Replacing Mercury Cells

Ever heard the phrase "Mad as a hatter"? It originates from the dementia which arises from exposure to mercuric organo-metallic compounds. Leather was "tanned" using mercury for centuries and the results were catastrophic for tanners.

Organic mercuric compounds interfere with development, brain function, etc. They can reside in the fatty tissues of the body for a very long time. If you have the opportunity to see W. Eugene Smith's photographic series entitled "Minamata" (Life Magazine, 1967) the devastation which these compounds can causes shown in all its tragic consequences. In this case, mercury discharge from electrolytic diaphragms used to separate salt into sodium and chlorine polluted the water source for the community of Minamata in Japan. Common sources of mercury in the environment include building wastes such as mercury switches and fluorescent lamps, burning coal and, of course, batteries.

The point of this article is not to debate the merits of controlling mercury pollution. Strict regulations are in effect and the sale of mercury batteries is prohibited in most of the world. While there are replacements, these tend to be expensive. Miniature integrated circuit regulators have become available which can take the 3.1 Volts from two LP66 cells and convert it to a stable 2.70 volts.

	My wife had given me a Gossen N100 meter as an anniversary present almost 30 years ago. The meter sat in a drawer since the PX625 Mercury Cells were no longer available in the United States. While wandering through the Digikey catalog one day I came upon a series of surface mount linear voltage regulators from Toko. The TC1015-2.7VCT13 takes 3.1 volts from a pair of photo batteries, reduces and regulates the voltage down to the 2.700 volts which would have been supplied by the PX625 mercury cells. Editor's Note -- the TC1015 is now manufactured by Microchip Technologies and the part is available from Digikey and Mouser. The regulator is inserted between the switch and meter movement. No current flows through the regulator unless the switch is on. When the switch is depressed 2.700 volts flows to the meter and the dial is rotated until the needle centers. Working with such small devices is a bit challenging, however. You will need a fine-tipped soldering pencil (I use a 12 watt Weller) and fine wire, such as the "wire-wrap" wire sold at Radio Shack. In order to solder the device and capacitor I mounted both upside down on a drop of epoxy which had been placed on a small piece of paper, making sure that the terminals were not inadvertently insulated with the resin. When the resin had dried I taped the wires into position so that each of the 3 leads were in contact with their respective terminals. I then epoxied the wires onto the paper. The wires were later soldered for a good fit and the entire assembly covered in epoxy resin (it only takes a drop.
	As seen to the left, the regulator is about the size of the "M" in a dime. Given its small size, the device can be placed easily inside the body of the Gossen meter. Its small size would also allow it to be placed within the body of a camera or other such device.
	This photo shows the small epoxy blob placed within the body of the Gossen meter.
	I've been asked if other battery modifications are available. While I haven't used the Linear Technology LT3020 devices these ultra-low dropout regulators could also be employed.