LM regulators, high power LEDs, building a good circuit

sgjames

Joined Dec 12, 2012
1
Pre thanks for stopping, reading, and even helping, it's all appreciated!

I've been tinkering with electronics for years, DC always made more sense to me then AC (which I still don't fully understand. LEDs have been driving my curiousity lately, and it's come to a head, or at least i think so. I've come to an 'epiphany', or understanding, that i hope is correct.... Looking to check my facts today, and hope for the best. So building little LED lights with the old school 20mA LEDs was fun, and great for learning. I found sites like http://ledcalculator.net/ for helping to figure out the design and calculations for simple LED circuits. I also happened upon http://www.the12volt.com/ohm/ohmslawcalculators.asp which is a regular tool in my arsenal in both fine electronics and home electrician work.
So from the basic 3mm "old school" LEDs i learned about forward voltage. Which is like saying, "The amount of voltage required to make that thing to light up at it's designed settings."* So for my ancient 3mm RED led, it takes roughly 1.9V to make it shine it's brightest, safely. AND when it's doing that safely, it's going to draw 20mA of current. I also learned that they are tolerant, meaning i could supply them with what ever power source i had, so long as i put in a resistor large enough to handle the max voltage. If i wanted to build a 9v battery model, no sweat, just lower the resistance from the 12v model, or use the same model and expect a reduced output.... If i had a AA battery model and wanted to make it work in the car, well i just 'up-ed' the resistance. SIMPLE, and sorry for boring the experts i'm looking for advice from but that's where's i'm coming from....
Then high power LEDs came around, and boy aren't they cheap now? I started with, and am still working with, 1 watt , while 3, 5, 10 and how ever bigger models they are coming up with exist. But my angle of attack is from the the 1w perspective. I like their minimal wattage (and relatively their low current draw) versus the lumen output. Once i got my hands on some, i made a bunch on screw ups, looking back at it.
One model in front of me, uses one LM350, two 10 ohm 1 watt resistors, and is running four groups parallel of 3 1watt LEDs in series (that ground directly to the power supply). The experts are probably saying "This jerk wad doesn't know that those LM's shouldn't have a program resistor of less then 100 ohms max!" ....little did i know. But I DID know that the new high power LEDs do need some sort of steady, constant power, which my first couple models learned me on They make "buck drivers" but at the time seemed too complicated to make, and too expensive to buy. I found this source, http://www.instructables.com/id/Super-simple-high-power-LED-driver/?ALLSTEPS which is what got me onto the LM317s/350's etc. I learned about LM's through this, and I did use a LM350 on purpose, it's rated for 5A, as opposed to the 1.5A through the 317. More amps means more driver power/ability.* Are these LM drivers still a good option, i mean without splitting hairs, generally, are they a sound, decent option? ("Option for what? boat lighting, or flashlights?" is what the experts might ask... but I'm looking to use these 1w HP LEDs for room lighting, indoors. Nothing fancy, nothing to critical in the design...)
1w leds deliver average of 100 lumens, where as 3w leds deliver 245 lumens. mathematically, i get more lumens per watt, with the 1w high powers, and at a lower cost per lumens as well. (getting in to the light topic for a second, lumens arent everything, there is also candela, but how does the average hobbyist get into that? because the angle of the beam change either or and i only see lumens rated in specifications...)
Basically what I was doing with the LM350 in past projects was to regulate the voltage to exactly what the forward voltage of the LED is rated for. It worked. Can't say for better of for worse, but i did make up a few of them, and to date, i think i've only had on string fail-due to surge i suspect. I thought that high power LEDs are drastically differnt then the old school ones. And that with the new ones, the driver was needed, and that was that. Now i've come to realize that they are the same.* They are both diodes, just rated for more, or larger. I thought the old school LED calculator(i sourced above) was only for them, but I dont think so any more. I think i didnt see it all before. I need to separate the driver from the LED circuit, which is the same designing it as a 20mA or 1A LED circuit.
The driver is just that constant insured power "controller", supplying a safe, steady stream, never fluctuating even though the power source may. And that LEDs are all the same(for my purposes right now anyway. So now i think I've got it, and i'm here for some review.
Here are the parts I am using:
LM350 (even though the design says 317, and the design below can be powered for less then 1.5A)
32mm 3-LED series PCB (D4,3,2 will be place on one PCB, D8,7,and 6 will be on one PCB, etc... each PCB, i refer to as a string)
1W high power bright white LEDs (D1-all of them)
110, 820, and 1.5 ohm resistors (R6, R5, then the R1-4 are the same at 1.5)
0.1, and 1 uF capacitors (C1 which may not be needed, and C2)
and besides some solder, wire, and hardware, that's the electronics of it.

