Home     Forum     Projects     Experiments     Circuits     Theory     BLOG     PIC Tutorials     Time for Science

### Author Topic: Pcb design for car led lights  (Read 10444 times)

0 Members and 1 Guest are viewing this topic.

#### filarhos

• Newbie
• Posts: 22
##### Pcb design for car led lights
« on: October 25, 2013, 12:05:00 PM »
Hello pcbheaven.
My original intention was to replace the conventional light bulbs with multiple leds covering the hole compartment. But when I saw the crypton project I liked it very much and some technical difficulties I faced, trying to open the compartments, lead me to decide to use 1 watt and 3 watt high power leds.

So to the lights I have to design
1 white for the reverse
2 yellow/orange flash lights (one each side)
1 for fog light
2 red for position
2 red with dual use stop and for position.

Till now I am facing some problems

First of all what led driving method to use, at crypton project Panagiotis used mosfet but he said that if he design it again he would use Buck-Regulating LED Driver.
Secondly, for the motorcycle he used 1w for the flash lights and 3w for the stop/position, I dont know if it would be enough for the car (I am thinking of trying that and if it is not sufficient change it to higher wattage leds).
The power supply will be ~12.5v when the engine is off and ~14.5v when the engine is on. Will it make any difference in the design?
About the danger of something (e.g. mosfet) getting really hot. The pcb will be inside the trunk compartment with little airflow (there is an option to be glued between metal away from the carpets).
Last but I am very troubled with that is the fog light. I have used it only 3-4 times the last decade but will the led have the same result through the fog with the conventional light?

The level of my expertise in electronics are bellow zero  so my first design for 12 leds per compartment was very simple with just resistors so I am trying to understand the schemes and the theory Giorgos write in "LED driving and controlling methods".

Any help will be welcomed.

#### kam

• Hero Member
• Posts: 1849
##### Re: Pcb design for car led lights
« Reply #1 on: October 25, 2013, 18:40:51 PM »
Hello filarhos, welcome to our forum,

I definitely suggest buck regulators. You can find constant current regulators at ebay for 1.5 each (yes!)
http://www.ebay.com/itm/Driver-1-3-1W-3W-LED-for-DIY-300mA-Constant-Current-waterproof-high-power-/171083776288?pt=LH_DefaultDomain_0&hash=item27d5634120

Select the proper one for your design (300mA i suppose). This way you save yourself from the heat dissipation headache!

Quote
The power supply will be ~12.5v when the engine is off and ~14.5v when the engine is on. Will it make any difference in the design?
As long as your LEDs have less than 12V forward voltage and you use a buck constant current regulator, no there is n o problem

Quote
Last but I am very troubled with that is the fog light. I have used it only 3-4 times the last decade but will the led have the same result through the fog with the conventional light?
I suppose so. I'm not quite sure though. You will need to test this... But... I wonder where are you going to find fog in Greece to do the tests

Quote
I am trying to understand the schemes and the theory Giorgos write in "LED driving and controlling methods".
Actually with the buck regulators you do not need to understand the theory. Using resistors for this project is not the right way to go. Too much heat and the voltage difference will cause more troubles.

#### filarhos

• Newbie
• Posts: 22
##### Re: Pcb design for car led lights
« Reply #2 on: October 25, 2013, 19:47:26 PM »
Thank you for the welcoming!!!

Quote
I definitely suggest buck regulators. You can find constant current regulators at ebay for 1.5 each (yes!)
http://www.ebay.com/itm/Driver-1-3-1W-3W-LED-for-DIY-300mA-Constant-Current-waterproof-high-power-/171083776288?pt=LH_DefaultDomain_0&hash=item27d5634120

Select the proper one for your design (300mA i suppose). This way you save yourself from the heat dissipation headache!

Those are for 85v-265v they are not going to work with the car voltage or something i am missing.
Do you mean something like
But they have a great disadvantage they are ready to installed. I want to create something on my own (I am a huge fan o diy).

