LM317, Can a single Unit regulate voltage & Current?

[toxsick]

I want to build a simple driver for a LED light.

It is 3watts.. and runs around 3.3v so thats about 990ma

I want to know if I require the Voltage set before entering the LM317 to regulate the 900ma Current.

Or the LM317 can be configured by using 2 resistors, 1st @ the adjust pin and 2nd at the output pin!

that way Resistor 1 Sets Voltage to 3.3v, Resistor 2 Sets 900mA

anyone please confirm this for me..

 

[Nigel Goodwin]

You don't need to set the voltage, only the current - you can't set both, it wouldn't work.

 

[toxsick]

so I would require 2 regulators...
first to set the voltage then 2nd to set the current?

P.S. thanks for the reply

 

[ericgibbs]

so I would require 2 regulators...
first to set the voltage then 2nd to set the current?

P.S. thanks for the reply

hi,
No, look at the datasheet for LM317, the current limited app notes.
Just a single resistor in the output that will limit the current to 990mA.

The output 'voltage' will become 3.3V

Do you follow OK.?

EDIT:
1.27R at 3W to 5W

 

[toxsick]

your example above does it matter what input voltage is?

so if I put 12v in, the output will go down to 3.3v becuase the current is being limited?
that makes no sense to me!

i thought that the 317 would take input + drop voltage (usually 1-1.2v) output @ 990ma
eg: 12v in, out would be about 10.8-11v @ 990ma?

or does this work same how a resistor+LED works, using a 180ohm resistor the voltage at the LED would be 3.3v!!!

I am sorry.. I thought I understood abit about electronics.. but this lm regulator is confusing me so much!

 

[Vizier87]

your example above does it matter what input voltage is? so if I put 12v in, the output will go down to 3.3v becuase the current is being limited? that makes no sense to me!

That is why it is called a regulator. It has internal circuitry which compensates any changes of the input so long it is in an acceptable range. How it works is way beyond you if you're not into electronic circuit theory.

Take a look here:
https://www.electro-tech-online.com/custompdfs/2010/01/3_3vto5vAnalogTipsnTricksBrchr-1.pdf

I dislike the idea of producing 3.3V using a variable regulator. There are so many better ways. :/

 

[ericgibbs]

your example above does it matter what input voltage is?

so if I put 12v in, the output will go down to 3.3v becuase the current is being limited?
that makes no sense to me!

i thought that the 317 would take input + drop voltage (usually 1-1.2v) output @ 990ma
eg: 12v in, out would be about 10.8-11v @ 990ma?

or does this work same how a resistor+LED works, using a 180ohm resistor the voltage at the LED would be 3.3v!!!

I am sorry.. I thought I understood abit about electronics.. but this lm regulator is confusing me so much!

hi,
The voltage drop across the LM317 will have to be about 2 to 3 volts minimum, so say 3V.
The voltage drop across the resistor will be 1.25V, the Vdrop across the LED is 3.3V, so in total equals 7.85V

That means the minimum input to the LM317 must be say 8V.

If you put 12V input and say you have 9V drop across the LM317 at 1A that is 9Watts heat dissipation in the LM317.

It maybe necessary to fit the LM317 to a small heatsink.

NOTE: without the LED connected the LM317 will rise to about Vinp -3V approx...

 

[audioguru]

An LED sets its own voltage because it is a diode, not an incandescent white hot resistor.
You can power an LED from 1000V or more if the current is limited properly.

Your LED might not be 3.3V. It might be 3.3V but it is spec'd with a range of voltages maybe from 3.0V to 3.6V.

 

[mvs sarma]

while the regulator gives a constant voltage, you can always limit the output current.
In this mode, the voltage is constant till the current reaches the designed point, there after the voltage droops in order to limit the current.
hope a study of the datasheet would clear off the clouds, as already suggested by Ericgibs

 

[toxsick]

hi,
The voltage drop across the LM317 will have to be about 2 to 3 volts minimum, so say 3V.
The voltage drop across the resistor will be 1.25V, the Vdrop across the LED is 3.3V, so in total equals 7.85V

That means the minimum input to the LM317 must be say 8V.

If you put 12V input and say you have 9V drop across the LM317 at 1A that is 9Watts heat dissipation in the LM317.

It maybe necessary to fit the LM317 to a small heatsink.

NOTE: without the LED connected the LM317 will rise to about Vinp -3V approx...

that makes alot of sense to me..

So even if I had 2 LEDs in series the voltage is not an issue.. because you so called 8v + 3.3v =11.3v so that would be about a 5-6w heat dissipation

my other question is... the resistor is choosen based on Voltage input & current output? or does voltage output have to be calculated also?