I still haven't figure out the power supply (expert help here, as i'm looking for cheap, heavy duty, efficient, cost effective, a unicorn! but for now it's a 12v wall wort that actually puts out about 13. Learning about the LM's I should have 2-3 volts more then I want the driver to supply-that's been covered in many forums, and wont get into it, bu i learned that too.* With the LM350, and by assuming that the LED calculator, and the LM calculator found here http://diyaudioprojects.com/Technical/Voltage-Regulator/ are correct, I could power upto 42 LEDs, or 14 strings of 3 leds in series!! with one driver!!... may get hot huh?, so I'm shooting for less anyway... Thinking at least 10 strings of 3 in series, total of 30 LEDs, and by the internet's wisdom that should draw 3.5 amps. NOW for some expert guidance, i know about resistors, and adding them in seies, and deviding them in parrell, right? do LEDs do some thing different? becuase just one LED draws 350 mA, and even a string of 3 LEDs in series, and a 1.5 ohm resistor, it only draws 350mA-internet told me so! The resistor needed and the wattage changes, but the circuit still draws the same amps....

I compared these to some LED 4ft T8 tubes i bought from Green light depot. The green light product was not bad at all, and at about half to almost the same price as making my own, that doesnt account for my labor or anything. BUT per foot, my home made ones generate more then 300% (3x's) the lumens/foot then the manufactured ones. In one foot, their 18w 4ft t8 replacement puts out about 450 lumens, where as in one foot, my home made is rated for 3000 lumens! Are my calculations right? Thanks for your help and time!

*these are "understandings" that i've been stringing together, and if they are wrong, PLEASE correct me! i'm basing assumptions off stuff like that

magnet18

Joined Dec 22, 2010
1,227
That is quite the wall of text! You might consider breaking it up and adding headings for separate ideas.

Picking the first thing I saw that looked like you could use some input-
So from the basic 3mm "old school" LEDs i learned about forward voltage. Which is like saying, "The amount of voltage required to make that thing to light up at it's designed settings."* So for my ancient 3mm RED led, it takes roughly 1.9V to make it shine it's brightest, safely. AND when it's doing that safely, it's going to draw 20mA of current.
The forward voltage drop of a diode (LED included) is not the voltage that makes it light up as desired, it's the voltage that the semiconductor junction physically MUST drop. Lower and it won't conduct, higher voltage than that and the energy from the electrons is released as heat when they lose energy to meet the voltage requirement. (that said, LEDs are more flexible about the voltage drop than other diodes, in that they turn on at lower than the max voltages. For manufacturing reasons (I think inconsistent doping densities)) Look into energy band diagrams and the like for more info if you're curious.

The current value is the one that can be adjusted, more current = more electrons losing energy = more photons emitted = brighter.

Last edited:

GopherT

Joined Nov 23, 2012
8,012
That is quite the wall of text! You might consider breaking it up and adding headings for separate ideas.

Picking the first thing I saw that looked like you could use some input-

The forward voltage drop of a diode (LED included) is not the voltage that makes it light up as desired, it's the voltage that the semiconductor junction physically MUST drop. Lower and it won't conduct, higher voltage than that and the energy from the electrons is released as heat when they lose energy to meet the voltage requirement. (that said, LEDs are more flexible about the voltage drop than other diodes, for manufacturing reasons, not physics reasons) Look into energy band diagrams and the like for more info if you're curious.

The current value is the one that can be adjusted, more current = more electrons losing energy = more photons emitted = brighter.

I'll wait for the Cliffs Notes version to come out.

#12

Sensacell

Joined Jun 19, 2012
3,064
Could not bear to wade through all that text, you'll get much better advice with a more reductionist approach.

...but looking at the schematic, I see a voltage regulator directly connected to a transformer secondary, you need a bridge rectifier and capacitor to convert the AC output of the transformer to DC.

ronv

Joined Nov 12, 2008
3,770
Well, you are pretty close, but you need some more data sheet reading.
If you look at the data sheet for the white LED the Vf voltage may have a range like this one, from 3.2 to 3.8 volts.