Desk Lamp Emitter Required, HELP!

[konadawg]

I have an LED desk type lamp - one of those things with a flexy adjustable stem and a clip at the end of it.

To cut a long story short, it appears that the emitter is damaged. I took it apart and there is still approx 2V DC going to the emitter base (sorry not sure what the correct term may be) but the actual emitter only works if, after switching on, I fiddle around with pressing it directly.

Could anyone advise what part I should buy to replace this?

The circuitry is separate, and apparently fully functional (as per the above testing showing 2V) the base and emitter have no electronic parts attached.

A base that is a nominal 20mm in diameter will fit.

Thanks in advance for any advice!

 

[Steve K]

I'm not sure how much help the forum can be, but some detailed photos wouldn't hurt. You'll probably need to host them on a separate site (I use Flickr), and then provide a link or use html functions to insert them into a post.

With the data you've provided, I wouldn't assume that the circuitry is working. 2V isn't enough to make a white LED work; most require 3v at a minimum. 20mm is pretty big for a single LED, so I would expect that it is actually a number of white LEDs connected in series and then covered with phosphor, causing it to look like a large single LED.

Since you have a meter which you can use to measure the voltage, maybe you could take a picture showing how the meter is connected to the circuit when you measure the 2V (approx)?

 

[konadawg]

Hi, thanks for your reply.

I don't have photos but the part I have that is faulty is very similar in appearance to this;

When I press down directly on the actual LED globe (after a bit of judicious pressing) the lamp goes on 100% and I can use it for a long period of time. Therefore I conclude that the circuitry is fine, as there is no way that my pressing could affect it, it's wired to the base and is about an inch away. 20mm is the diameter of the base / star not of the LED.

Voltage tested with a multimeter.

Thanks again.

 

[Steve K]

sounds like it could be a cracked or cold solder joint. Applying a soldering iron and reflowing the solder would seem like a good thing to do. If the solder joints are on the bottom of the LED, this might not be easy to do. Having the LED star attached to a decent sized heatsink could make this hard to do too. You might have to remove the star from the lamp to reflow the solder.

 

[konadawg]

May be worth trying. however there is voltage right up to the tabs that attach to the actual LED.

An update re the voltage - I was measuring when the led was not working i.e. energised but not emitting. This gave me a constantly fluctuating nominal 1.8 - 2v

I tested with the led emitting, after the massage

I am getting a very constant 3.8V.

Electrics / LED knowledge is very basic but this seems odd to me it would this mean that there is more voltage drop when the LED is not working? Just curious really.

Aside and more importantly I take it that I now (probably) need to find a "star" that is rated for 3.8V, or is there something else I don't know?

It seems I can have some fun soldering from just $1 off Ebay!

 

[Steve K]

pictures would help folks understand exactly what you were doing. Perhaps you were measuring the voltage downstream of the bad connection??

3.8V is a bit high for the forward voltage of a modern white LED, but this could be a cheap, crappy LED using old technology. My guess is that you could drop any nominally equivalent LED in its place, assuming that it is the LED that is bad and not a connection. Have you tried reflowing the solder joints?

 

[konadawg]

I measured the voltage across the LED itself, also at the solder joints where the supply wiring is soldered to the star. With the LED not emitting wherever I measured I always got a nominal 2V. Since what I do to get it to work does not involve the wiring to the star I suppose the solder joints are actually fine and the issue is with the LED itself. I have no idea how the actual chip/diode that does the illuminating is "wired up" so to speak but my best guess is that it has come detached from its "connections" at the rear and that is why pressing down on it and wiggling it makes it work.

 

[Julian Holtz]

If you can remove the whole star, I'd buy a new one, like this:

http://www.leddna.com/cree-xml-xm-l2-led-t5-5c3-neutral-white-4000k-4250k/

Choose the 20mm star option.
There are also other color temperature options available in that shop, choose wht you like best.

I assume it sits on a kind of heatsink in the lamp, so attach a bit of thermal paste when mounting the new star.

 

[konadawg]

Hi Julian, I'm pretty sure I'd have to read my books through a welding helmet / mask if I fitted that :laughing:

I ordered a 20mm star off Ebay. Whatever! Will be fun mucking around, nothing to lose everything to gain if it works out.

Yes it is seated on the metal body of the lamp "head" which is very effective as the whole head gets quite warm after some use. There already seems to be thermal paste which I will try to re-use as I don't have any and I don't want to get too deep into this... can't risk becoming a flashaholic, I am already -aholic to many other sports and hobbies!

 

[Julian Holtz]

Oh don't worry, I have almost 40W of LED power in my desk lamp and I like this very much.

The 2-5W of your emitter won't blind you. But I guess any LED would be ok for your application, if you don't fancy a specific light color.

Best of luck with your repair and show us a photo when you are done.

Oh, and while we are at it: When you remove the old star, heat it on a cooking plate to reflow the solder. Maybe you will repair it this way, and if you don't, well it was broken anyway.