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-   -   LightCraft Magnifier Lamp LED upgrade (https://www.vintage-radio.net/forum/showthread.php?t=178917)

beery 13th Apr 2021 10:43 pm

LightCraft Magnifier Lamp LED upgrade
 
2 Attachment(s)
Hi all,
When I started my current job 4 1/2 years ago I purchased a LightCraft LC8076 bench magnifier lamp from RS to use on my work bench. I liked it so much that I bought one for use at home, this time from Rapid Electronics.
The lamp at work was on almost every day and it was not long before the tube wore out. What actually seemed to happen is that the electrolytics on the electronic ballast failed causing the tube heaters to remain on until they burnt out, which also discoloured the white plastic of the lamp. I re-capped the ballast, though neither RS or Farnell had 105 degree caps with the same small dimensions as the originals (that may be why they were rubbish). I then got a replacement tube from Farnell (RS didn't stock it) and it has been fine so far.

Now to the lamp at home. Well I should have got round to changing the caps on the ballast before they failed, but I didn't. The result of course is that it failed in the same way as the one at work, with the same discolouring of the plastic moulding. However, this time round I found that the tubes are no longer manufactured. Lightcraft now sells the entire bench lamp as an LED version which is much more expensive than the fluorescent version was. What they don't sell of course is an LED conversion kit, so I had to make one...

I drew up a ring shaped PCB using Altium. It has 8 parallel groups of 7 series connected LEDs. Each lot of 7 LEDs has a 150 ohm current limiting resistor and the whole thing runs at 24V DC 160mA. I made the initial prototype by printing out the component layout (top silk screen) on to an adhesive label and sticking it on to some stiff card. Using a compass point I created the holes for the 56 LEDs. I pushed the component legs through the cardboard, bent them flat and soldered them up. I used 2 rings of tinned copper wire for the power tracks. To power the LEDs I cut off the 2 pin Euro mains plug and connected an external 24V PSU. It all worked well, so far so good, but I was not finished yet...

Cheers
Andy

beery 13th Apr 2021 11:15 pm

Re: LightCraft Magnifier Lamp LED upgrade
 
5 Attachment(s)
Hi all,
Here is the final part to the story...

Having tested the PCB layout in cardboard, I now wanted a proper PCB. I also wanted an inbuilt 24V PSU so that the mains switch on the top of the lamp would actually disconnect the input of the PSU from the mains. This was less to do with saving energy, but more to do with having fewer devices permanently switched on that might be a source of modulation hum...

To make the PCB I used a prototyping PCB routed to make the board out of copper clad SRPB. I realise that if anyone wants to get a PCB manufacturer to make it, then it would be cheaper to make 8 identical curved sections and then link them together to make the whole circle.
Next the PSU. I had a look through the box of things I had rescued from the WEE bin at work and I found one suitable PSU. This turned out to be a 6W constant voltage/constant voltage LED driver. By this I mean that it can deliver 24V at up to 350mA, beyond this point is will deliver a voltage of up to 24V at a constant current of 350mA. Typically these drivers have lots of 100Hz ripple in the output. This is intentionally not smoothed out at the primary side of the PSU in order to keep the power factor close to 1 and in any case, hum on LED lamps is fine for most applications. I don't know the exact pedigree of this LED driver, it would have been purchased for evaluation and then the casing discarded when it was put in the scarp PCB pile. Clearly it hadn't really been used though.
The PSU/driver board is smaller than the original fluorescent ballast and simply sits in the casing in the same way that the original ballast did.
So there we have it, quite a fun little project.

Cheers
Andy

Wendymott 14th Apr 2021 3:56 pm

Re: LightCraft Magnifier Lamp LED upgrade
 
Nice one Andy. Makes my upgrades look a bit shabby... but work...I used strips of Led's on a bandolier.....yours is MUCH Neater.

beery 15th Apr 2021 9:17 am

Re: LightCraft Magnifier Lamp LED upgrade
 
Hi All,
Well there's an embarrassing typo in my first post, I meant to say 250mA, not 350mA. Ohm's law, what's that?

Wendy, I hadn't thought of a bandolier of LEDs, not a ad idea. I've got lots of LED tape, but of course, it can't be bent round in the way required here!

Cheers
Andy

Wendymott 15th Apr 2021 7:44 pm

Re: LightCraft Magnifier Lamp LED upgrade
 
Andy. I glued mine to the inner edges so the tape was adhered to vertical surfaces. I get 3 lots of leds... all in series / parallel.. to a 12v walwart :D


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