Old NiCd Charger Resurrected As Slotcar Power Supply

How to re-use an obsolete NiCd Battery Charger

From time to time I cannot refrain from checking the electronics trash container of one of my employers, and it happens that I find things that can be of further use. One day I spotted several different battery charger units, and I didn't have the heart to let them go to the landfill. I'd like to tell you about the upcycling process of one of them.

This unit is a NiCd battery charger made in the early 1980s by 'Titan', a German manufacturer that no longer exists. Rechargeable NiCd batteries fell from grace some time ago (banned by the European Parliament in 2006) since they contain cadmium, a toxic heavy metal, and therefor the charger was trashed by its former user.

The charger featured a large rotary knob operating a mechanical (!) timer that terminated the charging process after the set amount of time. After checking the unit - it worked nicely - and then opening the case by drilling out five brass rivets, I decided to re-purpose the unit as a power supply for an electric H0-scale slotcar set, re-using as many of the parts as possible. A quick internet search showed that these chargers can still be found from time to time at online auctions for next to nothing, so my information might be useful for you, too.

In order to upcycle such a unit, you need some electronics tinkering skills as well as some basic tools, such as

- Soldering iron

- Desoldering pump or desoldering wick

- Screwdrivers

- Flat-nose plier

- Drill and drill bits

- Multimeter

- etc.

Of course you also should know which end of the soldering iron gets hot.

Target specifications

- Regulated, short-circuit-proof output voltage, adjustable in steps (6 V - 9 V - 12 V - 15 V)

- Output current in excess of 1 A

- Sockets provided for directly plugging both lanes and both speed controllers

- Re-use of as many of the original components as possible

- As far as possible, using only standard components available from my junk box

- Nice new front panel label

After checking what was inside the charger I decided to use an LM317 adjustable voltage regulator IC instead of the current source built around a TO-220 power transistor. The LM317 in the same TO-220 case could be installed at the location of the former output transistor on the PCB and on the unit's heat sink. I provided a four-position rotary switch to set the ouput voltage to 6, 9, 12 and 15 V, mounted at the location of the discarded mechanical timer.

The LM317 is simple to use but needs an input voltage being approx. 3 V higher than the highest desired output voltage for proper regulation. The rectified voltage of the mains transformer was a little bit short of the required 18 V, so I used four Schottky diodes in the bridge rectifier, having a voltage drop of only about 0.6-0.8 V instead of the 1.4-1.6 V drop of a standard silicon rectifier, giving me a benefit of almost 1 V.

An other benefit of using the LM317 is its internal current limiting, preventing the unit from being damaged if the outputs should accidentally be shorted.

In order to re-use the PCB I unsoldered all components from the PCB and cleaned it.

Fitting the components of the new circuit to the existing PCB required some thinking. Of course the copper tracks didn't match the new circuit. So I tried to arrange the new components on the PCB accordingly, which works out ok but looks somewhat strange. All remaining information - circuit diagram, component layout, wiring details of the rotary switch, and the formula for the resistors defining the regulator's output voltage - can be found in the attached PDF document.

The re-used components:

- Enclosure

- Mains transformer

- Mains cable

- Filter capacitor (C1)

- PCB

- Heat sink

- Mounting hardware for the voltage regulator IC1.

The enclosure needed some cleaning and upgrading. I drilled out the ugly, riveted brass sockets and added some more mounting holes for eight 4 mm banana sockets (2 each for both tracks, plus 2 each for both controllers) and an LED 'power on' indicator.

Once everything was installed within the enclosure, correctly wired and tested, I closed the enlosure with some M3 screws, washers and nuts, using the holes where the former rivets had been.

Materials used (re-used components are not listed here):

Some tinned bare wire for the 5 wire bridges on the PCB (marked yellow in the component layout diagram)

R1 - 240 Ω

R2 - 2.7 kΩ

R3 - 9.1 kΩ

R4 - 3.3 kΩ

R5 - 1.3 kΩ

R6 - 10 kΩ

C2 - 100 nF (Polyester or similar)

C3 - 1 uF/25 V (electrolytic)

D1-D4 - 1N5822 (Schottky, 3 A, 40 V)

D5, D6 - 1N4001

LED - 5 mm, 2 mA, red

U1 - LM317 (TO-220)

Rotary Switch with 4 positions + matching knob

8 pcs. 4 mm banana sockets

Rough estimate of all the additional components: less than 25 CHF/EUR/US$.

One more word about the front panel:

When I design a front panel label as a replacement for an existing one, I first scan the old one which gives me information on the dimensions, labeling and any holes or cutouts. I open this scan in a photo editor such as Photoshop and rotate it until it is completely parallel with the drawing layer. I then place/import this bitmap in a vector graphics drawing software, such as Adobe Illustrator or Corel Draw. After locking this layer I can draw contours, trim marks, the new text and any desired graphics elements on different layers, using the original contours, holes etc. as a guideline. Once completed, the original layer can be hidden so that only the new label is displayed.

After converting the file to PDF this is then printed on some self-adhesive material, either paper or printable plastic film. Self-adhesive plastic film is also available in weather-resistant, heavy duty versions and in different colors including 'aluminum-look' silver, for either laser or ink-jet printers. In order to protect the printed label furthermore, I run it with lamination film through my laminator.

Right, I do this before removing the backing sheet - so the upper side of the label and the backing sheet at the bottom are covered with lamination film. Only after cutting the label to the correct dimension I remove the backing sheet, together with its (unused) laminated coat, which is by the way much easier than removing the backing sheet alone. Then I attach the new label to its intended location.

Please note that, unfortunately, not every make of lamination film does permanently adhere to every make of these printable plastic films; some trial-and-error might be necessary.

A picture of my upcycled slot car PSU is shown at the beginning of this instructable. Renato, the name I gave it, means 'born again' in Italian, by the way.

Apologies for not having more pictures - I did this project some time ago and did not think about documenting it better.