Quote
As long as your LEDs have less than 12V forward voltage and you use a buck constant current regulator, no there is n o problem

So when I am calculating lets say the mosfet how much volt am I going to count 12.5 or 14.5? (for dissipation purposes)?

Quote
I suppose so. I'm not quite sure though. You will need to test this... But... I wonder where are you going to find fog in Greece to do the tests
I think you are right but I want to be perfect. For testing purposes only I am going to invite my smoking friends and I will have to boxes of cigars.

Quote
Using resistors for this project is not the right way to go. Too much heat and the voltage difference will cause more troubles.

I figure it out with a slightly burnt finger when a resistor got very hot.

#### filarhos

• Newbie
• Posts: 22
##### Re: Pcb design for car led lights
« Reply #3 on: October 25, 2013, 20:23:58 PM »
Is it better to make one pcb for each led?
The flashing leds, the fog light and the reverse have to be separately but for the pairs of the positioning and the brake/positioning I can make only two pcbs for each pair.

For the first test I am thinking in using
1watt for flashing leds (yellow), reverse led (white) and positioning led at about 200mA
and 3 watt for stop/positioning (500/200 mA) and fog (500 mA)

For the brake/position led at the mosfet schematics I will replace Rs with two in parallel Resistances? I am a bit confused about that.

For the fog light I see that is very powerful so I will give the same as the stop led.
(I haven't completed with the calculations)

Just for curiosity how can I use a buck regulator like
with the gold screw adjust the amp or the volt?
And for the led that has dual action brake/position how can I use it?
« Last Edit: October 26, 2013, 01:06:02 AM by filarhos »

#### kam

• Hero Member
• Posts: 1849
##### Re: Pcb design for car led lights
« Reply #4 on: October 26, 2013, 12:23:18 PM »
Ok, here is the fact. There are 2 types that you need first to learn about power supplies - The constant voltage and the constant current. Since LEDs are current devices and are extremely sensitive to slight (literally slight) voltage changes, you need a constant current power supply. This type of PSU provides always the same current regardless of the voltage that it needs to develop. So for example, you have a 1W LED and a 3W LED, both rated at 350mA. The 1W led needs 3volts and the 3V needs 10V (example). If you connect each one of them on the 300mA constant current PSU, it will always deliver 300mA, but for the first LED it will have 3V output and for the second it will have 10. The best part is that the voltage is automatically gained by watching the current (that's why it is called constant current).

So now that you know what you're looking for, you can search again. The LM2596 PSU is constant voltage. By default, when we say "power supply" we mean constant voltage.  A constant current power supply must declare that it is constant current. So i meant something like this:
http://www.ebay.com/itm/DC-Converter-Constant-Current-3A-Voltage-2-30V-LED-Driver-Battery-Charger-LM2596-/370899815350?pt=LH_DefaultDomain_0&hash=item565b5a0bb6

But if you want to make something yourself, here mess around with the LMN2596-Adjustable version. This is a DC-DC buck regulator. There is no schematic for constant current, but here are 2 tips for you:
1. You can make a constant voltage with the LMN2596 to about 1.5 volts higher than your LED requirements. Then make a constant current (like Panagiotis) from this output to drive your LED
2. Be a pro! Be unique! Find a way to provide the proper feedback to the LMN2596 and turn it into a constant current. I haven't tried this myself but i bet my leg that it will work. Actually, this sounds so much interesting that i will try it myself!

