Converting a Magnifying Desk Lamp’s Donut Fluorescent to LED Strip – Part I

I’ve had a magnifying desk lamp for years, of which starter plug, and donut fluorescent lamp had died last year. I wanted to replace the lamp with a LED block made for this purpose, but I could not find one anywhere here. So, I decided to make my own. I had a 3m 12V white LED strip (50 x 3 LED strips, merged together) in stock from another project, which I could use for this hack. The lamp had an ID of 150 mm and an OD of 207 mm. All I needed was a circular cardboard piece of the same size, so I could stick the LEDs on it.

Then I had an idea! Why not wrap the LED strip around the old fluorescent lamp? I made some calculations, and realized the LED strip was the perfect size. The diameter of the fluorescent lamp was about 180 mm at the center of the cylinder (28.5 mm thick) forming the donut. So the circumference was about 565 mm. Each 3 LED section of the strip (10 mm wide) perfectly covered one turn around the cylinder. So I needed roughly 565 / 10 = 56 of these 3 LED sections. I had 50 all together, which  was enough 🙂

I started with disassembling the desk lamp, removing the fluorescent lamp and the ballast transformer at the base. I decided to keep the already mounted cables and the switch.

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After unscrewing a couple of screws, the bare-bones of the lamp was ready for the hack.

The plastic part of the fluorescent lamp and the piece holding the starter were crumbling to pieces, probably due to heavy UV exposure from the lamp over time. I replaced the plastic part of the fluorescent lamp with duct tape, and started winding the LED strip around the donut.

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It took me a while to nicely wrap the adhesive backed LED strip around the donut lamp.

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Quarter way through.

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And here’s a nice donut LED lamp with spiraling LEDs all over.

I stabilized the final winding with duct tape as well 🙂 Next, I inserted the leads of the LED strip to the terminal block in the lamp’s head, and and placed the lamp in its grove. Finally I placed the huge magnifier in its place, and screwed the transparent lid back in its place.

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Looking good 🙂

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Here’s the final assembly. Still needs a LED driver, and a part to stabilize the PSU.

I used a 12V 5A switching power supply for this lamp. I still don’t know the current draw (should be about 1500 mA), and I’ll measure it tonight. Eventually, I may get a smaller PSU, so I can fit it better where the ballast transformer was. I also made a simple PWM driver with an N-MOSFET, for driving high powere LEDs. I’ll also use it with this lamp for adjustable lighting. All will be explained in the next part.

Here are some shots of the lamp while it’s on. Although it’s hard to tell from the pictures, I’m rather pleased with the amount of light the LED strip puts out.

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See you in the next part 🙂

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