Regulated DC to DC car battery converter

t06afre

Joined May 11, 2009
5,934
If one LED need 3 volt why not wire 4 of them in series, and then hook the whole thing to the car battery. And end of story. I do not think 4 LEDs will trigger any cops. Even if they are low on donuts and hot-tempered. But is not such lightning somewhat cheesy. We call it horelys in Norway. I guess someone can translate it ;)
 

SgtWookie

Joined Jul 17, 2007
22,230
But the voltage shows 12V,what will happen if i fix a LED at the output?
Before you try it with an LED, try it with a 100 Ohm resistor connected between the ADJ pin and GND. You should measure about 2v across the 100 Ohm resistor.

See the attached simulation; it's basically three copies of the same circuit with only the type load changed. The leftmost circuit uses a 100 Ohm resistor as a load. Since there is a current of 20.96mA flowing through a 100 Ohm resistor, there is a 20.96mV drop across the resistor.

The circuit in the middle has a red LED for a load; it's forward voltage is 1.856v when 20.96mA is put through it.

The circuit on the right has a blue LED for a load. The current is still 20.96mA, but the blue LED's Vf at that current is much higher; 3.717v.

Note that the simulations' LM317 model has a Vref of 1.3v, which is at the high end of the manufacturer's specifications. Vref is measured from the ADJ terminal to the OUT terminal. Typically, Vref is 1.25v, so a 62 Ohm resistor from OUT to ADJ would give an output current of about 20.16mA
 

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Thread Starter

Lanz

Joined Oct 30, 2008
153
If one LED need 3 volt why not wire 4 of them in series, and then hook the whole thing to the car battery. And end of story.
Yes,you're correct if the current is not an option(4x3V=12V)
But for LED,we had to pay attention on its Voltage and the Current bcoz V is proportional to I.
If not,as Sgtwookie said,it will not be the end of the story,but it will be the end of the LED's.



See the attached simulation; it's basically three copies of the same circuit with only the type load changed. The leftmost circuit uses a 100 Ohm resistor as a load. Since there is a current of 20.96mA flowing through a 100 Ohm resistor, there is a 20.96mV drop across the resistor.

The circuit in the middle has a red LED for a load; it's forward voltage is 1.856v when 20.96mA is put through it.

The circuit on the right has a blue LED for a load. The current is still 20.96mA, but the blue LED's Vf at that current is much higher; 3.717v.

Note that the simulations' LM317 model has a Vref of 1.3v, which is at the high end of the manufacturer's specifications. Vref is measured from the ADJ terminal to the OUT terminal. Typically, Vref is 1.25v, so a 62 Ohm resistor from OUT to ADJ would give an output current of about 20.16mA
Is "com" means ADJ pin?
I would like to use this circuit for Blue and White LED's.
 

SgtWookie

Joined Jul 17, 2007
22,230
Yes, in the schematic, the LM317 model in that library have the ADJ pin mislabeled as "COM". I can't correct the library models.
 

Thread Starter

Lanz

Joined Oct 30, 2008
153

The circuit worked well as the picture above.
I want to use blue n white LED.
But when i connect another LED parallel,seems the LED get dimmed.
I want to connect many LED's and want to use maximum brightness.
How to avoid the dimness?​
 

Audioguru

Joined Dec 20, 2007
11,248
The LM317 current regulator circuit provides 20mA. If there is one LED then it is fairly bright with 20mA.
But if you connect a second LED in parallel then the LEDs share the 20mA and have only about 10mA each which looks dimmed.

You are not supposed to connect LEDs in parallel because they are not made exactly the same. One will hog most of the current and will quickly burn out. Then the current will be too high for the next one which will also quickly burn out.

Connect two or three LEDs in series and in series with a current-limiting resistor to make a string. You could use an LM317 current regulator circuit instead of the resistor. Then make hundreds of strings.
 
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