Lm317t

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If you have 3 LEDs in series with a 2.2 ohm resistor and the resistor has 0.32V across it Then the current for this string is 0.146A.
You have 4 strings so the total current is 0.58A.

If you are using the 6V battery then the regulator is operating at its dropout limit so it will dropout causing the voltage to drop when the battery voltage drops.
 

Ok I'm confused again. So the current for the entire string is .146A? So each of the LEDs in the string is only getting .048A? So how are they running at pretty much full brightness at only .048A when the current is supposed to be .140A per LED? I know there isn't much of a difference in brightness on the higher side of the current like if you run .120 or .140. Or am I just thinking they are running at full brightness, and can go alot brighter?

So based on that, I don't really need 2 regulators?

So basicaly my question is should the string be at .140? or should it be at .420? .140 per LED in the series?
 
ReallyConfuzed said:
Ok I'm confused again. So the current for the entire string is .146A? So each of the LEDs in the string is only getting .048A? So how are they running at pretty much full brightness at only .048A when the current is supposed to be .140A per LED?
Click to expand...
No.
The 0.146A of current in the string flows through the resistor and all 3 LEDs. There is only one current when parts are in series. So each LED has a current of 0.146A which is a little higher than its max allowed continuous current of 0.14A.

I know there isn't much of a difference in brightness on the higher side of the current like if you run .120 or .140. Or am I just thinking they are running at full brightness, and can go alot brighter?
Click to expand...
Your eyes' response to brightness is logarithmic. So if the current is 10 times then it looks twice as bright. If the current is 1/10th then it looks half as bright. Some LEDs compress their brightness increase at high current.

You will burn out your LEDs and maybe also burn out your eyes if you increase the current.

So basicaly my question is should the string be at .140? or should it be at .420?
Click to expand...
A current of 0.14A is the absolute max allowed for your LEDs. The datasheet will say if that is when it has a heatsink. If the ambient temperature is warm then the current should be reduced.

The current in all parts of a series string is the same. So each string should have a max current of 0.14A.

One regulator will be fine. The input voltage must be high enough for it.
 
So if I run 3 LEDs and 4 strings, a total of 12 LEDs. Should the Amps drawn also equal 2 LEDs and 6 strings? or 1 LED x 12?

but based on what I'm understanding.

3 LEDs with a current of .140 x 4 is .56A

But 2 LEDs with a current of .140 x 6 is .84A?

and if I did 12 single LEDs each would be .140 x 12 = 1.68A?

When I go to switch to the LM317. If I'm lucky I will be able to use 2 LEDs in a string. The Specs on the web for that chip say it needs a extra 3 volts. So based on that I will probably only be able to get about 2.5v - 3v to work with.
Which means each LEDs will be ran in paralle.
 
1 LED with a current-limiting resistor has a current of 0.14A.
1 million LEDs in series and with a current-limiting resistor for 0.14A all draw 0.14A and only 0.14A from the power supply. The current flows from one LED to the next LED to the next LED. The current does not add in a series circuit, the voltage adds.

Each series string draws 0.14A. Two strings draw a total of 0.28A.
Don't you understand??

It is dangerous to connect LEDs in parallel. One will have a forward voltage that is lower than the others and the remainder might have a slightly higher forward voltage. Then the one with the lowest voltage hogs all the current and burns out. Then the next LED with the lowest voltage will burn out, then another and another.
Each LED or series string of LEDs needs its own current-limiting resistor.
 
I think my terminology of parallel might be wrong. I mean each LED and resistor running parallel with the battery, and the LED and resistor in series. Like what you have in your diagram a few post back.

So should I run it at the 4.06v that you have in the diagram? Which my battery voltage my not be high enough? or should I switch it to 2v and just run 1 LED in series with whatever ohm resistor is required per string?

Now going to 1 LED and resistor, will I still only be able to run 8-10 strings? and I would need a second regulator if I wanted to run a total of 16 LEDs?

I will have to test the 4.06v and see if I can live with the dropout before the current gets to low to power the LEDs. Will have to test the LEDs to see what’s the lowest current I can run them on and still produce the light I need.
 
