Repair and Upgrade a Magnifying Desk Lamp

This is my first Instructable and it reads more like a blog entry than an instructable, so please forgive me for that. I hope though, that you can be inspired by my project, and hopefully get some ideas for your own.

My elderly mother loves to do craft work, but her eyesight isn't what it used to be, so many years ago I bought her a bright desk lamp that had a magnifying glass built into it.

The light served its purpose for many years, but recently it's fluorescent tube died and I've been unsuccessful in securing a replacement.

I decided to repair it myself and figured I'd upgrade it to LEDs while I was at it.

Step 1: Disassembling the Lamp

The first step was to take apart the old lamp and see how it worked, as well as remove any unnecessary parts.

The globe was a little bit tricky, but with some gentle force and a screwdriver, I managed to take it out and identified what had actually happened, the starter had blown. I could have found an appropriate starter and called it a day (maybe), but I decided to continue on with the upgrade anyway.

After removing the globe I then opened the bottom of the lamp stand and removed the ballast inside, reminding myself that this was an AC lamp and potentially very dangerous even though it was unplugged.

I also removed the wiring that went up to the light socket and set it aside for later use. I decided to keep the original power cable for three reasons.

- It was undamaged

- I couldn't remove the gland no matter how hard I tried anyway

- I didn't want to modify the casing by cutting a hole in it for my planned replacement electronics

Step 2: Installing the LEDs

I bought a cheap solar powered white LED strip light to use as my replacement for the fluorescent tube as well as a reliable 5v 5amp USB power supply.

In my haste, I assumed the LED strip would be powered by 5v like other RGB LED strips I have bought, but it wasn't. In fact, it wasn't even DC, but AC. The solar charger and battery supply contained an inverter and it took me an unreasonably long time to work out that the unit was providing 3v AC and not 5v DC like I expected.

After discovering this, I added an additional LM2596S based DC-DC buck converter I had lying in a drawer to convert the 5v to the required 3v. Not ideal, but it would do the job.

The first thing I did was to cut the LEDs into individual LEDs. Since neither the strip nor the solar power supply had any resistors, I assumed I would be okay to use the LEDs without a resistor but just to be sure I measured everything several times and settled on under powering the LEDs slightly just to be safe.

I know you should never power an LED without a resistor, but personally I don't trust the evidence for why. Supposedly, if the voltage changes slightly, the LEDs can draw excess current and burn out, but I was always taught that in any electronic circuit, the sum of its components will only draw the current it actually uses, so a circuit made up of nothing but LEDs will still draw only what the LEDs use, and not more just because the voltage fluctuates a tiny bit. Maybe I'm wrong, but I've been powering LEDs without resistors by supplying the exact voltage for a few decades and haven't had one blow yet.

Add a resistor if you're worried, but in my own experience, its not necessary.

After determining how to power the LEDs, I then stuck them on a scrap of aluminium roof flashing. This probably wasn't necessary, and I might have got a better result if I had added a wooden or similar disk to attach them to instead, which I could then screw in place of the hooks that held the globe in, but its what I did and it ended up working fine.

Once I had aligned all the LEDs in an approximate spacing, I pressed them down hard so the double sided tape would stick them firmly and then used a multimeter to check for any short circuits between the copper the LEDs were soldered to, and the aluminium I had stuck them to, and found a single short, which I fixed before continuing.

After that, I cut some copper electrical cable and resoldered all the connections, making sure to leave the last LED not connected.

Using a salvaged USB cable which had 24AWG copper wires, I cut the end off and attached that to the start of the LED string I had made, and fed the other end down the lamp's shaft.

The light was then reassembled and the clear cover screwed back on.

Step 3: Powering the LEDs

I decided to put the power supply I had chosen, in the same place the ballast I removed was. I had to drll two holes in the PCB, so after carefully checking there were no internal wires and everything aligned well, I made my holes and screwed it into place.

After checking that everything fit and still worked I removed the USB power supply and soldered the blue wire from the power cable to the supply side. I cut the cable I set aside earlier and soldered it to the other power input. this was so I could still use the switch the lamp had.

There is a bridge rectifier on the PCB, so it doesn't really matter which way around the wires are soldered.

The power supply was then screwed into place again and the switch reconnected.

Using the remains of the USB cable, I attached the LM2596S buck converter and hot glued it in place. I also hot glued the variable resistor just in case it somehow managed to be adjusted. It would probably be a better decision to replace the varistor with a resistor, but this was all I had.

Step 4: Testing the Light

Once I was satisfied all my wires were safely secured and the electronics couldn't move, I reassembled the lamp and turned it on.

The lamp lit up once more and my mother can return to her craft work.

Thanks for reading. I know this was less informative than it could have been, but I posted it as encouragement and hopefully inspiration for your own broken, neglected lamps.