How can i improve on my design?

[Grayfox]

I am making a LED brake light for my car, i added the components, but would like to know if i can improve it any further

[c4757p]

With 12V and all those LEDs in parallel, you're wasting more power in the resistors than you're using in the LEDs! Put them in series groups of 3 or 4 (depending on the voltage drop of the LED, it needs to add up to less than 12V and still give a bit of room for a resistor), each group having one single resistor, and put the groups in parallel.

Like in the attached schematic:

[c4757p]

Also, upload the board in a more friendly format. Not everybody wants to mess around with your Gerbers or has KiCad installed. Export to PDF.

[David_AVD]

Also, upload the board in a more friendly format. Not everybody wants to mess around with your Gerbers or has KiCad installed. Export to PDF.

+1 to that.  Uploading a schematic or layout as anything other than an image or pdf will get your files ignored big time.

[digsys]

+1 to that.  Uploading a schematic or layout as anything other than an image or pdf will get your files ignored big time.   

+1 and besides, ZIP files are the devils advocate !! Never open a zip file from people you don't know. Not even with protection.
I'd add to C47#s suggestion - Assume 1.2V drop, put 8 in series = 9.6V. Use an LM317L as a constant current limiter, to say 20mA.
That adds 1.2V = 10.8V, which ends up giving you fixed brightness from a flat battery to fully charged.
With higher OP Leds, just redo the calcs for a minimum 11.0V cutoff. If your battery gets lower than that, you have other problems.

[David_AVD]

Assume 1.2V drop, put 8 in series = 9.6V. Use an LM317L as a constant current limiter, to say 20mA.

Using a Vf of 1.2V is way too low for visible LEDs.  For red LEDs, I'd use 1.6V - 1.8V range, so that would allow for only 5 in series with enough headroom for voltage variations.

[Corporate666]

Constant current is the way to go for stuff like this.

Car electrical stuff is supposed to have a range of 8 to 17 volts.  10 to 15 is a more realistic range, and 11-14.4 is probably realistic for a healthy vehicle in good condition.

So let's say your red LED's are 2.5 volts at 20mA. 

Working with a max realistic voltage of 14.4, you need to drop 11.9 volts on the resistor, so 595 ohms.  But that means each resistor is dropping ~1/4 watt!  For 30 LED's, it's over 7 watts.  It's going to get toasty hot and for a PCB the size of yours, that means you need heat sinking.  But the other issue with resistors is at 11 volts, you're only getting about 14mA, which would probably be a noticeable difference in brightness.

So let's say you decide to put the LED's in series strings of 4.. for a forward voltage of 10.  Max 14.4V means you need to drop 4.4V, so you use a 220 ohm resistor.  But now at 11 volts, you're getting a vastly smaller amount of current... because less power is being dissipated in the resistors, and because the volts/amps curve of an LED is very high (small voltage difference = big current difference), your LED's will go from dim to bright over the range of voltage they will see.

In general, you can count on a voltage more like 12.5-14.5 but it can still make for noticeable differences in brightness, especially when you string your LED's together to increase LED voltage drop.

You could go with a simple regulator set up as a current limiter and "equalizing" resistors on each string.  Would be cheap... not ideal, but better than resistors on each LED for sure.

It's late and I am still at work and probably have made some calculation errors - so pedants, feel free to correct me 

[ignator]

The LED you've chosen, QTLP690C (assume this from c4757p schematic insertion), the data sheet indicates this for cell phone backlight.
Is this going to be bright enough for brake lights?  You need sunlight visibility.  If you look at a car with LED brake lights, most are pulsed (wiggle your eyes and you will typically see it's not a solid line, but a dashed line indicating the LEDs are pulsed). 90Hz will generally not be seen by your eyes from persistence of vision (with your eyes still).  But I believe to make them sunlight readable, you need to find a different LED, and build a simple power switch.  In this case they will not be in series, but single, maybe 2 series LEDs  so you can get enough forward pulsed current.  The other issue you will have is LEDs are matched (or they were back when used in a traveling message signs, ~1976) for brightness (as well color).  But I have not stayed current with sunlight readable technology, that I see now on signs used for this purpose.

[c4757p]

I chose it out of the LTspice library for convenient V_F. I've no idea what LED he is using.

[Grayfox]

I will be using a 12,000mcd 5mm LED that runs at 1.8~2.0v @ 20mA.

