "backing offirnost of the voltage
with a stable supply, say, from an alkaline battery source
stabilised by a zener diode. A better alternative is to incorporate
a multiple LED display into the flash housing which would
indicate the state of charge continuously when the unit is
switched on. This would eliminate the need for the reed switch
across the charging diode. Lack of space precluded this being
done with the Oceanic.
*Note
2 The current taken by a torch bulb is dependent on the resistance
of the filament. At the moment of switching on the resistance
is low and the current higher than it is a few hundred milliseconds
later when the torch is at its brightest. Measurements on
a MINI Q40 diving torch showed that the "switch on"
current is three times that of the running current.
Though
the intention is to produce a non-leaking flash it seemed
desirable to fit a sensitive leak detector capable of detecting
traces of water too small to immediately affect operation
of the circuit. Concentric rings of bare wire, mutually insulated
and closely spaced were attached to the inner circumference
of the housing. Initially a sensitive circuit was arranged
to switch an ultrabright LED and a piezo sounder if a conducting
path was established between the rings. Tests revealed the
difficulty of establishing satisfactory acoustic coupling
between the sounder and the body of the unit and the sounder
was replaced with a small electromechanical buzzer. The buzzer
had a much higher current demand which required a further
transistor being added to the sensing circuit.
There
is no reason why most flash units should not be modified to
allow external charging without altering the other switching
arrangements. The work took about half an hour in the case
of a MORRIS AQUA Fill. Substitution of Ni/Cd cells for alkaline
does, however, reduce the number of flashes from 130 to 40
(manufacturer's data). In the case of a simple non - TTL circuit
the substitution would reduce the energy of the flash to about
60% of the original (less than one stop). In the case of the
low power Oceanic this seemed too great a sacrifice but by
using five smaller (2/3 Af size) Ni/Cd's in place of four
alkaline cells the full power was retained.
Quite
apart from the greater reliability of the permanently sealed
unit the writer would like to claim another advantage - that
it is a time-saver. Anyone who has taken part in survey expeditions
knows the pressure on individual members, writing up results,
filling cylinders, cooking, processing films, maintaining
equipment, especially photographic equipment, where an error
can be costly. A flash unit requiring no mechanical maintenance
must surely be an advantage.
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