# Looking for a constant current LED driver for 12V dc

#### leche

Joined Apr 1, 2011
23
Hello everybody.I have been trying to find the best solution for me for a while but i am confused a little bit.
The goal it to make a custom led light for my car's tail light.I am gonna drive 3 x 1w power leds in series (about 300-320 ma at 9 VDC) or i might try to wire 3 x 3w power leds in serial (about 900 ma at 9 VDC).First i look for lm 317 ic but then i figured it out that i need a constant cureent.Without it leds are gonna heat too much and it means lower lifetime.LM317 does not limit current i think.So i look for some constant current led driver ics like NCV7691 etc.But i am confused to decide.So i wanted to ask you about it.

Which way would be the best to limit led voltage and current for my specs?
It might not be any ic if there are more options i am open more ideas.
The board should be about 13 mm x 40 mm. (0.51 inches x 1.57 inches) should put the circuit on this size of board.

Thank you.

#### crutschow

Joined Mar 14, 2008
27,399
Which way would be the best to limit led voltage and current for my specs?
You could use the LM317 in a constant-current circuit (from data sheet below):
R1 can be a fixed resistor to give the current you want (1.2v / R1).

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#### LowQCab

Joined Nov 6, 2012
734
When doing this, remember that the Pot in the Schematic, as well as the 317,
are going to be dissipating some significant Heat,
depending on how much Voltage-Drop you have through the LEDs.
A standard 1/4-Watt Pot is not going to survive.

All of that ~14.5-Volts has to go somewhere.
If it doesn't go to the LEDs, it must go to the 317, and the Resistor, and be dissipated as Heat,
so, more LEDs in the string is better, and more efficient overall.
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#### crutschow

Joined Mar 14, 2008
27,399
When doing this, remember that the Pot in the Schematic, as well as the 317,
are going to be dissipating some significant Heat,
Good point.
For 900mA, R1 will dissipate about a watt (use a 2W resistor), and the LM317 will dissipate about 5W (will need to be on a heatsink).

To reduce that power, you would need to go with a constant-current switching-regulator type design.

#### LowQCab

Joined Nov 6, 2012
734
Or add another LED to the string.
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#### BobTPH

Joined Jun 5, 2013
3,484
Or you can buy one. They are quite common.

Bob

#### leche

Joined Apr 1, 2011
23
Thanks to everybody who responded.I did not expect such a fast help from you =).

When doing this, remember that the Pot in the Schematic, as well as the 317,
are going to be dissipating some significant Heat,
depending on how much Voltage-Drop you have through the LEDs.
A standard 1/4-Watt Pot is not going to survive.

All of that ~14.5-Volts has to go somewhere.
If it doesn't go to the LEDs, it must go to the 317, and the Resistor, and be dissipated as Heat,
so, more LEDs in the string is better, and more efficient overall.
As i mentioned i use that circuit on 12V supply.And if i wire 3 leds in serial it is 9V in total.Voltage drop would be 3V.So it doesn't seem to much to me.Do you think it causes too much heat?Also i calculated the resistor value and it advices 0.4W one but i thing to use a 2512 smd 1w one.So it works %40 capacity and it too would be okay for heating problem right?Also probably i am gonna use a metal sheet as a heatsink on the chips.Do you think theese will bee enough to keep the system stable?

Good point.
For 900mA, R1 will dissipate about a watt (use a 2W resistor), and the LM317 will dissipate about 5W (will need to be on a heatsink).

To reduce that power, you would need to go with a constant-current switching-regulator type design.
If i try to use a constant current swtiching regulator, can you please advice me a good and a common ic for my needing just in case?

Thank you again.

#### LowQCab

Joined Nov 6, 2012
734
3-Volts X 0.300-Amps = 0.9-Watts,
in your car, the Voltage will be closer to ~14.5-Volts,
which is 5.5-Volts X 0.300-Amps = 1.65-Watts,
which will be divided between the 317 and the Resistor.

If your calculator says the Resistor will handle 0.4-Watts at 12-Volts Input,
then the rest of the Heat (which is 0.5-W) will be dissipated by the 317 Regulator.

You need to re-do your calculations using 14.5-Volts Input.