[ignator]

I will be using a 12,000mcd 5mm LED that runs at 1.8v @ 20mA.

Do you have a vendor designator.  I'm curious. I assume it does not need to be pulsed. And only 36 milliwatts of dissipation.

[Grayfox]

vendor designator?

It is these i will be getting
http://www.ebay.com.au/itm/100p-4-8mm-RED-LED-LAMP-LIGHT-12-000mcd-FREE-RESISTORS-/190400581196?pt=LH_DefaultDomain_0&hash=item2c54c24e4c

Or may choose these
http://www.ebay.com.au/itm/20-pcs-x-5mm-Super-Bright-RED-Leds-15-000-mcd-Round-Top-light-Us-Seller-/330942810458?pt=US_Car_Lighting&hash=item4d0dba895a

And my initial design was in series but if one failed(rare i know) all the ones within that group no longer work and it would look cheap to have a missing light when it is lit.

[mariush]

Those are 5mm leds with 1.9-2.3v forward voltage and 20mA of current.

Keep in mind the battery may give you more than 12v, let's say up to 13-14v. So you don't want to do the math considering the input voltage fixed at 12v and the current at 20mA, you should leave some room for variations... the leds will probably be very bright even at 15mA.

here's two pages that can help you figure out what resistor to use when putting leds in series or parallel :

http://led.linear1.org/led.wiz
http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator

With 12v, you can put 5 leds in series with a resistor. For 20mA current, a standard 27 ohm resistor will do.  For about 15-16mA, a 33 ohm resistor will work - both standard values

later edit:  if dying leds are a concern and you want each led to be independent, forget about resistors, you'll waste too much energy in them.
Use led drivers, they're more expensive but they work with wide input voltage and don't waste as much power as a resistor.

For example these are even recommended for use in car lightning and similar stuff:

10mA : http://www.digikey.com/product-detail/en/NSI50010YT1G/NSI50010YT1GOSCT-ND/2409698
20mA : http://www.digikey.com/product-detail/en/NSI45020AT1G/NSI45020AT1GOSCT-ND/2096225
30mA : http://www.digikey.com/product-detail/en/NSI45030AT1G/NSI45030AT1GOSCT-ND/2096228

You basically put them as if they're a resistor... only problem you might have is the size, but it's still solderable by hand.

[Corporate666]

vendor designator?

It is these i will be getting
http://www.ebay.com.au/itm/100p-4-8mm-RED-LED-LAMP-LIGHT-12-000mcd-FREE-RESISTORS-/190400581196?pt=LH_DefaultDomain_0&hash=item2c54c24e4c

I didn't check the second one, but I have to laugh at the specs on that LED.  They are total, total bullshit.

It says Vf 1.9, If 0.02A, view angle 120 degrees and 12,000mCd brightness.

That is over 37 lumens of light output per LED.  From ~0.04 watts of power.  1000 lumens per watt.  Wow, those are the most efficient LED's ever made - 15 times more efficient than the most efficient LED's, and they produce more light than the maximum mathematically possible amount of light from 0.04 watts.  Quite an achievement   I wonder if the seller includes some magic beans with the order?

Probably the brightest 5mm LED's around would be a high bin Cree which would *maybe* get 2 lumens each.  60 for the whole string of 30 if you use top shelf LED's and drive them right at their limit.

A single Cree XBD or XP-E will get you over 200 lumens.




Having said that, putting them in strings is OK, but using a resistor to drop the voltage in an automotive application is not, most especially if you have put your LED's in series strings.  You need to control current, not voltage.

[Grayfox]

Can one of those current limiters run all LEDs or do i need one for each LED.


May get these instead, there cree and they have good price too
http://au.element14.com/cree/c503b-ras-cy0b0aa1/led-red-t-1-3-4-5mm-12cd-630nm/dp/1855555

[digsys]

   Can one of those current limiters run all LEDs or do i need one for each LED.   

You need one only for every series string. You use them just like a resistor.

[mariush]

Those current drivers I linked to limit the current to a particular value.  A 10mA one will limit the current to 10mA, so if you put one led or several in series after it, each led will get 10mA  (in a circuit, each element in series gets same current).

But the forward voltage of the leds limits the number of leds you can put in series.  The current limiter chip does work with wide range of input voltage, but if a led has a 2v voltage drop (the forward voltage), you can't power 10 leds from a 12v input  because 10 leds x 2v drop per led = 20v , which is more than 12v.

