Holding Wire with Spool and Fixture

My brother has an old analog light meter designed for running on a mercury coin cell. Due to the environmental hazards of mercury, consumer batteries were switched to formulations based on lithium, zinc, and silver. Unfortunately, these newer chemistries are not entirely compatible, particularly in nominal voltage, voltage stability with load, lifespan, and working temperature range.

For example, silver-oxide batteries have a nominal voltage of 1.55 volts, instead of mercury batteries with 1.35 volts. In an uncompensated analog circuit, this 0.2 volt difference is enough to produce incorrect readings.

Some people use Schottky diodes to drop a little of the excess voltage, but the effect is inconsistent across current load and temperature. Other people have created more complicated circuits, but which take up a lot of space.

I figured that modern voltage regulator technology must be able to address this need. And indeed, Toshiba Semiconductor has a fixed regulator (TCR2EE135LM) with a 1.35 V output and an input voltage range of 1.5 V to 5.5 V. This can convert the voltage of silver oxide and lithium button cells, or a pair of alkaline cells.

Tiny Regulator

The Toshiba TCR2EE regulator is tiny. It requires few additional components (a couple of capacitors), meaning the entire circuit is also tiny.

Tiny chip makes a tiny voltage regulator actual size

Tiny chip makes a tiny voltage regulator (actual size)

Depending on your monitor’s pixel density, the above image is actual size. Let’s magnify it.

Tiny chip makes a tiny voltage regulator

Tiny chip makes a tiny voltage regulator

On the circuit board, there are input holes on the left, chip in the middle, capacitors on the back side, and output holes on the right. The capacitors are 0.1 µF (such as CC0805KRX7R9BB104) for input and 1 µF (such as CC0805KKX7R7BB105) for output.

The ESV-package chip is only 1.6 mm x 1.6 mm. That’s smaller than the thickness of a standard PCB.

Chip is smaller than PCB thickness

Chip is smaller than PCB thickness

The chip is smaller than the break-off tabs that hold boards together from OSHPark.

Spot the chip

Spot the chip

In fact, the entire circuit is smaller than the minimum size (quarter-inch square) required from OSHPark. So, I had to artificially increase the board size and then machine down the PCBs after receiving them from OSHPark.

A set of three boards only cost 35 cents from OSHPark.

Battery regulator from OSHPark

Battery regulator from OSHPark

I felt guilty, so I ordered enough to make it cost at least $1. Even then, I worried that OSHPark would bounce the order because free shipping probably costs more than that. Perhaps I should have added the $89 super swift service to make the order that much more ridiculous.

PCB Layout

The regulator chip only uses 35 microamps. This is insignificant during usage, but will drain a button cell after a couple of months of storage. So, you need to remove the cell or have a power switch. The circuit is small enough to stuff into the device if the device has a power switch. However, I made an alternative PCB layout that includes holes for a power switch for when the circuit needs to be external.

Mercury battery regulator PCB layout

Mercury battery regulator PCB layout

For space savings, the capacitors are soldered on the back of the board. This is not optimal for manufacturing. But, in that case, the capacitors could be smaller packages and incorporated on the front side in a denser layout due to industrial soldering capabilities. For a really advanced manufacturer, I’d like to see this circuit stretched out onto flex cable to wrap around a small coin cell.

Soldering

Yes, my soldering job is ugly, but I did it by hand with a soldering iron. I didn’t have solder paste, stencils, pick-and-place, or a reflow oven. Here’s how I did it:

Soldering the PCB

Soldering the PCB

1. Snap apart the boards

2. Mill the remaining perforations. Sanding or filing would also work. I accidently chipped the surface finish of the board edges by using a milling bit with relatively large flutes. I should have used a narrower bit, routing bit, or deburring bit.

Select the correct size flutes for the material

Select the correct size flutes for the material

3. Tin the pads with a tiny bit of solder. Suck up the solder if necessary (the remaining solder will be enough to tack the chip in place). Attach the chip to the board with adhesive (such as DAP silicone adhesive). Let it dry. Now roughly apply blobs of solder to the pads. Generously apply flux and allow excess solder to adhere to the soldering iron tip.

4. Repeat step three for the capacitors on the back of the board.

5. Add a switch, either directly to the board or wired to a remote switch.

To prevent the wire holes from becoming clogged with excess solder, Kapton tape was applied to the sides during soldering. This was marginally successful because I had difficulty getting the tape to stick.

Kapton tape prevents solder in wire holes

Kapton tape prevents solder in wire holes

Result

In the end, what really matters is whether this works. Indeed!

Regulation works

Regulation works

With a 0.5 mA load at room temperature, the regulator works down to about 1.42 volts. Even so, that’s only enough head room to support a fresh alkaline coin cell. Stick with silver oxide or lithium. I wonder whether someone could create a tiny voltage-doubler circuit (LED charge pump?) followed by the TCR2EE to support the ubiquitous alkaline coin cell.