11 November 2018

Reviving a ceiling lamp from CFL to LED

We've had at home a ceiling lamp for about 10 years, and for the last 4 years it has misbehaved. It contains two 55W CFL tubes with an external HV power supply by OSRAM (Quicktronic Professional QTP-DL 2x55 GII).

The defect was that it would switch off after about 15 minutes of operation and it would restart only after a rather long while. In any case it was not usable at all.

The original configuration.
I tried replacing the gas-filled elements without success, so the culprit must have been the embedded power supply. Since I am still developing my spider superpowers, working upside down on the ceiling is not my thing so I pulled down the lamp on the table. Still about 15 minutes of operation even without the front glass (in case it was a heat issue).

I took apart the power supply case and looked for the obvious, that obviously wasn't that obvious. I simply reconnected everything with the power supply out of its case and it would run for hours. HA! Maybe a cold solder joint failed with heat cycles over time and moving the circuit around sort-of fixed it.

I took my chances and put everything into the metal case, and still working. So I secured the case back to the lamp and ... 5 seconds of operation!

Enough. Either I got another identical power supply or I had to find something else. My readers probably already guessed which one I picked! "Something else", of course.

The lamp base is metallic and hidden against the ceiling so it can be drilled as needed to accomodate a different setup.

Few searches later I learned that there are LED replacements for neon tubes, and they include the power supply too. I chose two circular replacements whose total equivalent power would get close to the original 110W.

Of course I had a look inside the power supply, that is very simple since it has to light up a series of LEDs:

The LED strip power supply.

And the final look, up on the ceiling before placing the cover glass:

Final config. Notice something "light"?
Now the two light sources are independent, so if one fails the lamp will still work at 50% luminosity. If you look closely at the last picture you will see LEDs are "on", dim, but "on". Even with a single wire attached, the culprit is probably some leak of the long driving wire towards ground. I couldn't get rid of it by swapping the two cables, so I consider it as a night light undocumented feature.

We don't use that lamp very often, so hopefully it will last another 10 years!

06 November 2018

Elka 135, calculator from Bulgaria

Meet the Elka 135 scientific calculator! I bought this marvel in FN 2018 fair. Batteries had leaked inside, so this was the fault to fix (yet again).

There are few pages online that describe this calculator made in Bulgaria. I like the 8-digit LED display. It can be powered from an external 6V DC source through a 3,5mm audio jack (!!).

Using a variable voltage power supply I could confirm that it works down to 4.5V and a small "L" appears in the leftmost position: this means I can use this calculator with USB power, without fully restoring the battery contacts.

Few keys are bouncy resulting in double-triple press, but I am not sure whether they were like that in the original, non-damaged calculator.

More complex operations take some time, and the left position in the display keeps updating until the result is ready.

This device is quite power hungry, because of the LED display, but modern batteries (or USB power) can handle it.