Driving and dimming numerous LED strings together

Discussion in 'The Projects Forum' started by MythicalCoder, Apr 6, 2016.

Aug 28, 2015
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I have several LED strings that I need to be drive from a car battery with as few components as possible, while dimming them all together. I'm using these CREE SMD LEDs, which are rated at 50mA with a Vf of 3.8v, and am, at the moment, hooking them up in strings of two. I have found a bunch of schematics and techniques for driving one string of LEDs with dimming capabilities, but I can't seem to find any that address multiple strings. I was originally pursuing PWM dimming, but this requires the LED strings to be driven to 50 mA at all times, which itself requires an IC and a lot of external components per string. This is a problem because I am limited on available board space, and have a crowded space with the LEDs already (I'd like to use a total of 800 mA for this project).

Can anyone recommend a good, simple method of regulating multiple LED strings' power while also allowing dimming capabilities for the entire board?

2. k7elp60 Senior Member

Nov 4, 2008
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I would recommend hooking them up is strings of three, which would require 11.4 volts for the strings. If the car battery is not being charged at the time the car battery is close to 12.5 volts. If it is higher then battery v-led string/.05amps= the series resistance needed.
for 12.5 volt battery 12.5-11.4=1.1/.05= 22 ohms So each string needs a 22 ohm resistor in series. The resistor could be a 1/4W.
P=E x I, or 1.1 x .05 = .055 watts. I have a circuit that I use for dimming LEDs. It will handle much for than 800mA. It is a PWM type of circuit.
For full brilliance the strings of LED's would be on for about 95% of the time. For minimum brilliance the strings would be on for about 10% of the time. In both cases the voltage to the strings is very close the the applied voltage. I you this this what you want please let me know and
I will post the schematic for you

3. John P AAC Fanatic!

Oct 14, 2008
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I wouldn't recommend 3 LEDs in line with a single resistor, because that would give you quite a variable brightness according to the battery voltage at the moment. Look up "two transistor constant current" and you can find a circuit that gives you pretty good current control, and one transistor in the setup would also be usable for PWM control. Just drive all those transistors in parallel, and you're done.

Aug 28, 2015
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Thank you for the responses so far guys! Really appreciate it

I'm not a fan of the one resistor approach as the voltage is unknown. I've also seen wild fluctuations due to the chronic electrical problems this car is known for, dipping down to 10 volts at times (it's a common problem with this car), so I'm sticking to strings of two unless I have a way to bump up the voltage first, which itself seems excessive.

Two transistor approach sounds fairly simple. Is this the one you meant?

That question mark is where the LED string goes. If it is, how do I hook up multiple strings? Is there a way to share components, or slim down the overall count with a transistor array?

5. mcgyvr AAC Fanatic!

Oct 15, 2009
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maybe you should address the cars electrical problems before blinging it out with LEDs..
All the LEDs in the world won't make it look cool when you are pushing it home..

Aug 28, 2015
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That was horribly uncalled for and quite dickish for you to say that. For your information, the car in my garage runs as it was designed to and has undergone a complete nut-and-bolt frame-off restoration, and is now one of the most pristine examples in existence. As I stated before, it is a design flaw with the car's electrical system, not a flaw specific to the one I own. Voltage drops are extremely common in these cars, even mine.

And also for your information, I am not "blinging out" my car, and I don't appreciate your condescending attitude. Leave it at the door.

7. mcgyvr AAC Fanatic!

Oct 15, 2009
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relax there guy... just a joke.. No need to get your panties in a bunch..
What car BTW?

So now..
How many LEDs do you need total? 2 per string.. got that.. but how many strings?
How do you intend to dim them? via a potentiometer (knob), via an analog signal, via a PWM signal?
If analog or PWM describe the signal (0-10V analog, 0-5V analog, 5V PWM, 10V PWM) ?
Describe your space constraints with details?
Do you want a through hole solution or smt or mixed?
What are your prototype capabilities? Soldering/reflow/min part size (0402?) ?,etc..
Will you design and purchase a circuit board after a schematic has been defined?

Aug 28, 2015
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Didn't seem like a well-placed one to be honest, but understood. Sorry if I went overboard. Long day at work.

Anyways, moving forward:

Looking at my prior schematic (which wound up being too component-heavy), I'm using 24 strings, or 48 individual LED pieces. The math works out to 1.2 amps max current draw, since each string is 50mA each.

Yes, a pot is what I'm after. It doesn't have to be appealing, so a small screw-turn guy would work fine.

I'll have to get the measurements for you, but I'm cramming it all into a small space.

Through-hole is vastly preferred. I tried SMD last go around, and I found out the hard way that I suck at it.

Not sure what this means, but I am very handy with a soldering iron. The part has to be big enough for me to get a hold of, but most through-hole parts are, so I don't see that as an issue.

Yes, absolutely. I also need to test the light output to define a minimum so that it doesn't go any dimmer than an incandescent, so that will be another board order right there.

9. mcgyvr AAC Fanatic!

Oct 15, 2009
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What is the intention of these LEDs?
Would a single higher power LED be ok vs 48 smaller ones ?
Just trying to look at ways to drastically reducing part count/size.. 24 drivers is going to take space and I wouldn't recommend paralleling series strings which would drastically reduce part count but open the potential for thermal runaway/strings blowing out
Any reason for the 800mA limitation too?.. (So you want to run each string at 30mA maybe now vs 50mA)

The more detail you include the better the answers will be..

Aug 28, 2015
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For this one in particular, to illuminate what's behind me when the car is in reverse. I hope to expand this design with my other thread to accomplish a custom replacement for the failure-prone taillight setup (the white LED units I selected will serve as the reverse section of this).

