How can i improve on my design? - Page 1
Forum section: electronics / general

author: Grayfox
author_rank: Regular Contributor
timestamp: July 17, 2013, 01:07:13 pm
content: 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

author: c4757p
author_rank: Super Contributor
timestamp: July 17, 2013, 02:06:07 pm
content: 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:

author: c4757p
author_rank: Super Contributor
timestamp: July 17, 2013, 03:09:24 pm
content: 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.

author: David_AVD
author_rank: Super Contributor
timestamp: July 17, 2013, 10:59:11 pm
content: Quote from: c4757p on July 17, 2013, 03:09:24 pm+1 to that. Uploading a schematic or layout as anything other than an image or pdf will get your files ignored big time.

author: digsys
author_rank: Unknown
timestamp: July 17, 2013, 11:26:07 pm
content: Quote from: David_AVD+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.

author: David_AVD
author_rank: Super Contributor
timestamp: July 18, 2013, 12:09:33 am
content: Quote from: digsys on July 17, 2013, 11:26:07 pmUsing 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.

author: Corporate666
author_rank: Unknown
timestamp: July 18, 2013, 12:12:52 am
content: 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

author: ignator
author_rank: Regular Contributor
timestamp: July 18, 2013, 12:26:30 am
content: 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.

author: c4757p
author_rank: Super Contributor
timestamp: July 18, 2013, 12:29:45 am
content: I chose it out of the LTspice library for convenient VF. I've no idea what LED he is using.

author: Grayfox
author_rank: Regular Contributor
timestamp: July 18, 2013, 12:36:26 am
content: I will be using a 12,000mcd 5mm LED that runs at 1.8~2.0v @ 20mA.

author: ignator
author_rank: Regular Contributor
timestamp: July 18, 2013, 12:42:43 am
content: Quote from: Grayfox on July 18, 2013, 12:36:26 amDo you have a vendor designator. I'm curious. I assume it does not need to be pulsed. And only 36 milliwatts of dissipation.

author: Grayfox
author_rank: Regular Contributor
timestamp: July 18, 2013, 12:47:09 am
content: vendor designator?It is these i will be gettinghttp://www.ebay.com.au/itm/100p-4-8mm-RED-LED-LAMP-LIGHT-12-000mcd-FREE-RESISTORS-/190400581196?pt=LH_DefaultDomain_0&hash=item2c54c24e4cOr may choose thesehttp://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=item4d0dba895aAnd 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.

author: mariush
author_rank: Super Contributor
timestamp: July 18, 2013, 01:26:07 am
content: 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.wizhttp://www.hebeiltd.com.cn/?p=zz.led.resistor.calculatorWith 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 valueslater 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/240969820mA :http://www.digikey.com/product-detail/en/NSI45020AT1G/NSI45020AT1GOSCT-ND/209622530mA :http://www.digikey.com/product-detail/en/NSI45030AT1G/NSI45030AT1GOSCT-ND/2096228You basically put them as if they're a resistor... only problem you might have is the size, but it's still solderable by hand.

author: Corporate666
author_rank: Unknown
timestamp: July 18, 2013, 01:43:48 am
content: Quote from: Grayfox on July 18, 2013, 12:47:09 amI 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 achievementI 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.

author: Grayfox
author_rank: Regular Contributor
timestamp: July 18, 2013, 03:50:06 am
content: 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 toohttp://au.element14.com/cree/c503b-ras-cy0b0aa1/led-red-t-1-3-4-5mm-12cd-630nm/dp/1855555

author: digsys
author_rank: Unknown
timestamp: July 18, 2013, 04:22:55 am
content: Quote from: GrayfoxYou need one only for every series string. You use them just like a resistor.

author: mariush
author_rank: Super Contributor
timestamp: July 18, 2013, 04:23:47 am
content: 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.QuoteThose 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 example3mm diameter0.05$ eachhttp://au.element14.com/multicomp/mcl034srt/led-3mm-32-super-red/dp/158111820mA 1.9V 250mcd Round 643nm T-1 (3mm) 36° Through Hole5mm diameter$0.119 each, $0.08 for 100 -http://au.element14.com/multicomp/mv5752/led-5mm-red/dp/170350120mA 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/158121720mA 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/760573020mA 2V 80mcd Round 635nm T-1 3/4 (5mm) 30°

author: David_AVD
author_rank: Super Contributor
timestamp: July 18, 2013, 04:55:22 am
content: QuoteQuote from: mariush on July 18, 2013, 04:23:47 amThose LEDs are 12Cd, not 12mCd.

