I have a couple different leds that I want to place with. I do not yet have drivers for all of them and was wanting to possibly convert a desk lamp or 2 into led
What should I use for a driver ?
How much voltage can the Led’s see ? say if I had a 5v dc 2amp power supply could I hook that up direct drive ?
or do I need to get it down closer to the vf of the led ?
have an xml, xre, maybe an mce to use. My small area in the garage has a 2’ flourescent above me mounted under a shelf but I find it is always blinding me and kinda a sucky light anyhow gives me headaches.
Would I be better off getting a proper driver and then feeding that with a wall wart dc power supply first ?
due they make 120v ac drivers ?
Thanks
you have a whole bunch of options.
the cheap and cheerful would be to scrounge a USB wallwart and just direct drive the LED. The LED pulls the supply voltage down and it ends up at a somewhat reasonable current output (800mA for a Nichia 219 I think). Something like the XR-E would work a treat as it has a relatively high Vf. Bolt it to an old CPU heatsink and you’re done 
I’ve been meaning to do that with a WW XR-E I have spare, but I haven’t had time to machine the heatsink for it.
slightly less cheap but equally cheerful would be to buy a simple 12V 1 LED driver from Fasttech for a couple of bucks (MR16 fitment I think), then scrounge a 12V wallwart - I found one in Goodwill for $2 - and you’re good. You can hunt around for different current level drivers, but anything between 500mA and 1A output should be more than enough.
not cheap but much more sophisticated - anything Texaspyro does with home lighting. He’s done a couple of work/ desk lamp retrofits, with all kinds of whizzbang wizardry that looks very nice but requires these odd things called money and knowledge
would the excess voltage go away as heat?
millerman, I’ve used old cell phone chargers to power single LEDs (or two in parallel for the older LEDs). The unloaded voltage of one charger was just over 8V but, the max current provided by the charger is 800mA so in the end, the LED gets just around 800mA or less. I find even the ‘350mA’ nokia chargers I have power single XPE LEDs very well. If you want different modes, hook up a Nanjg driver. You can also buy an AC-DC converter like this one DX has other options.
not as far as I’m aware. The reports I’ve seen on them use the standard 5V 500mA wallwart. Hooking an LED up to one pulls the voltage down to that of the LED and raises the current proportionally, usually to around 800mA, depending on the LED. As I said, i haven’t had a chance to do it myself yet, but plenty of other people have done it without problems.
What you guys are doing with these 5V etc. weak powersources and direct driving an emitter is overloading the powersupply and forcing it to drop the voltage. If the voltage is too high when starting it can and will blow up the emitter before it drops down. Plus overloading a powersource is not a good idea.
Just use a buck, limit output current.
sure, a buck driver would be better, but better and more knowledgeable minds than mine have done this and are using lights built like this with with no problems. There’s even a guy who runs a machine shop and designs his own drivers who makes work lights like this for his own use.
Anyway, if this is done as a cheap no-cost mod and it doesn’t work, what have you lost? Make sure you have a fire extinguisher handy (as all garages should) and you’ll be fine
Edit: and if that gets anyones’ knickers in a twist, just slap an appropriately large resistor between the wallwart and LED to drop the voltage to LED Vf.
A 5 VDC regulated power supply is very nearly an ideal voltage source but LEDs want to be driven by a current source. That is, as the LED voltage varies the current through the LED should stay constant because LED light output is better predicted by LED current than by LED voltage.
A resistor in series with a voltage source that gives you the recommended LED current can be used to approximate a current source. The internal resistance of these wall adaptor power supplies may or may not be the right value for your LED.
The higher the voltage and the higher the resistance, the better is the current source approximation. 120VAC passed through a diode will give you about 50 VDC but depending on the LED current, the series resistor needed to make this into a decent current source may need to dissipate many watts. You can use a series inductor or capacitor instead but the necessary value is more difficult to compute.
If you post the LED datasheet some supply voltage options & resistor options can be figured out.
They are new to me and I want to use one to figure out how they work. I know 4 dies inside, and they can the be ran all in series all in paralell. Series would be better for not burning it up incase one die fails ? In series the vf would be 4x 3.5 roughly or 14v ? versus all in paralell would be 3.5v ?
Or maybe an ssc p7 with a vf of about 3.5f at 1400ma.
Obviously I have more reading to do.
So if I ran the P7 at 1400ma with a 5volt ps would I get
R=Vs-Vf/I
(5-3.5)/1.4
= roughly a 1ohm resistor ?
okay gonna go read more
In this post it is mentioned that they are using a 3v 4A powersupply to run an MCE - why did that work ?
Wiring in series would assure that all see the same current. In parallel the current through each die is anyone’s guess.
Yes, ~one ohm at ~one watt so choose a 2W resistor to allow for derating.
You may need to tweak the value so if you have some #30 wire wrap wire, 10’ equals about one ohm and you can cut it to length to adjust the resistance.
Yes. Why did that work?
The voltage from the transformer was measured @3.21V. Although I didnt measure current to the LED, total power consumption was 3.7 watts through the 120VAC transformer.
I upgraded the long thin wires that came with the stock MCE to 20 gauge and the wattage measurement jumped from 3.7 to 7.6.
A cell phone charger is an independent current source current source until it maxes out at 4.2V, so there’s zero risk of blowing an LED. The charger will supply the max current it would provide during the CC charge phase of a Li-ion cell, and do so at the forward voltage of the emitter.
See texaspyro’s post below
No, many cell phone chargers are 5.0V (USB voltage compatible). There is a circuit in the phone that uses that voltage to charge the battery. What keeps the 5.0V charger from cooking the LED is that it gets overloaded (which can burn it up) and cannot supply enough current.
I stand corrected—I always assumed that the CC-CV circuit was integral with the charger. Thanks!
The transformer FlashPilot used with his MC-E was 3V@4A(actually measured at 3.21V) I have a few 5V@2A USB chargers but I’m assuming that’s still not enough current for the MCE?
Transformer with say 3V output, rectified with rectifier bridge and a capacitor puts out 4.24V, then an LM350 wired in CC mode set to 3A.
That won’t work… the LM350 has a dropout voltage of over 2.0V (the input voltage must be at least 2V higher than the output voltage). For 4.2V out (at 3A) you would need at least 6.7V in.