Think of this led driver as a smarter resistor... you can give the circuit anything from 1v to 45-50v and the chip will make sure the whole circuit will get 10mA.
A plain resistor will not do that, you have to compute a fixed value for the resistor which will only give that amount of current at a fixed input voltage.  If input voltage changes, so will the current going to the circuit.
Also, a plain resistor will be affected by heat - when it heats up, the resistor value changes and current going through the led will increase or decrease - in extreme cases you may burn up the led. That led driver has circuitry inside to take care of these cases.

May get these instead, there cree and they have good price too
http://au.element14.com/cree/c503b-ras-cy0b0aa1/led-red-t-1-3-4-5mm-12cd-630nm/dp/1855555

Those leds aren't impressive in any way. Only 15 degrees angle and only 12mCd of luminous intensity. For car lights, you probably want about 30 degrees at least and you can do better when it comes to luminous intensity.
For example

3mm diameter

0.05$ each http://au.element14.com/multicomp/mcl034srt/led-3mm-32-super-red/dp/1581118  20mA    1.9V    250mcd    Round    643nm    T-1 (3mm)    36°    Through Hole

5mm diameter

$0.119 each, $0.08 for 100  - http://au.element14.com/multicomp/mv5752/led-5mm-red/dp/1703501  20mA    2V    100mcd    Round    635nm    T-1 3/4 (5mm)    30°    Through Hole
$0.133 each , $0.065$ each in 100 pcs - http://au.element14.com/multicomp/mcl053srdd/led-5mm-36-red/dp/1581217  20mA    1.85V    75mcd    Round    643nm    T-1 3/4 (5mm)    45°    Through Hole
$0.16 each, minimum 200 pcs - http://au.element14.com/lumex/ssl-lx5093it/led-red-t-1-3-4-5mm-80mcd-635nm/dp/7605730  20mA    2V    80mcd    Round    635nm    T-1 3/4 (5mm)    30°

[David_AVD]

May get these instead, there cree and they have good price too
http://au.element14.com/cree/c503b-ras-cy0b0aa1/led-red-t-1-3-4-5mm-12cd-630nm/dp/1855555

Those leds aren't impressive in any way. Only 15 degrees angle and only 12mCd of luminous intensity. For car lights, you probably want about 30 degrees at least and you can do better when it comes to luminous intensity.

Those LEDs are 12Cd, not 12mCd.   

[mariush]

F**k... that's amazing in that case.

Serves me right for posting at 6-7 am with no sleep. My bad.

[Psi]

Can one of those current limiters run all LEDs or do i need one for each LED.

They will limit the current to their rating (20mA etc) 
So you can have as many leds as you want in series as long as you have enough voltage to drive them plus an extra ~2.5V for the regulator itself.

Assuming 12V and 1.8V led you could have..
12V - 2.5V= 9.5V left for leds

9.5V / 1.8Vled = 5.27
So you could have a max of 5 leds per string plus one of those regulators.

And if the input rises to 14V the current stays around 20mA.


Personally i think you should be using http://dx.com/p/30w-1500-lumen-rgb-led-emitter-metal-plate-39960
 

[poorchava]

I wonder where does to OP live. In most EU countries making modifications to car exterior lighting is strictly forbidden. You are supposed to use only lighting which has homologation for your vehicle. And as long as various bigass spoilers, wings, low profile tires, monstrous exhausts and the like usually won't get you a ticket, modified head or tail lamps will, and so will excessively tinted windows without certification.

[Grayfox]

Luckily i do not live in any country that is governed by the EU.

I live in australia.

This is going to be a center brake light upgrade for my car.

on my car it only lights up 2/3 of the way and it is light that from factory.
This will make it light up all the way.

[Grayfox]

May get these instead, there cree and they have good price too
http://au.element14.com/cree/c503b-ras-cy0b0aa1/led-red-t-1-3-4-5mm-12cd-630nm/dp/1855555

Those leds aren't impressive in any way. Only 15 degrees angle and only 12mCd of luminous intensity. For car lights, you probably want about 30 degrees at least and you can do better when it comes to luminous intensity.

Those LEDs are 12Cd, not 12mCd.   

I do not like it when the Si prefixes change for candle power.

[Grayfox]

Replaced resistors with CCRs
Doubled amount of LEDs

[David_AVD]

Sorry, still a fail.  Go back and reread the part about putting the LEDs in series strings.