author: mariush
author_rank: Super Contributor
timestamp: July 18, 2013, 05:02:10 am
content: F**k... that's amazing in that case.Serves me right for posting at 6-7 am with no sleep. My bad.

author: Psi
author_rank: Super Contributor
timestamp: July 18, 2013, 05:49:47 am
content: Quote from: Grayfox on July 18, 2013, 03:50:06 amThey 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 leds9.5V / 1.8Vled = 5.27So 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 usinghttp://dx.com/p/30w-1500-lumen-rgb-led-emitter-metal-plate-39960

author: poorchava
author_rank: Super Contributor
timestamp: July 18, 2013, 06:25:24 am
content: 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.

author: Grayfox
author_rank: Regular Contributor
timestamp: July 18, 2013, 11:33:31 am
content: 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.

author: Grayfox
author_rank: Regular Contributor
timestamp: July 18, 2013, 11:36:59 am
content: Quote from: David_AVD on July 18, 2013, 04:55:22 amI do not like it when the Si prefixes change for candle power.

author: Grayfox
author_rank: Regular Contributor
timestamp: July 19, 2013, 08:26:12 am
content: Replaced resistors with CCRsDoubled amount of LEDs

author: David_AVD
author_rank: Super Contributor
timestamp: July 19, 2013, 08:47:29 am
content: Sorry, still a fail. Go back and reread the part about putting the LEDs in series strings.

author: Grayfox
author_rank: Regular Contributor
timestamp: July 19, 2013, 09:22:02 am
content: I really want to avoid using them in series.But it looks like i have no choice.

author: David_AVD
author_rank: Super Contributor
timestamp: July 19, 2013, 09:39:39 am
content: Why are you against putting them in series? If you design this properly, the LEDs won't fail.

author: digsys
author_rank: Unknown
timestamp: July 19, 2013, 10:01:59 am
content: Quote from: GrayfoxJust about every single Vehicle LED lamp, Traffic light, house and factory light etc etc run at least 2-3++ parallel strings of series LEDs.They wouldn't be doing it in millions upon millions of systems if it was a problem - at all. And use a constant current device for each,as has been pointed out - it's just an LM317L and 1 resistor.

author: Grayfox
author_rank: Regular Contributor
timestamp: July 19, 2013, 10:14:27 am
content: My old cars center LED brake lamp had LEDs fail one at a time

author: David_AVD
author_rank: Super Contributor
timestamp: July 19, 2013, 10:47:20 am
content: Quote from: Grayfox on July 19, 2013, 10:14:27 amEarly LED spoiler lights were somewhat dodgy.

author: Grayfox
author_rank: Regular Contributor
timestamp: July 19, 2013, 11:28:55 am
content: It was from 1987.And it did use through hole LEDs.

author: mariush
author_rank: Super Contributor
timestamp: July 19, 2013, 12:11:19 pm
content: If you're that concerned about it, just put them interleaved1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5It's just going to be more traces and probably some jumper wires in some places. this way if one segment dies, it's just the brightness that will go down a bit.Or put 2-3 strings in parallel on the pcb, but each independent and limited by its own current regulator:1 1 1 1 12 2 2 2 2If one dies, the other will still light up the spot, it's just going to be a bit less bright.

author: Grayfox
author_rank: Regular Contributor
timestamp: July 19, 2013, 12:56:08 pm
content: Put them in series

author: David_AVD
author_rank: Super Contributor
timestamp: July 19, 2013, 10:11:57 pm
content: Much better. Now you need to beef up all the tracks as they appear to be hair width at present. I'd also do each LED-LED track as a dog-leg instead of a single (angled) line.

author: Grayfox
author_rank: Regular Contributor
timestamp: July 19, 2013, 11:00:18 pm
content: the tracks are 0.512mm wide.Even if i made them 2mm wide they come out the same in the PDF.Since i am using Current Regulators would the current not be now divided by 5 to give 4mA for each LED.

