Mercury cells were banned some years ago due to environmental concerns. Many cameras and other low power electronic items relied on the constant voltage of these cells during discharge. The only chemistry with a similar voltage and flat discharge curve is the Zinc Air chemistry which is nominally 1.4V.. Zinc air unfortunately has a limited lifetime of typically 60 days once the cell is activated by allowing air into the cell.
Silver oxide chemistry has similar flat discharge characteristics to the mercury cell, but at the higher voltage of 1.55V. In many circuits this voltage leads to significant error, particularly in camera exposure meters. To work correctly the voltage must be reduced to the correct 1.35V that the circuits are designed to use.
The required characteristics for such a circuit are:
1. Constant output voltage with varying current drain
2. Very low power consumption
3. Small size
4. Low temperature coefficient of voltage
A commonly used solution in a camera is to insert a schottky diode in series with the cell. This provides an exact solution at one current drain and temperature, but does not satisfy 1) and 2) well, errors of 1/2 to 2/3 stop build up across the brightness range.
This circuit is a very low power consumption active potential divider. The operational amplifier buffers a simple resistive potential divider that reduces 1.55V to 1.35V. Output voltage is essentially constant up to an output current of approx 3mA.
Quiescent current drain is less than 2uA which means that the circuit can be used in an "always on" arrangement without significantly affecting battery life.
This design is an adapter into which an SR44W cell is fitted. It is designed to fit into the battery compartment of an Olympus OM1(n) camera in place of the original PX625 cell. The circuitry is fitted to a small sub PCB mounted to the circular contact PCB. he cell is retained by phosphor bronze wire or strip contacts.
It is important to use a high quality Silver Oxide cell such as a Renata as some cells have a poor discharge curve.