You have series strings of two LEDs plus a resistor. Each string draws 126mA from the LM317 that is set to 4.06V.
The LM317 can supply 1.5A if it has a big enough heatsink.
Simple arithmatic calculates 1.5A/126mA= 11 strings.

Your battery is pretty small so its charge won't last long. When the LEDs begin to dim then the lead-acid battery is beginning to be destroyed.
 

I have not looked into this, but how hard or easy is it to add something to shut off the current once the current hits a certain point?
 
You could make a comparator and a voltage reference detect when the voltage from the battery drops too low and then it can turn off a Mosfet that gives the circuit its power.
An LM10 is an 8-pins IC that has an accurate voltage reference plus an opamp that can be used as a comparator.
Use a "logic-level" Mosfet that works at your low voltage.
 
audioguru said:
16 of your LEDs draw more than 2A which is more current than an LM317 can supply.
Click to expand...
If he is not intending to disconnect any of the LEDs then he could connect a large power resistor from the LM317's input to output to by pass it so it only passes 1.5A maximum.
 
Ok, I'm testing the LM317T, Just wondering about some things. Just testing with 1 LED and current limiting resistors.

Here are the numbers.

Battery 6.57V

LM317 Output is 2.275V
R1 = 100ohm
R2 = 82ohm
(82/100 = .82)
(.82 + 1 = 1.82)
(1.82 * 1.25 = 2.275V)

Current Limiting resistor 6.6ohm (2.2+2.2+2.2)
Voltage across current limiting resistor .83V
(.83/6.6 = .125mA)

Voltage across LED 1.43V


Is it normal that my voltage across my LED is 1.43v? Everything was figured out to be 1.6v. The specs for the LED say 1.5-1.6V? Should I switch to metal instead of carbon resistors? Would that help?
 
The output of your LM317 is 2.275V.
the voltage across your 6.6 ohm current-limiting resistor is trying to be 0.82V but then only 1.445V will be across the LED and maybe it needs more voltage.

Don't cut the voltage to the LED so low that one with a high forward voltage won't work.

Try it again with the LM317 giving an output of 3.0V. Then if the LED needs 1.6V the current-limiting resistor will have 1.4V across it.
Of course you will need to re-calculate all the resistors.
 
Ok I redid it.

R1 = 100
R2 = 150
1.25*(1+(150/100))=3.125V Which is what is coming out.

CLR = 12.2ohms with 1.67V and 137mA
LED = 1.43V (didn't change)

I also tested it with different LEDs (but the same type).

Now if I drop to 10ohms, I get 1.68 on the CLR but 168mA

The voltage on the LED goes up to 1.45V.

So dropping the resistance forces the voltage up on the LED, but also the current....
 
Of course.
Increasing the current of an LED increases its voltage a small amount.

Don't exceed the max allowed continuous current rating for the LED.
Its brightness and how long it lasts depends on the amount of current you feed it.
Its temperature also determines how long it lasts. Don't fry it.
 

Corrent, but do I need the voltage to run between 1.5-1.6? or do I just need the current at .140? or am I supposed to have it at both in that range, which I can't seem to get?
 
ReallyConfuzed said:
Corrent, but do I need the voltage to run between 1.5-1.6? or do I just need the current at .140? or am I supposed to have it at both in that range, which I can't seem to get?
Click to expand...
The LEDs have a wide range of voltages but their current is exact.
It is the current that determines their brightness and how long they last.

A resistor and an old incandescent light bulb increase their current gradually when their voltage is increased. But not a diode and not a light emitting one.
Their current skyrockets when their voltage is increased a very small amount.

LEDs need a regulated current, not voltage.
 
So could I run the LEDs at .36v? as long as the current is around 140ma? by running 4 LEDs in the string? or does it still need to be within a certain range of the specs for the LED? Like 10-20% of the specs?

I'm going to test it to see the result, but was just wondering? Trying to understand voltage a little more.
 
On the first page of this thread, he says the spec's for the LED is 1.5V to 1.6V and 140mA continuous, 700mA peak.
 
Yes, these are large and high power LEDs. 10mm, and a 3 chip design (3 little lights inside). There are also Infrared LEDs so not sure if they really have a color.
 
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