And a last word about heat dissipation. Very briefly - You have something (an LED) that draws 300mA. You want to make a linear Constant Current driver (we all linear whatever is not switching mode (like buck), so linear is the solution that panagiotis uses). Linear drivers will dissipate any excess of power as heat. So, what is this excess of power? Since current is constant, it only depends on the voltage. Multiply the difference between your power supply and the LED voltage with the current:
Power supply: 12V
LED voltage: 3 V
LED current: 300mA

Voltage diff: = 12-3 = 9
Power dissipation = 9 x 300 = 2700mW
That is 2,7W!!!
As a rule of the thumb, up to about 1.2W the TO220 package works fine without heatsink. At 2W you get a slight burn. At 4W you can cook,

So, what you wanna do is this: use a buck regulator to drop voltage from 12 down to say 5volts. Then use a linear constant current (like panagiotis). The power dissipation is radically decreased:
Voltage diff = 5-3 = 2
power dissipation = 2x300 = 600mW

The TO220 will barely heat up...

#### filarhos

• Newbie
• Posts: 22
##### Re: Pcb design for car led lights
« Reply #5 on: October 26, 2013, 15:22:57 PM »
Now I can understand about the LM2596.
Let me tell you what I understood.

From the car we have ~15V (max). We connect the LM2596 and regulate it to lets say 4 V (2.2Vf and the 1.8V higher). Then we connect the mosfet circuit and give to the two leds in parallel(with double the A) (or one led) 200mA (for 1w led) or 500mA (3watt led).
So I need 5x LM2596 and the 2A are more than enough for the max 2 leds in parrallel at 0.5A each.

I am also thinking if I should use relays and use 2 or 3 LM2596. I will need to have a line from the battery (not very good idea for the plastics inside). But the LM2596 are really small so ...

For the brake/position led of 3watt we do the same but we change the mosfet circuit with two Rs resistors? (I am trying to understand the Panagioths circuit). (I am confused about the jp1 jp2 jp3 and If you could tell me what connects to what and if the scheme is for the rear red led I would be grateful). On the other hand I have a solution with a relay to add a resistor in parallel to make from 200mA to 500mA for the brake (a 12v 5A max will be perfect I think, I also have to see the signal is 12v or negative).

Question: The LM2596 (or buck converter) gives always the Voltage we choose 1.23V-30V and provides the current that the circuit will demand with max 3A?

You bet your leg that it will work but not bet it that I will do it to work

Thank you very much for time I am in really deep waters but I am stubborn and I want to make it work. Next I will try a flasher for my 7-pin flasher, so I will be around a lot!!!

Έχω αρχίσει και το γουστάρω πολύ. Σε ευχαριστώ πολύ Γιώργο!!!
« Last Edit: October 26, 2013, 18:57:15 PM by filarhos »

#### filarhos

• Newbie
• Posts: 22
##### Re: Pcb design for car led lights
« Reply #6 on: October 30, 2013, 11:57:03 AM »
Hi again, I managed to do a design (attached).
Again some questions the code names of the mosfet (TO220 ? )and the transistor.
What resistor should I put to RG 10K will be good?

Can I use relay like this http://www.ebay.com/itm/New-Mini-SONGLE-VDC-Power-Relay-PCB-type-12V-DC-coil-SRD-12VDC-SL-C-/281161305588?pt=LH_DefaultDomain_0&hash=item4176850df4 ?

And the wattage of the resistors if my calculations are correct for the Rs we have 0.14watt and for the Rs' 0.3watt.

How do you think?
« Last Edit: October 30, 2013, 12:11:34 PM by filarhos »

#### kam

• Hero Member
• Posts: 1849
##### Re: Pcb design for car led lights
« Reply #7 on: November 30, 2013, 12:36:32 PM »
10K will be fine. After all the mosfet is a voltage controlled transistor, so current does not really matter. TO220 is the package type, not the mosfet code. For example, the TIP142 has package TO220...

This relay will be ok for your design.

RS is 3.5 Ohms for 200mA = 0.2x0.2x3.5=0.87 mW
So you need at least 1W. I'd rather say that you overpower this resistor. use at least 2W. If you have space get a 5W resistor. The more the merrier

Same stands for RS'. Always overpower if you make a one-time project with no commercial value. I mean, if you plan to make it for yourself you do not need to consider cost if the difference is pennies and dimes. 5W resistor are kinda big, so make sure the fit first.

