Sequential LEDs - Voltage regulation with low dropout

Thread Starter

tomk825

Joined Dec 9, 2018
11
Hi everyone,

I'm looking for some information on how LED tail lights traditionally regulate the battery's 12 V signal or ideas on how to solve my specific problem. I'm looking to regulate the battery's voltage at a current of 1.2 Amps while still having a low dropout voltage (<=1.5 V). (My apologies if the answer is easy, I've been researching linear and switching regulators for too many hours straight so my brain is a little fuzzy, I'm going to take a break for a couple of hours to clear my mind!)

My projects involve sequential LED turn signals therefore requiring a lot (I'm using 12 pairs) of LEDs in parallel. Total current drawn (1.2 Amps) by the 12 pairs of LEDs is too high for linear regulators but most DC/DC switching regulators have too high of a dropout voltage to output 10.5 V for my LEDs when the engine is not running (my TI LM22678-ADJ buck converter needed close to 13 V to output 10.5 V but the battery's voltage is lower than that when the engine is not running). I pulled open a set of non-sequential LED tail lights and it looks like all they use is a diode, a transistor, and a capacitor without a controller..could I make something simple like this? I assumed I absolutely needed a regulator in order to always illuminate the LEDs at the same brightness and protect against voltage spikes, or is there another option?

More information: I'm designing my project to survive the absolute worst case scenario:
-ambient air temperature of 60 C (a car interior can reach this temp in the sun);
-peak current of 1.2 Amps with average of 0.6 Amps (HIGH for 350 ms, LOW for 350 ms, repeat);
-input voltage of 14.5 V (engine running);
-output voltage of 10.5 Volts (the LEDs' forward voltage);
-maximum regulator junction temp of 125 C

The thermal resistance for the regulator I use needs to be under about 30 C/W while still satisfying the characteristics I just mentioned. Don't get me wrong, I'm not asking you all to to find a regulator that works for me. Instead I'm trying to give as much detail as possible to illustrate my difficulty. Most linear regulators either can't handle the input and output voltages I'll be working with or can't handle the current.

My previous plan was to use LEDs with a forward voltage < 3 V and a forward current of 250 mA. The plan was to use 12 pairs of LEDs in parallel, each pair itself was two LEDs in series consuming 250 mA and dropping a total of about 7 V (3.5 V each LED). My DC/DC step down converter had too high of a dropout voltage and couldn't handle the power dissipation anyway despite its very good thermal resistance of 22 C/W. My new LED design cuts the power dissipation by more than half but the 2W is still too much for many linear regulators.

Thanks in advance for any help.


Tom
 

dl324

Joined Mar 30, 2015
11,013
Why don't you just use the 12V from the alternator/regulator? Use a MOSFET or BJT to switch the LEDs low side.
 

Thread Starter

tomk825

Joined Dec 9, 2018
11
Why don't you just use the 12V from the alternator/regulator? Use a MOSFET or BJT to switch the LEDs low side.
Do you mean getting rid of the regulator in my circuit altogether and just using the 12V signal as is? Is this what most LED tail lights do?

If so, the LEDs would illuminate at different brightness depending on whether the alternator is putting out 13.7 Volts or 14.5 Volts..

And I would still need some kind of over-voltage protection, no?
 

dl324

Joined Mar 30, 2015
11,013
Do you mean getting rid of the regulator in my circuit altogether and just using the 12V signal as is? Is this what most LED tail lights do?
AFAIK, they use a relay that's switched by some control logic.
If so, the LEDs would illuminate at different brightness depending on whether the alternator is putting out 13.7 Volts or 14.5 Volts..
I think you're worrying about something that you won't be able to see. The human eye response to light is logarithmic, so it'd take a difference of about 2X to be discernible.
And I would still need some kind of over-voltage protection, no?
It depends on the circuit, but likely because there can be spikes of several times battery voltage.
 

Thread Starter

tomk825

Joined Dec 9, 2018
11
AFAIK, they use a relay that's switched by some control logic.
I think you're worrying about something that you won't be able to see. The human eye response to light is logarithmic, so it'd take a difference of about 2X to be discernible.
It depends on the circuit, but likely because there can be spikes of several times battery voltage.
The difference in voltage would be noticeable with the LEDs I was planning to use:
For the sake of calculations, let's say the input voltage is 12.5 V when the engine is off and 14.2 V when the engine is running.
The goal is for each LED to pass 50 mA, at which the forward voltage is 10.5 V. So, for the 12.5 V scenario, the resistance needed is R=(Vf-Vi)/I= (12.5V - 10.5V)/0.050A=40Ohms. If this same resistor were used when the alternator is putting out 14.2 Volts, the current in each LED would be I=V/R=(14.2V-10.5V)/40Ohms=92.5mA. That's over an 80% increase in current, which translates to nearly 80% increase in brightness.

