Dear List,
Read on only if you are a technocrat.
There seem to be a lot of questions on Batteries and Mercury battery
replacements , diodes etc on the list lately.
I have just been looking at alternatives for modifying an OM1N for non-Hg
battery use, so some of the questions on the list are ones I have been
investigating. At the risk of clogging up the list with more techno-cratic
information here are some comments and web resources.
BATTERY VOLTAGES:
The '625 type mercury cells used in the OM1 (N ) have a very constant 1.35V
voltage and a bit higher capacity (mAh) than the silver oxide cells used in
later OM's. The '76 or 357 silver oxide cells are often referred to as 1.5V
cells (on this list) but actually have a specified voltage of 1.55V and
typically have a slightly higher open circuit voltage especially when new
(some types are closer to 1.6V) The adapter needs to drop around 0.2V at
"typical" meter current drain. The voltage of the silver oxide cells is not
quite as constant as that of mercury cells. Using a schottky diode and Silver
oxide cell will not work as well as a mercury battery particularly over a
wide temperature range and it will add a small nonlinearity to the metering
system. Luckily film sensitivity variation is probably a bigger variable and
the accuracy required for photography compared to most electronic measurement
applications is very low. I have calculated and measured some voltage errors
for the measured meter currents in the section on current (below).
Unfortunately I have not converted this into fractional stops of linearity
error which would require some more tests.
OM1 (N) CURRENT CONSUMPTION
Checking an OM1N with unknown heritage (but it appears to be
functioning normally) I measured the following currents (with meter switched
on!):
Bright flaslight straight into 28mm lens very approximately 0.25mA
Covered lens and covered eyepiece very approximately 0.007mA
The only OM1 I currently have is a parts unit and has a non-functioning meter
circuit. Has anybody else measured an OM1 ??
Note : Mount a lens when checking the current.
>From the curves for the low power shottky diodes referenced below we expect a
voltage change of about 60-80mV for the measured range of currents. Since
the currents are probably over a narrower range for realistic EV values the
voltage variations will be less in actual application. I did a quick check
and diode voltage varied by about that amount for the camera conditions
tested above.
NOMINALLY COMPATIBLE SILVER OXIDE CELLS:
(Note: cell capacity in mAh, is the product of current drain times time. That
is a 180mAh battery would power my OM1N with a bright light shining straight
into the lens (0.25mA) for 180/0.25=720 hours, approximately or with the case
closed in a dark cupboard for about 180/0.01=1800+ hours.
Duracell (in IEC standard SR44, ANSI 1131SO etc):
http://www.duracell.com/OEM/index.html
model D303 / 357 165 mAh @1 kOhm load
model MS76 180 mAh @1.5 kOhm load
Eveready:
http://data.energizer.com/
model 303 175mAh @6.5kOhm load
model 357 175mAh @6.5kOhm load
model EPX76 200mAh @6.5kOhm load
model S76E 196mAh @6.5kOhm load
(test conditions may account for capacity differences between brands)
(note '76 cells have slightly higher ~1.6V open circuit voltage and a more
sloping end discharge voltage curve (bad!) ) Note Although the '357 battery
seems to get the nod for use with OM cameras (Camtech and list) the '76 cells
may be better at low temperature since they seem to be aimed at higher
current applications and have a lower internal resistance. The downside again
for the '76 cells is a more sloping voltage-discharge curve and a slightly
higher initial voltage.)
Note there are actually two types of Silver -Zinc ("Silver Oxide ")
batteries:
The higher current batteries usually use Potassium hydroxide for electrolyte
and work better at really low temperatures.
The lower current types use Sodium Hydroxide but have better shelf life and
may be less likely to leak. (see Eveready Website tech documents for
excellent discussion)
ARTICLES
Articles referenced by Duracell on battery alternative replacements etc. (I
have not read these)
Popular Photography®.
Oct 1993, page 130
January 1994, page 144
September 1994, page 144
August 1995, page 66
(follow link on Eveready site: titled Cross Reference for brief Mercury cell
discussion)
SUITABLE (?) LOW POWER SCHOTTKY DIODES FROM HP:
http://www.hp.com/HP-COMP/rf/1n5711.html
(1N5711, 1N5712, 5082-2300 Series, 5082-2800 Series, 5082-2900 )
The 1N5712 or 5082-2810 look best for very low current meter applications
<60uA.
The 5082-2900 and 5800-2300 look better at higher currents up to about 0.2
mA.
A list member suggested possibly using a high power Schottky diode like a
1N5819/1N5818 etc These will have a bit lower forward voltage drop (good)
than the HP diodes but are likely to be completely unsuitable for this
application as they have very large leakage currents which may make operation
erratic especially at high temperature. If you must use a device such as this
choose the highest rated voltage device (1N5819) as it will tend to have
lower leakage than the lower voltage devices.
INSTALLING A SCHOTTKY DIODE
Installing a low power schottky diode requires modest care in handling as
they are small geometry low current low voltage devices and susceptible to
electrostatic damage. If concerned just check the camera side voltage (with
OM switched on ) after installing the diode (2.3-2.35V) if you are worried
you have damaged the diode. The diode needs to be connected in series with
the battery negative lead either before or after the meter on/off switch.
(The battery positive lead goes to the case and is harder to intercept!) The
diode should be installed with the marked bar side (anode) going to the lead
connected to the negative of the battery. Use only an electrical rosin cored
solder. NEVER use plumbing solder and NEVER use acid flux.
MR9 BATTERY ADAPTERS FROM CRISCAM ($30):
I have not used these devices but they are apparently just a schottky diode
in a hollow battery case that takes a (recommended by criscam) '76 silver
oxide cell (or a '357). These devices are rated well on this list : Great
convenience, but this seems pricy for a 50 cent diode in metal can! But
considering what members are willing to pay for lenses?maybe a bargian!
http://www.criscam.com/cris2i.htm
I am still considering putting in something more sophisticated than a
schottky diode to modify my OM1N, but anything else will increase current
consumption so it is a trade-off with accuracy. One very simple modification
is to add a resistor on the meter side of the diode to increase minimum
current consumption a little which reduces diode forward voltage variation
with light level. If you do this you must put it on the off side of the meter
switch else it will kill the battery while the camera is in storage.
Good Luck in your Modifications. Don't blame me if anything goes wrong.
Tim Hughes
Hi100@xxxxxxx
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