author: mariush
author_rank: Super Contributor
timestamp: July 19, 2013, 11:31:51 pm
content: No, each led gets the same current.

author: Corporate666
author_rank: Unknown
timestamp: July 19, 2013, 11:54:27 pm
content: MUCH better.5 times the voltage, same current = same power to each LED.Is this PCB going inside an existing housing? And is this something you are planning to make commercially or is it just a one-off for yourself? If commercial, you need to handle overvoltage events, reverse connection and (most importantly) load transients.Only one suggestion - not sure what your CAD program has for capability, but you may as well make the tracks as big as you can. I'd make the power and ground rails 1.5-2mm wide, then use copper pours to connect each LED to the next. This will help with heat dissipation. I know 5mm LED's don't dissipate a ton of heat, but that which they do dissipate goes through the legs. 60 LED's at, say 2V/20mA is 2.4 watts of power. If those LED's are 20% efficient (wild ass guess), that's still about 2 watts that need dissipated. You have a lot of free board area - may as well get that heat into the PCB where it can spread out a bit - doesn't cost you anything and fatter traces might be more reliable too.

author: Grayfox
author_rank: Regular Contributor
timestamp: July 20, 2013, 01:46:47 am
content: It will go inside this housinghttp://www.ebay.co.uk/itm/2006-2011-Honda-Civic-8gen-FN2-Type-R-HIGH-LEVEL-ROOF-MOUNTED-BRAKE-LIGHT-OEM-/290942149735?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item43bd80e467I am using Kicad.

author: Grayfox
author_rank: Regular Contributor
timestamp: July 20, 2013, 01:43:16 pm
content: Like this?Used zone fill in kicad.

author: Corporate666
author_rank: Unknown
timestamp: July 20, 2013, 06:52:40 pm
content: Quote from: Grayfox on July 20, 2013, 01:43:16 pmNo, that one has the copper isolated from the connections. You want the maximum amount of copper connected to the LED leads as possible. Like this:

author: Grayfox
author_rank: Regular Contributor
timestamp: July 21, 2013, 11:38:35 am
content: Either kicad cant do that sort of thing or i do not know how to do that.Fill zone dont go near pads.

author: Bloch
author_rank: Unknown
timestamp: July 21, 2013, 01:41:21 pm
content: Quote from: Grayfox on July 21, 2013, 11:38:35 amJust make the tracks 1 or 2mm thick that will be fine.

author: Thor-Arne
author_rank: Unknown
timestamp: July 21, 2013, 02:07:01 pm
content: Quote from: Grayfox on July 21, 2013, 11:38:35 amYou need to select the net that the zone is going to connect to in the copper zone properties dialog.Also, those nets should be named in eeschema.

author: Grayfox
author_rank: Regular Contributor
timestamp: July 21, 2013, 09:37:29 pm
content: No nets available, and never used a schematic for it.

author: Thor-Arne
author_rank: Unknown
timestamp: July 21, 2013, 09:49:11 pm
content: Then I assume you skipped making a schematic for this and just draw it directly in pcbnew.You really need to follow the proper steps to be able to use all functionality in pcbnew.For the zones to connect to pads it has to know which pads.This is achieved by making a schematic and naming the nets (Vcc and GND), then generating a netlist, assigning footprints to the schematic symbols.And then importing this into pcbnew.EDIT: Typo

author: Grayfox
author_rank: Regular Contributor
timestamp: July 22, 2013, 01:33:58 pm
content: I had made this design in Pad2Pad.But since it can only save in its own version of .pcb i just used the same kind of template for the layout.I plan on making less than 10 of these boards and needed a easy to use program that was free and could save in gerber

author: mariush
author_rank: Super Contributor
timestamp: July 22, 2013, 02:32:47 pm
content: Diptrace is free for non-commercial purposes as long as you don't have more than 300 pins (and I don't think you do here. Dave can probably hook you up with a free license that raises the limit to 500 pinsThe free 300pin limited version (and unlimited 30 day trial versions) you can download from their site:http://www.diptrace.com/download.phpKiCad is otherwise a good open source program that lets you do all of the above but it may take you a bit more time to learn to use it, compared to diptrace.