#### filarhos

• Newbie
• Posts: 22
##### Re: Pcb design for car led lights
« Reply #8 on: December 21, 2013, 01:20:20 AM »
10K will be fine. After all the mosfet is a voltage controlled transistor, so current does not really matter. TO220 is the package type, not the mosfet code. For example, the TIP142 has package TO220...

This relay will be ok for your design.

RS is 3.5 Ohms for 200mA = 0.2x0.2x3.5=0.87 mW
So you need at least 1W. I'd rather say that you overpower this resistor. use at least 2W. If you have space get a 5W resistor. The more the merrier

Same stands for RS'. Always overpower if you make a one-time project with no commercial value. I mean, if you plan to make it for yourself you do not need to consider cost if the difference is pennies and dimes. 5W resistor are kinda big, so make sure the fit first.

Thank you!!!

You have lost me!!!
P=I^2*R so 0.2*0.2*3.5 (accually I change it 3.3ohm for 0.21 A) is equal to 0.14W

I have 1 watt resistances and I can use four of them for the 2watt.

#### kam

• Hero Member
• Posts: 1849
##### Re: Pcb design for car led lights
« Reply #9 on: December 22, 2013, 08:51:14 AM »
ops! stupid of me I have this slight problem with numbers that comes back to me every now and then from when i was a child and some times i make such fatal mistakes.

#### filarhos

• Newbie
• Posts: 22
##### Re: Pcb design for car led lights
« Reply #10 on: February 27, 2014, 20:16:09 PM »
After almost two months a managed to continue with this project.
I have alread made my first testing leds and I am trying to find a way to mount them in my car. The great thing it that I did!!!

Some notes that I cannot explain:
Fisrt of all I am not sure about the voltage of the leds every supplier gives his own voltage.
red 1w  1.8v to 2.4v or 2.2v to 2.75v ?
white 1w 3v to 3.2v or 3.2v to 3.8v ?
the same with 3watt leds.

For the 1w red led I have measure with 3.3ohm resistanse, 4.14v from the LM2596 to the led: 0.12A and 1.93. My calculations give that the resistanse only draws 0.396v instead of 0.7v I though.

I also purchased some LM2596 with current and voltage regulator to drive 1watt led white (I have a problem with space and without the extra pcd they fit). So I am trying to regulate first the voltage without the led on it at 3.2v and then the current on the circuit but I cannot control it; it stops to rise and I have to raise the voltage in order to increase it. I had to go to 3.33v to have 0.2A. I f I want 0.3A I have to raise the voltage to 3.6v or so. Why that is happening, I am thinking something with the led but I am not sure.

The instructions of LED constant current driver:
1. Make sure LED current and the maximum operating voltage;
3. With a multimeter ( to 10A) Measuring output Short-circuit current, Also adjust the constant regulator allows Output current reaches a predetermined LED working current value;
4. Adjust Constant regulator allows Output voltage Reach LED maximum working voltage;
5. Connected to the LED, test.
(1,2,3,4 take steps input connect power, output do not connected LED.)

I am following the steps at
s2-> 5v  s3-> 0.3A  s4->3.2v and when I use the multimeter (cutting between the + of the led) to check the amp of the circuit I have ~0.11A. I should have 0.3A?

P.S. The Lm2596 with the three functions (volt, amp and charge) has leds of giving feed about the charge. Can I use those indicator leds and the charge regulator to understand something for my driving leds? Really the best way to describe what I am thinking.

