Desklamp conversion to 12v DC 20w power supply

Picked up a halogen desk lamp from a street hard rubbish pile the other day. This particular desk lamp model has been a popular product for many years and appears to be still available. As I stood on what Melburnians strangely refer to as a ‘nature strip’, starting at someone else’s pile of junk, I faced a familiar dilemma– take it and do something with it, or walk away.

The weight of the base suggested an old fashioned power transformer (you know, with steel laminations and a primary and secondary of enameled copper wire) inside for the 12v 20w halogen globe in the shade. The dome shaped base had a nice design edge about it. The plastic was translucent blue, the kind that would be unmissable to any self respecting bower bird.  It was in clean condition. It had a nice rocker mains switch. It evidenced reasonable build quality. It could be made to work again. Or the transformer, base, switch, power cable could be liberated. Decision made.

Back at home, separating the dome and base plate revealed a 12v 1.67 amp transformer. The secondary connects directly to the two telescopic arms which convey 12v AC up to the bulb. A multimeter measured 12VAC at the base, but as I expected, extending the arms made the voltage bounce around, settling at 3.4V at half extension, then back up to 12V at full extension. The globe was missing so I couldn’t test it. Perhaps the bulb would have flickered on and off, and may still be unreliable at full extension, which would explain its dumping.

I can see why the idea of conveying power up two telescopic arms would appeal to a product designer. It’s a designerly concept — the arms fulfill multiple purposes, physical support of the bulb, extension, and delivering power without wires. The design however is flawed for two reasons.

Firstly the telescopic pieces make unreliable electrical conductors as my test showed. And secondly it presents a shorting hazard. A small sticker on the bottom advises you to keep metal objects away from the arms. If a steel ruler or scissors, both likely to be nearby to a desk lamp, fell across the arms, shorting a 12V 1.6A secondary could make quite a bang. There are no external fuses, but a 130 degree thermal fuse built into the transformer, which would open the primary if the short persisted for long enough.

What to do with it?

Given a new one could be bought for $30 it had no commercial or collectible value. Being used, I suspected the electrical connection may be intermittent which would quickly get annoying. But the 20 watt power transformer and dome-shaped base was too nice to ditch.

I considered removing the arms and putting two banana plug sockets in their place, with a 12V DC power rectifier and regulator inside — a simple utility linear power supply for homebrew projects. Or the halogen bulb could be left in place so the unit would function as a desk lamp, DC power supply or both at the same time, subject to current draw limits. Another option would be to replace the halogen with a LED component and power it from the 12VDC source.

Building in a compact linear rectifier and 12V DC regulator seemed like a good idea. I started by sketching out a small rectifier and regulator on a board shaped to sit on top of the transformer under the dome. Only the DC power socket (a 3.5mm power socket) would need to protrude thru the dome. A LED mounted on the PCB would be visible through the translucent blue dome as a DC power indicator. A series switch in the 12VAC up the arms to the bulb would kill the lamp when a heavier 12VDC load was plugged in.

I chose a simple circuit with a 7812 setting 12VDC on the base of a 2N3055 emitter follower, with current/short circuit protection via an inline 2A fast blow fuse.

The only difficulty would be heat-sinking the TO3 power transistor in the limited space, which I did with a piece of aluminium cut to fit. Not a very efficient heatsink but better than nothing. The small board was cut to sit under the dome, in the space on top of the transformer. The toggle switch and 3.1mm DC jacked drilled for and mounted.

A 12V 20 watt supply (1.7 amps) is enough for many homebrew projects, radios, and QRP transmitters (which in my modes of operation have low duty cycles). And perfect for powering my Arduino GPS Clock. At an amp or so the 2N3055 is loafing. I thought of cutting a circular hole in the dome above the 2N3055 for ventilation and to allow a finger to be be poked thru to feel the transistor case temperature. But aesthetics got the better of me.

Repurposing other people’s junk brings a simple kind of joy to the maker or radio homebrewer. It’s kind of functional form of ‘found object art’. It’s not so much that you’ve got something for little or no cost, it’s more the creative challenge, the promise in a rejected object that few people can see.

What’s next? Who knows, the neighborhood’s nature strips hold the answer to that question.

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