Wayne writes:
<< For anyone interested in a battery replacement for a 1.35V mercury
cell, I just tested one of our (Linear Technology) LDO regulators (low
drop out regulator) and it seems to work. The part is the LT1761. (see
www.lineartech.com for the data sheet). For an input I used a 1.5V
source (1.5-1.55V is typical for silver oxide battery) and the
regulated output was 1.35V. The part draws 20uA of quiescent current
and easily handled 300uA load, which is probably more than enough for
an OM-1.
>>
Wayne,
The OM1 draws up to 0.8mA max according to the service manual.
It depends on the particular camera and the maximum aperture so
an F1.2 is worst case in bright conditions. With an F1.4 I measured
somewhere around 0.4-0.5mA when looking at this a long time ago.
You asked about AG cell capacity. The 357's run 165mAH - 185mAH depending
on vendor and his test conditions. The Energiser brand claim the slightly
higher values
but it may be academic.
The way I read the LT1761 spec it does not gaurantee operation at
1.5-1.55V input
although typical devices may work. In particular the part may not work at
lower temperatures
where drop out voltage increases.
Interestingly the original 1.35V Mercury cells drop off to something closer
to 1.3V if
operated continuously at low current for 24hours, so it is always worth
switching off
your OM1 with a mercury battery even though you may not kill the battery
leaving it on,
it reduces accuracy to do so.
The LT1761 guaranteed drop out voltage at 1mA is 150mV at 25C just ok
(1.5V-150=1.35V ) and with typicals running 100mV and actual peak current
less than 1mA this should be fine.
Unfortunatelty this guaranteed spec is with a higher input voltage.
Interestingly like many regulators when you run them on the edge of
drop out the current
consumption increases as they drive the output transistor into saturation
even with no load.
The data sheets seem to show the part could draw up to 45uA instead of 20uA
with no load under these conditions.
This would be like an extreme power hog OM4.
This is also shown in the anomalous current increase at low input voltages
shown in the typical current vs input voltage graphs. This probably makes it
impractical
to run this from an alkaline battery witich has a more variable voltage. It
also means
the battery will die very quickly once the voltage starts to droop at all.
An alternative if you are prepared to open the top cover, is to add a
shunt resistor
to the standard meter to adapt the meter to the higher voltage of a Silver
cell.
This requires absolutely no recalibration, does not affect linearity (like a
diode adapter)
and does not add any standby current so you get very long battery life
something
like 700f a mercury cell. This is my preffered fix.
Unfortunately it is only possible to do this if you remove the top cover
something
most owners should not do unless they have pretty good mechanical/repair
skills.
Regards,
Tim Hughes
>>Hi100@xxxxxxx<<
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