Thank you!!!
« Last Edit: February 27, 2014, 22:59:34 PM by filarhos »

#### kam

• Hero Member
• Posts: 1849
##### Re: Pcb design for car led lights
« Reply #11 on: March 03, 2014, 11:18:19 AM »
w/b filarhe,

About LED voltage and current, LEDs are current devices. Which means that you always want to work with their current. The voltage is way to sensitive for one to work with. Plus it is not always stable. Use a current supply for the LEDs and the voltage will adjust by itself. As for the specifications, i've noticed myself big differences which i suppose they come from the material used for the PN contact. To measure the voltage yourself power the LED and measure the current instead. When it reaches the required level (say 20mA or 18mA) then you measure the voltage across it. Especially high power LEDs! Only with current supplies! Normally you should not even care about the voltage of the LED, only the current.

Regarding your steps. Step 4 is just a remark, you should not do something on step 4, i mean you should not adjust the voltage to the LED voltage. The 3.6 volts that you mention are normal for a 1W white LED. So, you follow steps 1 and 2 and 3. Then connect the LED. You should get 300mA at this point through the LED.

Finally, the Lm2596 is a step down voltage regulator. How are you using it as a current regulator? Do you have a circuit?

#### filarhos

• Newbie
• Posts: 22
##### Re: Pcb design for car led lights
« Reply #12 on: March 03, 2014, 12:19:53 PM »
Judging from the values I saw at ebay sellers and the papers of the known manufacturers the former have lower quality so the later have better tolerance leds.

Right now I am having to types of led drivers:

The first one is the design circuit from the 6th post with a LM2596 voltage regulator.
So from the calculations something is wrong. Should I give to the circuit more voltage in order to supply with more current? I have calculated 0.22A (with 3.3 ohm res) and I measure 0.12. The light I think is enough but the question stays

The second one is just a LM2596 current and voltage regulator like this one;
http://www.ebay.com/itm/301057888314?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

So dispite the working voltage of the led 3.6v if I give to the led 5v it will lower to 3.6?

Another question I had after some tests the dissipitation of the led increases expodentially as the current increases, the same happens at the life of the led? Because the the light that produces at 0.2A is enough for the purposes I want to use it.

Thank you Kam!!!
I am trying to understand how it works and when I think I am close something happens and I am thinking that I don't know anything.

#### kam

• Hero Member
• Posts: 1849
##### Re: Pcb design for car led lights
« Reply #13 on: March 03, 2014, 17:05:17 PM »
First of, you do not use a resistor if you use a constant current driver. The constant current driver has some quiescence voltage (say 5V). When you connect load, it will automatically drop the voltage to whatever value needed so that you get the required current. So, wen you put the 10A ammeter and you set to 300mA, then regardless of the load or the voltage of the driver it will ALWAYS drop down the voltage (hence "buck regulator") to achieve the 300mA

#### filarhos

• Newbie
• Posts: 22
##### Re: Pcb design for car led lights
« Reply #14 on: March 03, 2014, 22:01:24 PM »
With the Lm2596 with the current and voltage regulator I am not using any resistanse. I did some test and I am ok (better than ok because the new leds are brighter than the older).

With the other with only voltage regulator I am using a circuit constant current but the measures are not what I was waiting to find. Also did some tests and when I increase the voltage from the lm2596 at 5v I get 0.16A closer to the 0.22A I had in mind. But still I cannot understad why I am "losing" 0.08A like the resistor only draws 0.528v and not 0.7v from the theory.Tomorow I will try with the other resistance 2.2 ohm and also in parallel to check the results.

To be fair the 1watt red led with the 0.16A gives enough light but the numbers does not match with the result. I will be very pleased if I measure If_led=0.22A and have this "light" output but now something is wrong and I dont now what.

Let's recap:
5v from the LM2596 to the circuit.
Vf_led=1.9v                     Vm=5-1.9-0.7(resistance)=2.4v
so with 3.3ohm resistance and 0.7 I should have 0.212A at the led am I right?

P.S. I just reread the theory and Kam at your example I saw that you calculated 0.03A and you got 0.245A and at the other example 0.28A and you got 0.26A. What happened?
« Last Edit: March 04, 2014, 11:33:00 AM by filarhos »