Iout = 1.25
-------------
r. sense

where did the 1.25 come from? if I use any resistor it will always be 1.25?

so confirm my calculate

2 Leds In series
3.3v+3.3v + 1.25 = 3v (10.85 minimum v input)

current @ 900ma
can you help calculate resistor value
current @ 320ma
can you help calcuate resistor value?

thanks...
P.S. message from audioguru TRUE thanks

 

[ericgibbs]

**broken link removed**,
The 1.25V is an internal voltage refrence of the LM317.

The formula for Current limit is 1.25V/R1

So R1= 1.25V/Icurrent

If you wanted sat 100mA, then that would be R1 = 1.25/0.1 = 12.5R

For 1amp say R1=1.25/1 = 1.25R and so on...OK.?

Your point is valid for two 3.3V LED's in series plus the voltage drop of 3V across the LM317 and 1.25V across the resistor

 

[toxsick]

**broken link removed**,
The 1.25V is an internal voltage refrence of the LM317.

The formula for Current limit is 1.25V/R1

So R1= 1.25V/Icurrent

If you wanted sat 100mA, then that would be R1 = 1.25/0.1 = 12.5R

For 1amp say R1=1.25/1 = 1.25R and so on...OK.?

Your point is valid for two 3.3V LED's in series plus the voltage drop of 3V across the LM317 and 1.25V across the resistor

great, thank you for the help... much appreciated..

Shaun, good night all.

 

[mvs sarma]

great, thank you for the help... much appreciated..

Shaun, good night all.

a constant current mode is better for your design. never work at 100% wattage.
always confine and work at around 80% so that the Lamp lives longer.

 

[toxsick]

yes thankyou MVS.....

the lamp I selected is around 3 watts and i never use LEDS to the full ma. to last longer. but the constant current regulator will allow me to go near the value required! as resistors in the past have proven dodgy! ie... 12v and 13.8v in a car.. that extra 1.8v usually means more ma as well! so the lamp will burn quickly.

that is store brought High Brightness LED and resistor in packages for GAUGES and Meters.

does anyone know typical luminous output relation based from 100% and say 85% current.

 

[Grossel]

The question as I understood it was how to make a device if output voltages make the conected device (assume resitive load) consume the same amount of wattage whatever resistance it might have.

Answer to this is that you can't easilly solve this with just a voltage or current regultor. It must me more "inteligent" than that. But interresting problem, I'll do a search on net
Cheers

[edit]
Sorry, no relevant hits for google search on "constant wattage output"

 

[mvs sarma]

yes thankyou MVS.....

the lamp I selected is around 3 watts and i never use LEDS to the full ma. to last longer. but the constant current regulator will allow me to go near the value required! as resistors in the past have proven dodgy! ie... 12v and 13.8v in a car.. that extra 1.8v usually means more ma as well! so the lamp will burn quickly.

that is store brought High Brightness LED and resistor in packages for GAUGES and Meters.

even 7808, 7809, 7805, 7806 can also be used in constant current mode

please see the respective dtat sheet

such cases the common can not be connected to ground . the heat sink, if used should be insulated from the device.

 

[mvs sarma]

The question as I understood it was how to make a device if output voltages make the conected device (assume resitive load) consume the same amount of wattage whatever resistance it might have.

Answer to this is that you can't easilly solve this with just a voltage or current regultor. It must me more "inteligent" than that. But interresting problem, I'll do a search on net
Cheers

[edit]
Sorry, no relevant hits for google search on "constant wattage output"

If we study the initial postt, it is clear what the OP wants
it is certain that LEDs are current devices and not voltage ones
thus , he more needs a current source.
let us suppose we have a 3.3V supply, we cant connect straight to a white High power LED.

 

[toxsick]

i want to askanother question!!!!!!!!!!!!!!!!!!!!!!!!!!!, again about LEDs...

If I wanted to power a heap of them...

it would be cheaper & Easier to USE LM317T because It can regulate to a voltage precisly!!
the led I would use is say 3.3v forward v. & Forward of 20ma

If I was to use the Regulator set at 3.0volt and use wow! how amny.. 20ma/ LM317 capacity 1500MA

thats a HEAP of leds.. without requiring resistors!?!?!?!?!?!?!?!?!1

 

[toxsick]

this is another way but involes so many parts! I would require so many transistors.

 

[Nigel Goodwin]

i want to askanother question!!!!!!!!!!!!!!!!!!!!!!!!!!!, again about LEDs...

If I wanted to power a heap of them...

it would be cheaper & Easier to USE LM317T because It can regulate to a voltage precisly!!
the led I would use is say 3.3v forward v. & Forward of 20ma

If I was to use the Regulator set at 3.0volt and use wow! how amny.. 20ma/ LM317 capacity 1500MA

thats a HEAP of leds.. without requiring resistors!?!?!?!?!?!?!?!?!1

As I said back at the start of this thread, you DON'T regulate the voltage, you regulate the current. Your idea would either not work at all, or blow the LED's.

For multiple LED's wire then in series, and feed them from the same constant current - the number of LED's depends on your source voltage.