With SMD construction, you may have trouble with getting rid of ~2-Watts of total Heat.
With an Aluminum-Box as a Heat-Sink,
a 317 Regulator in a TO-220 Package can easily handle that Power.
SMD construction is geared toward Automated-Mass-Production,
and is not very Hobbyist friendly.

Complete Current-Limiting-Regulators are "a-Dime-a-Dozen" from China.
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#### leche

Joined Apr 1, 2011
23
Hello everybody again.today I built a circuit by a lm317 to-220.used 2w resistor and they work good and no heating problem at all.but let me explain something.to-200 package is a little bit bigger than I expected.ı am expecting a really basic and small circuit to fit it on a small board for a 1156 bulb.can someone please help me about it?I think some constant current delivery can be built by just transistors right?is there any basic solution to build a small and efficiant way to drive the led without using any ic?ı think that way would be easier and I would not need some extra components to couple with ic.

#### Papabravo

Joined Feb 24, 2006
16,469
Hello everybody again.today I built a circuit by a lm317 to-220.used 2w resistor and they work good and no heating problem at all.but let me explain something.to-200 package is a little bit bigger than I expected.ı am expecting a really basic and small circuit to fit it on a small board for a 1156 bulb.can someone please help me about it?I think some constant current delivery can be built by just transistors right?is there any basic solution to build a small and efficiant way to drive the led without using any ic?ı think that way would be easier and I would not need some extra components to couple with ic.
You are always going to have a heat problem going from automobile voltage to LED forward voltage. Handling it is why they call it Engineering instead of DIY experimentation. In order to meet all of your requirements I would look into a prefabricated buck regulator that would take 14V in and put out something just above the LED forward voltage and enough current for acceptable brightness. You might not pay more than \$0.99 ea.

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,909
Use a real LED driver automotive qualified. They are available.

I'm going to try to make a 0-10v dimmer from a BCR430 and a CDM10. The BCR430 is good for 100 mA, but can be paralleled

#### leche

Joined Apr 1, 2011
23
Use a real LED driver automotive qualified. They are available.

I'm going to try to make a 0-10v dimmer from a BCR430 and a CDM10. The BCR430 is good for 100 mA, but can be paralleled
Thank you.ı built a circuit by lm317 and it seems that it works well now.i check temp regularly to make sure the components don't heat too much.
Let me ask a question to you.

-I have been already looking for a good constant current ic for my future applications.But there is a confusing point for me.mA limitations seem low for me generally.So i would like to ask ,

if a constant current ic rated max 100 ma output and if i need to wire 4x 50 ma leds on it, can i wire 2x lines of leds in parallel and every series has two leds?

What 100 ma means exactly?Is that the maximum current we can get from it without any exceptation or is that the rating for every led's maximum current?

For example if it is the maximum rating for the ic and if we can not wire 2x50 ma led in total, it means it is trash for a stop light a rear fog light etc. because it would be so weak what we get from 100ma output for that job.

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#### KeepItSimpleStupid

Joined Mar 4, 2014
4,909

mAH is millamps*hours and usually denotes battery capacity. Some fraction of C, where C is the battery capacity.
It's a way to compare batteries. but not absolute spec.

mA is the current the LED draws.

Vf is the voltage it drops. That has a range.

PWM is the most efficient way to control brightness. You can sometimes use a lower duty cycle and thus get a high peak output with less power. IR remote controls do this. Current regulated PWM per LED would use the least amount of power.

The LED output efficiency (lumans/Watt) also enters into the equation.

The eye has different perception of brightness sensitivites of colors.

#### leche

Joined Apr 1, 2011
23

mAH is millamps*hours and usually denotes battery capacity. Some fraction of C, where C is the battery capacity.
It's a way to compare batteries. but not absolute spec.

mA is the current the LED draws.

Vf is the voltage it drops. That has a range.

PWM is the most efficient way to control brightness. You can sometimes use a lower duty cycle and thus get a high peak output with less power. IR remote controls do this. Current regulated PWM per LED would use the least amount of power.

The LED output efficiency (lumans/Watt) also enters into the equation.

The eye has different perception of brightness sensitivites of colors.
Thank you that was a typing mistake.it is corrected.

#### bassbindevil

Joined Jan 23, 2014
168
I'd just use a resistor; it's simple, reliable, and just as efficient as a linear regulator. And add a rectifier in series to protect against negative transients.