Of course, I could use LEDs with lower forward voltage so that each pair only drops a total of 6.5 V instead, however this creates heat problems as you'll see. Low forward voltage LEDs require more current to be as bright, approximately 250 mA: R=(Vf-Vi)/I= (12.5V - 6.5V)/0.250A=24Ohms. At 14.2 V, the current in each LED would be I=V/R=(14.2V-6.5V)/24Ohms=321mA, or 28% more current than when the engine was not running. This would not be noticeable. However, now I need to dissipate 7.7V*0.250A=1.9W through resistors for each of my 12 pairs of LEDs. That's 23 Watts dissipated by resistors on a 9"x0.5" board so unfortunately that won't work. Could I dissipate that power any other way? Adding a third LED in series to each pair would be way too costly.
 

dl324

Joined Mar 30, 2015
11,013
The difference in voltage would be noticeable with the LEDs I was planning to use:
I doubt that very much. Post a datasheet so we can discuss actual parameters.
For the sake of calculations, let's say the input voltage is 12.5 V when the engine is off and 14.2 V when the engine is running.
The goal is for each LED to pass 50 mA, at which the forward voltage is 10.5 V.
Why do you care about the voltage when the engine is off? Turn signals would be of little use when the engine wasn't running. If they're for hazard indication, difference in brightness don't matter.
 

Thread Starter

tomk825

Joined Dec 9, 2018
11
I doubt that very much. Post a datasheet so we can discuss actual parameters.
Why do you care about the voltage when the engine is off? Turn signals would be of little use when the engine wasn't running. If they're for hazard indication, difference in brightness don't matter.
All of the numbers I used are legitimate but you can check them if you'd like, here you go (look at the series amber LED): https://www.cree.com/led-components/media/documents/XlampMLE.pdf

And really, you think hazards brightness isn't important when the car isn't running? Hazards brightness is 100% important when your car doesn't run and its sitting on the side of the road in daylight. Having 80% less brightness could make the difference between your car being seen or not in bright daylight.
 

dl324

Joined Mar 30, 2015
11,013
And really, you think hazards brightness isn't important when the car isn't running? Hazards brightness is 100% important when your car doesn't run and its sitting on the side of the road in daylight.
You quoted me out of context. I never said that brightness didn't matter. What I said was that the difference in brightness between the LEDs wouldn't matter.
 

Thread Starter

tomk825

Joined Dec 9, 2018
11
Actually, you misquoted me:
You're right, but when the difference is 80% it's affecting the brightness when the engine is off by too much, especially when stranded on the side of the road in daylight. I did misquote you but difference in brightness does still matter. Would a difference in brightness of 300% be okay with you when you're trying to be visible on the side of a road?

If we could get the thread back on topic that'd be fantastic. Is there any way I could have a smaller difference in brightness between engine on/off without introducing huge amounts of heat in power dissipation?
 

dl324

Joined Mar 30, 2015
11,013
Would a difference in brightness of 300% be okay with you when you're trying to be visible on the side of a road?
I can't see how you could get a 300% difference in brightness. We'll have to see when we get to the current differences at high and low voltage.
If we could get the thread back on topic that'd be fantastic.
As I am the only member out of hundreds of thousands that has responded to your post, it's not likely that anyone is going to get confused.
Is there any way I could have a smaller difference in brightness between engine on/off without introducing huge amounts of heat in power dissipation?
You could use a current source, but you're going to have to dissipate power in the current source to maintain a constant current.
 

Thread Starter

tomk825

Joined Dec 9, 2018
11
I can't see how you could get a 300% difference in brightness. We'll have to see when we get to the current differences at high and low voltage.
As I am the only member out of hundreds of thousands that has responded to your post, it's not likely that anyone is going to get confused.
You could use a current source, but you're going to have to dissipate power in the current source to maintain a constant current.
Seems like my options are (1) deal with the 80% brightness difference (last resort option), (2) add a 3rd LED in series to reduce the power dissipated by the resistors to a manageable amount (adds cost), or (3) regulate the voltage using this DC/DC step down I just found that can (almost) handle the 2.5 W of power dissipation and will have a dropout voltage less than 1.4 V: http://www.ti.com/lit/ds/symlink/lm2576hv.pdf
 

dl324

Joined Mar 30, 2015
11,013
Use a current source and make LED current independent of supply voltage.