Background on the OEM units: the manufacturer tried to make use of a cost-effective circuit board, which was made out of something that resembles cardboard with tin traces of randomly-varying sizes on it. Not even kidding either. Nuts and bolts are a common fix for the "bad connections" even! The whole car is riddled with cutting corners like this, so it quickly becomes apparent why electrical issues are so prevalent, and must be designed accordingly. Instead of simply replicating this design out of a sturdier material, I'm opting to create a modern, LED replacement.

(I know this is automotive in nature, but the only real part of the specification that makes this circuit automotive is the varying voltage. I should be able to power my test boards from a 10V DC jack, for example)

I've tried that approach first, but the high power LED units are so small, they don't disperse within the allotted space well enough, or are too small for me to handle. I need something spread out over the whole space to make it look good. I do have the option of using the back of the board for the drivers and the front for the LED strings.

Simple miscalculation -- I thought it was 800mA worth of LED strings I was using, but it appears to be 1,200mA plus whatever other current is being used to dim and/or control these guys. I'm not stringent on the amperage -- whatever we can comfortably max out at is fine by me.

As for size, the white section is 146 x 51 mm , which is the most cramped part of this unit I believe. This all being said, considering the sheer quantity of LED strings (the white ones I have mentioned make up about 1/6 of the total string count, the rest are identical red and amber units with 2.4 Vf at 50 mA) would it make sense to regulate the voltage to something like, say, 9V, then use a single resistor to control the current?

11. mcgyvr AAC Fanatic!

Oct 15, 2009
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So do you really need dimming or is that just for testing to determine desired output?
Can't you use a smaller number of higher powered LEDs then?
What about "COB" type LED setups? http://www.dx.com/p/6958-diy-10w-30...gclid=CPCNqMeB_csCFYNkhgodRFQEQg#.VwaTjfkrKHs
Do you have a lumen or mcd spec you are trying to achieve?
Do you intend to use some sort of optics on these LEDs?

again.. just trying to get you down from 24 LED drivers for something as simple as a backup light..

12. John P AAC Fanatic!

Oct 14, 2008
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Yes, but my suggestion is to drive the upper transistor off a logic-level input, not off the + supply. It would be able to control the LED current and also to function as a switch to let your PWM input (from LM555 oscillator? from processor?) control the brightness.

I think 330 ohms as the sensing resistor is wrong; to get something close to 50mA you'd want 12 ohms. (V/I = R, so .6/.05 = 12)

Aug 28, 2015
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I would like the light output to be adjustible, but it does not need to be infinitely so. Something between an OEM bulb and shy of temporary blindness. Not sure how to measure this exactly. Would it reduce part cost to use a switch instead of a pot to decide light output?

If size, power, and light distribution won't be an issue, then that approach will work. The problem I've been facing is light distribution (mostly pinpricks instead of full coverage), difficulties handling a small SMD component (I'd prefer something around "3mm round LED" size or bigger)., or large power requirements to fill in the gap.

I can visualize it, but until I can replicate it, I won't know the number offhand. (Not that I know how to measure either!)

Yes, although I'm not sure what would be best. Whatever provides nice, even coverage would be best.

14. mcgyvr AAC Fanatic!

Oct 15, 2009
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I don't understand that at all.. How are you only getting small spots of light from an LED?
Most LEDs will easily spread the light out over 120 degrees or more..
Create a drawing with a 120 degree cone coming out of each one.. Thats what you should be getting..

Aug 28, 2015
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Like this:

This was my first attempt, which utilized two 5mm round LED units to see if what I was wanting to achieve was possible. Later tests proved to be the same result. The texture is similar to tightly-packed bubble wrap.

Aug 28, 2015
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I do know what you're getting at, but it's limited by the very short distance between the LED and the lens that I am designing for, which is why I went with the array approach. I can change the lens pattern if it would help, but I don't know what to change it to exactly.

I also tried a circular-type lens, which reduced the total output (although that may be a side effect of the color mismatch -- white LED and red lens), but seemed to disperse the light better overall. Still has that dot in the middle I'm trying to avoid though. It should look like a rectangle of light, not a rectangle of a color with a dot of bright light in the middle. See here:

That second dot on the top-right is the second LED, which clearly did not get dispersed at all. Positioning the LED in the center of the circle array dispersed it, while anywhere outside of that gives it a similar effect as the above white lens picture.

17. k7elp60 Senior Member

Nov 4, 2008
481
69
I have built a number of signs with LED's in series, with a current limiting resistor in each string. The individual strings are in parallel with the dimmer output. The dimmer ckt is a 555 in a pwm circuit running about 118Hz. For maximum brilliance the output is positive for 90% of the time. For minimum brilliance the output is 0 for 90% of the time. One of the signs was made with red LED's and the individual LED's are showing a bright red circle in the center very similar to your last post. If you think this will be suitable let me know.

Last edited: Apr 7, 2016
18. mcgyvr AAC Fanatic!

Oct 15, 2009
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Ok.. I wasn't thinking you were talking about the cosmetic look of the light itself.. Thats mostly how LEDs are now besides the "optics" manufacturers mold into the lens to spread it.. Its a point light source..
Then COB LEDS might be your best bet.. Its tons of LEDS tightly packed covered to blend the output into a virtual bar of light like this..

19. Alec_t AAC Fanatic!

Sep 17, 2013
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Have you considered using a boost converter to power the LEDs? You could then use just a few LED strings, each having many LEDs in series. I think there would be a net reduction in components.