Hi Guys,
I have a somewhat unusual question: I own an old, broken desk light and I was thinking of converting it to LED.
I have a few Cree xpl v5 left and I was wondering, what would be the best way to power it?
The lamp doesnt have much heat sinking and I am not sure, how I would add that, so keep that in mind please.
I also dont need any modes, since it will only be used as a desk light.
Do you have any suggestions for me?
Stay healty
Devryd
Can you provide a picture of the lamp?
It’s a lot easier to mod some things using an inexpensive 12V DC led array so that you can power the lamp with a discarded 12V converter. I used a bunch of the lamps shown below to illuminate Christmas decorations. They come in 6, 9, and 12 SMD 5050 LEDs (white or warm white tint). Prices including shipping to the US is only $1.09 to $1.25. The G4 ceramic bases with about a 10 cm lead were 10 for a dollar from Chinese sellers on eBay. Plenty bright enough for a desk lamp. Since they run on 12V you can also include a dimmer in the circuit. You will not need a heat sink since the plate acts as a sink.
They look great for the job. Do you have any measurements for me?
I will post a foto in about an hour I guess
edit: I managed to take the foto directly, I hope this helps:
Propably a stupid question, but what happens, when I use a amc 7135 with a 5V USB Charger. It should still only provide 350 mA to the LED, shouldnt it?
This looks great @Itinifni.
I took a different approach.
I used a 5 V phone charger and a DC-DC stepdown to get the voltage to 2,90V and drive the LED directly. I know, that this is not good, because I have no current control, but it was pretty easy to do and the heatsink only gets warm to touch after a few minutes.
As heatsink, I used an old chipset heatsink from an old pc, that was lying around in my room.
I used a saw and some hot glue to make room for the heatsink and secure it in place. It looks a little frankenstein but works great.
If the Heatsink gets hot, but I can still touch it, is it too hot for the LED, or is that still ok?
If you can hold it without it burning you and you have a good thermal path it should be fine.
Are you doing the finger burn test after about 20 minutes?
I tested it for about an hour and it never got hotter. I used arctic mx 4 between the led and the heatsink and it sits directly on the heatsink
Sounds like its working fine. Note that if the room temp goes up so will the heatsink temp. Dont know what climate its subject too so thought I would add that.
Air movement would also help to keep it cooler. If your room temp is going to get hotter there you might want to check it again when it does.
Sounds like its running fine but it never hurts to be a little cautious.
My toom Was at about 22°C while I was testing, in summer it might be go up to about 27 or something. Will that make a hugr difference?
But it burns off alllllllll the excess as heat.
Eg, 5V in, LED voltage 3.2V, 7135 headroom 0.1V. 5.0V - 3.2V = 1.8V. 1.8V × 350mA = 0.63W.
From even a freshly charged cell at 4.2V, that’s now 1.0V instead of 1.8V, or 0.35W. Not quite half, but almost.
And the 7135 burns off less and less wattage as the battery voltage drops.
They might not burn out, but they’ll certainly cook twice as hot.
Stick 2 or 3 1N400x diodes in series to drop 1.4V/2.1V. You’ll be distributing the “excess voltage” to the diodes vs all in the 7135.
“In series” as in between charger and driver, to drop some volts. Think of it as super-RPP.
I will keep that in mind for a possible future build, but for now, I will leave the lamp as it is. It works fine, and I dont think I can overheat the LED, the USB charger, I used is pretty weak and doesn`t provide enough power for it. I tried to increase the power at the DC-DC stepdown converte a little bit yesterday, but it made no difference, so I guess the charger is just old and doesnt provide enough amps
The charger should have had a listed output current somewhere. Most of the older type where 500ma.
The voltage the led is runnig at is also directly related to the current. Just like when a battery voltage decrease in direct drive so does the current.
You can’t have a certain current unless you have the correlated voltage.
Heat build up can sway that a little or certain bins have lower vf’s but we use these graphs as a guide for a particular led.
Good mod devryd! Agreed it looks a little “Frankenstein’s monster” like, but that’s what makes it cool. 