Post a schematic of your proposed LED configuration, including the circuitry that will control the LEDs.
 

Reloadron

Joined Jan 15, 2015
5,513
All LEDs I have used in automotive applications have had just fine intensity anywhere between 12 and 14.6 volts. This includes converting my bike from incandescent lamps, including the headlamp and spots, to LED lamps. As to an alternator load dump it has never been an issue.

Ron
 

Thread Starter

tomk825

Joined Dec 9, 2018
11
As to an alternator load dump it has never been an issue.

Ron
Should I be OK without any overvoltage circuit protection in my design at all? The OEM turn signal in my car is simply a halogen bulb so I'm convinced there's some kind of voltage protection in the CECM I mentioned in my previous posts, otherwise that halogen bulb would blow when voltage surges. Thank you for your help.

And yes, after looking into the LED forward voltage vs. current plots in its datasheet last week and redoing my calculations with more exact values, I found that there would be only a 30% increase in LED current between voltages of 12.4V and 14.5V. That's good enough for me.
 

Tonyr1084

Joined Sep 24, 2015
4,738
A little out of my league here, but what is being said about brightness is that it is not directly correlated to current. 80% more current does not mean 80% more human perceived brightness.

Consider this: A 1 inch square has 1 square inch of area. A 2 inch square has FOUR square inches of area. Therefore, and here is where I'm reaching a little out of my league, an LED that is running 4 times more current will only appear twice as bright. So, and I'm guessing, 80% more current may only represent a 20% increase in brightness. Remember, I said "I'm guessing". But I think I'm either correct, or very close to being correct. Otherwise I would not offer my observations.
 

Reloadron

Joined Jan 15, 2015
5,513
Should I be OK without any overvoltage circuit protection in my design at all? The OEM turn signal in my car is simply a halogen bulb so I'm convinced there's some kind of voltage protection in the CECM I mentioned in my previous posts, otherwise that halogen bulb would blow when voltage surges. Thank you for your help.

And yes, after looking into the LED forward voltage vs. current plots in its datasheet last week and redoing my calculations with more exact values, I found that there would be only a 30% increase in LED current between voltages of 12.4V and 14.5V. That's good enough for me.
I wouldn't worry about it. Something I did observe with a string of 50/50 RGB LEDs was running them on PWM and monitoring the PWM on a scope I could not perceive any brightness change for the last 20% of my signal. Maybe even a little more. I converted my bike to several LEDs and never a problem and the bike voltage swings much like an automobile.

Ron
 

Reloadron

Joined Jan 15, 2015
5,513
A little out of my league here, but what is being said about brightness is that it is not directly correlated to current. 80% more current does not mean 80% more human perceived brightness.

Consider this: A 1 inch square has 1 square inch of area. A 2 inch square has FOUR square inches of area. Therefore, and here is where I'm reaching a little out of my league, an LED that is running 4 times more current will only appear twice as bright. So, and I'm guessing, 80% more current may only represent a 20% increase in brightness. Remember, I said "I'm guessing". But I think I'm either correct, or very close to being correct. Otherwise I would not offer my observations.
I guess maybe after we return from the holidays I could find a photocell and place it a fixed distance from a LED and see what the photocell sees. My observation in my above post is my eyes could not perceive the change in brightness from about 75% to 100% using PWM.

Ron
 

dl324

Joined Mar 30, 2015
11,013
A little out of my league here, but what is being said about brightness is that it is not directly correlated to current. 80% more current does not mean 80% more human perceived brightness.
The information is in the datasheet. The OP was talking about this LED in a different thread - Cree Xlamp MLE.

He eventually determined that voltage extremes would give him a 30% difference in current. Starting out at about 150% relative luminous flux (brightness), it would need to increase to about 300% RLF to be noticeable.
upload_2018-12-22_7-54-10.png
 

dl324

Joined Mar 30, 2015
11,013
find a photocell and place it a fixed distance from a LED and see what the photocell sees. My observation in my above post is my eyes could not perceive the change in brightness from about 75% to 100% using PWM.
Silicon photo detectors have a wider spectral response than the human eye, with a peak sensitivity not corresponding to the human eye.
upload_2018-12-22_9-38-46.png
Human eye range is 400-700nm with peak response around 555nm.
upload_2018-12-22_9-45-36.png
(From Wikipedia)

And conflicting data from physicsClassroom: https://www.physicsclassroom.com/class/light/Lesson-2/Visible-Light-and-the-Eye-s-Response
upload_2018-12-22_9-49-37.png
 
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