Caliper Capacitor:
  1. Purpose
  2. Pieces
  3. Soldering

2. Parts for a Capacitor Compatible with a Watch Battery

(article continued from previous page)

To recap, when using an external power source for a device that was designed only for battery operation, the chances of misoperation due to electrical noise may be reduced by replacing the battery with a capacitor.

Most of the inexpensive imported digital calipers use a single alkaline LR44 or silver-oxide SR44 power cell. It’s a 1.5 V (nominal), 0.46-inch diameter by 0.21-inch thick cell with between 145 mA and 170 mA capacity.

Note: The LR44 and SR44 are generally interchangeable with PX76A A76 RPX675 V13GA WS14/D303 303 SR44SW V303 R303/9 A SB-A9 280-08 10L14/D357 357 SR44W V357 R357/17 J 228 SB-B9.

Steps and parts to make a capacitor that can replace a coin cell.

Steps and parts to make a capacitor that can replace a coin cell.

Based on the official dimensions of the LR44 button cell, I created a round PCB (printed circuit board). This board is designed for a surface-mount capacitor. However, one could drill holes and solder a small through-hole capacitor as well.

The small circular base is roughly cut from the PCB using a hacksaw or milling machine. The base is then cut to shape using a rotary table. A copper or brass ring of the same diameter is machined. (Details about these aspects appear later in this article.)

Either a tantalum or multilayer-ceramic capacitor is soldered to the surface. If you use a tantalum or other polarity-sensitive capacitor, make sure the positive end (anode) connects to the outside ring. The inner ring connects through the center hole to the bottom side of the board, which is ground/negative (cathode).

Aside: For the purposes of using this in a digital caliper, I tried a variety of capacitor values. The voltage is not going to exceed 1.7 V. Lower maximum voltages allow for higher capacity capacitors in smaller size packages. In the end, I just took what I had lying around that would fit (4.7 µF to 100 µF). A quick test indicates the imported calipers come with about 1 µF of built-in capacitance, so I’m not sure adding lower values will help much.

The metal ring is then soldered onto the board (instructions appear on the next page).

Finally, the bottom edge is rounded with a Dremel or other grinding tool. This is only necessary in order to fit in some devices.

A metal ring cut from a copper tube with the outer diameter reduced in a lathe.

A metal ring cut from a copper tube with the outer diameter reduced in a lathe.

The outer ring can be made from any tube of conductive material that can be soldered. Brass and copper are the best choices. (Aluminum and stainless steel are difficult to solder, and ordinary steel can rust.)

A 12mm tube is almost the correct diameter. A little fine sanding may be all that is required. Or, if you have a lathe, then a 0.5-inch tube or rod can be turned down to the desired diameter of 0.46 inches.

You can cut the ring free from the tube using tube cutters, a hacksaw, or the lathe cut- off tool. In any case, rough edges can be sanded down or cut flush on a mill. Make sure the piece is equal to or slightly taller than required height of 0.21″ minus the printed circuit board thickness. (For me, that’s 0.21 - 0.06 = 0.15 inches). Any extra height can be sanded down or milled down after the ring is soldered into place.

A plastic plate with four clear bolt holes and one #2-56 threaded center hole secured to a Sherline rotary table.

A plastic plate with four clear bolt holes and one #2-56 threaded center hole secured to a Sherline rotary table.

To cut the PCB round, I made a fixture with a center hole that could be screwed onto a rotary table. I started by designed a template on the computer using the Microsoft Visio drawing program. I taped the template a thick piece of flat scrap plastic and drilled the holes.

The four outer holes attach the plastic plate to the bolt holes on the rotary table, and cause fairly good centering. The center hole is tapped for a #2-56 screw.

Cutting a round PCB on a milling machine using a fixture attached to a rotary table.

Cutting a round PCB on a milling machine using a fixture attached to a rotary table.

A couple of pieces of scrap material are inserted under the PCB pile to prevent the end mill cutter from cutting into the fixture or bolts. A #2-56 is inserted in the center and tightened down to hold the PCBs.

Note: Because only a single center screw secures the PCBs, the rotations on the milling machine may unscrew it. Proceed slowly and carefully. If the PCBs appear to be loose, power off the mill and re-tighten the screw.

As the rotary table is turned, the PCBs are cut into a round shape. Advancing the milling machine into the work will cut a smaller diameter circle. Continue until the desired diameter (0.46 inches) is reached.

Finally, let’s see how the ring is soldered to the circuit board...