LED driver

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Thread Starter

kaushizcute

Joined Feb 16, 2012
24
I am making an led driver circuit. My led specification is 3.6V(typical), 140mA. My supply is from the car battery, say Vin is from 9V to 16V.
So basically i want to use a regulator for the battery so that i can get 12V constant and then using LM317L i can get the led specification.
I want to know how to regulate the car battery voltage or 9V-!6V to 12V.

Initially i made a circuit using LM317Ls basic adjustable regulator circuit for Vin 12V (R1=200Ω, R2=500k potentiometer) but the problem was when i was increasing or decreasing the voltage the output voltage and current was differing which i do not want. I am looking for constant voltage and current circuit.
 

Felo

Joined Feb 20, 2012
91
Or use a 3,3V regulator only one IC and should have no problem powering your led
 

Felo

Joined Feb 20, 2012
91
Sure, If the input it's below the forward voltage the led will pull just what it need/can, I see no overcurrent scenario here go ahead and try it. I would bet on it.

Just the same way as you can light a red, green, yellow led with a AA battery, won't give out full brigh but still lit.
 

Audioguru

Joined Dec 20, 2007
11,251
Sure, If the input it's below the forward voltage the led will pull just what it need/can, I see no overcurrent scenario here go ahead and try it. I would bet on it.

Just the same way as you can light a red, green, yellow led with a AA battery, won't give out full brigh but still lit.
Not always.
LEDs have a range of forward voltage. My Fairchild red LEDs have a range at 20mA from 1.5V to 2.4V. one with a low forward voltage will be very bright when powered from one new alkaline cell but a "typical" one will be very dim and a maximum one WILL NOT LIGHT.

Old fashioned green LEDs were dim and had a tyicxal forward voltage of 2.2V so they will not light fron one alkaline cell. Modern green LEDs are extremely bright and have a typical forward voltage of 3.5V and they will not light from one alkaline cell.
 

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Felo

Joined Feb 20, 2012
91
I totally agree whit you, it is not the best way to light a led with a AA but the diffused ones do light, the point of real discussion here is:

will a led with 3,6 fv light whit 3,3 safely without current limiting resistor?
 

Audioguru

Joined Dec 20, 2007
11,251
You cannot buy a "3.6V LED" but instead you can buy a 3.6V light bulb which is completely different.

You can buy an LED that has a range of forward voltage at 20mA from 3.2V to 4.0V. It will survive being powered from 3.3V but if its forward voltage is actually 4.0V then it will be very dim.
 

Thread Starter

kaushizcute

Joined Feb 16, 2012
24
My led works between 2.6V to 4.1V but i want it to glow constantly at 3.6V. When i am using LM317 the problem is when input voltage is changing the output current is changing but i want constant current output(140mA) and voltage(3.6V) irrespective of input voltage variations.
 

Thread Starter

kaushizcute

Joined Feb 16, 2012
24
The lm317 speciality is that it drops a certain amount of voltage, generally 5V. So if input is 12V it will give 12-5=7V.. and as the input changes the output will change so the circuit attached to the lm317 output voltage will have variations.
I want something like if input varies between 9V to 16V the the ic or a circuit should regulate it to a constant 12V and the i believe the lm317 circuit can be attached.
 

John P

Joined Oct 14, 2008
1,756
You seem to have a fundamental misunderstanding about the LM317. It's generally used to produce a constant output voltage, not an output that's a constant amount below a varying input voltage. Alternatively, it can be used to limit current to a constant value. Both uses imply a varying voltage drop in the LM317 itself.

What the LM317 can't do is give you a 12V output for a 9V input; the input must always be at least 1.25V higher than the output. But why make a fetish of 12V? If you have to run off an input that can go down to 9V, set it up for an output of (let's say) 6V, and you'll always be able to get that. Or set it up for constant current, and it'll do that for you.
 

Audioguru

Joined Dec 20, 2007
11,251
My led works between 2.6V to 4.1V but i want it to glow constantly at 3.6V.
Its forward voltage drop will be whatever it needs, not what you want it to be. Its current determines its brightness.

I want constant current output(140mA) and voltage(3.6V) irrespective of input voltage variations.
The LM317 uses a single resistor to be a constant current source. The resistor needs 1.25V and the LM317 needs at least 2V so the input voltage must be at least 3.25V higher than the output voltage.

You cannot regulate the current and regulate the voltage at the same time.
The LED forward voltage is whatever it needs, not what you want it to be.
 

Thread Starter

kaushizcute

Joined Feb 16, 2012
24
Thanx John P.
If i want a voltage regulator for 9V-16V input and a stable 12V output which ic shall i use?
 

Thread Starter

kaushizcute

Joined Feb 16, 2012
24
My led driver will get the supply from car battery which fluctuates from 9V-16V so i want a stable output 12V.
Current can be 3A-5A
SMT
 

John P

Joined Oct 14, 2008
1,756
If you absolutely must have a voltage that sometimes is above, and sometimes below, the input, it's possible using a SEPIC circuit. You can look it up. But it would be much simpler if you decide to live with a voltage that is always below the input. So if the input can go as low as 9V, make your output voltage 7V. Can you make it work on that?
 

mcgyvr

Joined Oct 15, 2009
5,394
Why do your specs keep changing? Why don't you give us the full picture?
First it was 140mA now its 3A-5A..
 

Audioguru

Joined Dec 20, 2007
11,251
The LED voltage is "3.6V". Then why do you want a 12V power supply for it?

Maybe you calculated that two LEDs in series might have voltages at their max of 4.1V so their total is 8.2V. Then a 12V power supply can use a series current limiting resistor to limit the current to 140mA.
 

Thread Starter

kaushizcute

Joined Feb 16, 2012
24
@ mcgyvr
i want a constant voltage and current because i want to regulate the voltage first and then i want to make the led driver circuit using the regulated power
 
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