# Homemade 15 emitter high-CRI Rebel LED flood light with red and cyan boost



## jeffosborne (Jun 14, 2010)

For a while now I have been using neutral-white Rebel LEDs to build small portable lights to use for photography. They tend to be used for table top photography or as fill-in lights for existing lighting, with 500 to 1000 lumen output. When the new high-CRI Rebel emitters became available recently, I was inspired to build a light bright enough to use with a diffuser or umbrella as a primary studio light. I made use of a suggestion made by CPF forum member JTR1962 to add cyan and red emitters to increase the CRI. Thanks, JTR1962!

This new light uses 15 of the Rebel high-CRI (color rendition index) emitters, part LXM3-PW51, which have a typical CRI of 85. It also has two red and one cyan emitter to further boost the CRI. A toggle switch on top of the light sets the output to either 15 white emitters only, or 12 white with 2 red and one cyan emitter. I wanted to compare the difference that the color LEDs would render. The top panel includes a power on-off switch, rotary brightness dial, and four 3mm indicator LEDs: yellow power on, blue fan on, red battery low, and red overheat. The all-aluminum enclosure is 4.5” long, 2.5” wide, and 2.625” tall, less the feet and handle. It weighs 1.1 pounds. A lexan window covers and protects the emitters. All are hand made with simple tools. I did get a bench top drill press for Christmas last year (thanks Jim!) and that has improved my results over using a hand drill. 





In all-white mode the lamp produces 2500 lumens at the emitters, according to the manufacturer’s specs. With 12 white, 2 red, and 1 cyan, that drops to about 2250 lumens. I do not notice the brightness difference, and the color shift is slight but noticeable. The cyan and red LEDs are driven at 500ma maximum, all the others are driven at 850ma at full brightness. The light contains no batteries. It has a 6’ power cord that plugs into either a battery pack or 12 volt DC power supply. At full brightness it draws 4 amps. The output is continuously variable with the potentiometer, but I tend to use it on full. 

Both side panels have openings that allow air to exit when the fan is on. You can see the sides of the P4 heat sink fins there. The rear panel has several vent holes, and 3 small tapped holes used to mount the driver printed circuit board inside.




The bottom and top panels also have vent holes for incoming air. The hole near the bottom center has a threaded ¼” female light-stand fitting inside. My primary use of this light has been on a light stand, with a reflector attached. The clear rubber feet are 3M self-adhesive bumpers.





The emitters are wired as six sets of three in series. The cyan and red emitters are at the bottom row.





The first reflector I used with the light was an 18x18” sheet of textured Epson Heavyweight Matte paper mounted to foam core board. It made a pleasing soft light, but was not too efficient





Next I tried a light weight 48” umbrella reflector made by Paul C. Buff Co. with very good results.





Here’s a portrait made with only the high-CRI light. The shadows are very soft, and the skin tones look natural. The light is set to include the red and cyan emitters for this shot.





Building the light was fun, and a challenge. With about 40 watts to dissipate, I decided to use a large CPU heat sink and a 50mm, 12 volt cooling fan. Light-stand mounting is a must for studio lights, so a round aluminum adapter with the proper fitting is secured to the bottom. It is next to the fan below. My on-off switch and dimming potentiometer would need to be accommodated, as well as the control electronics, fuse holder, and some wiring. The handle was left over from a previous project, and it is included to keep the top side controls from being inadvertently reset. The plan is to use a ¼” thick aluminum rear panel, cut to the same dimensions as the heat sink, so I can drill and tap holes in each to mount the side panels. 





This is 30-06 aluminum, recommended by the salesman at the local sheet metal shop. Not too soft, but not too brittle either. It was easy to work with. I used 1/16” thick pieces for the side panels, and 1/8” thick pieces for the top and bottom panels. 





The heavier 1/8” thick pieces at the top and bottom help to make a sturdy enclosure. Once the holes are located and marked, I like to use a center punch to keep my drilling on track. Lots of filing is required after the pieces are cut and drilled, to smooth edges and adjust dimensions. 





The large heat sink and rear panel are drilled and tapped to accept #4-40 screws, which secure the top, bottom, and side panels. 





I use a grit impregnated nylon brush on my hand drill to smooth and finish the pieces. Here are all the enclosure pieces, which have been washed to remove any aluminum dust. 





The bottom panel has 5 MOSFET transistors mounted with arctic alumina epoxy. Each transistor is part of a circuit driving 3 LEDs in series. The round aluminum piece in the center is the light-stand fitting.





The LEDs are mounted directly to the heat sink with thermal epoxy. Near to the center a 50K ohm thermistor is also mounted. When the temperature reaches 140 degrees, the cooling fan comes on. If the thermistor reaches 210 degrees, the control circuitry shuts off the lights. If the fan were to fail this would protect the LEDs from overheating. The threaded spacers mounted to the corners are used to secure the lexan window.





Oh, I know it looks like a lot of wires, but it is all manageable. The 6 white wires with red ends will all be cut as short as possible and connected to one fat 16 gauge wire that connects directly to the power on-off switch. The other white wires attach to either the toggle switch or one of the transistors below. The braided wire set is the thermistor leads.





With additional assembly completed, we still have lots of wires. We added wires from the indicator LEDs and the potentiometer, and now all of these conductors will attach to the control PC board.





The control circuitry resides on a small piece of prototyping board with plated through holes at .1” spacing. The board slides into two nylon fittings at one end, and is secured with a single screw into a threaded spacer at the other end. This arrangement makes it easy to get to the PC board - I need only to remove one side panel and one screw to slide it out. 





Laying out and soldering the components to the PC board is made easier by having the schematic for the circuit drawn around a physical layout of the two comparator ICs. These quad comparators ICs are LM239N.





Five of the eight comparators are used for the linear current regulators driving the LEDs. Two comparators are used with the thermistor to turn on the fan as needed, and shut the light off if overheated. And one comparator looks at the incoming supply voltage and lights the ‘low battery’ indicator if it drops below 10 volts. My previous lights have used a small signal transistor across the sense resistor to regulate the MOSFET, which requires a .5 volt drop across the sense resistor – not very efficient. Here the small signal transistor is replaced with a comparator, so my sense resistor can be very low ohm - .05 ohm 1% is used. The voltage drop across this resistor is only .045 volts with the maximum 850ma current flow – much more efficient. The blue components at the top are trim potentiometers for the adjustment of low voltage, fan on, and overheat detection.





This is the completed unit with one side panel removed. The enclosure size worked out well for the space required. After making initial adjustments, it has worked without a hitch.





The quality of light is improved over my other standard neutral-white lights, for most subjects. I really notice the skin tones, lips, and any warm or brown tone to be improved. This has been a worthwhile project!

Before I built the light, I knew I would need a larger battery pack for portable use, so I built one in a Bud Industries NEMA rated, waterproof transparent enclosure, with nine 18650 Li-Ion calls in a 3S-3P configuration. It powers the new light for almost 2 hours on full brightness.




This pack has 3 switched battery outputs and two variable regulated outputs for 0 to 850ma. I will post this project separately in the batteries section of the forum.

Keep your batteries charged!
Jeff O.


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## darkzero (Jun 14, 2010)

Holy crap!  

Excellent work, I can't imagine how much time you've put into this one & your others. I always enjoy your extraordinary extreme builds. Hope to see more.


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## jamesmtl514 (Jun 14, 2010)

super nice and clean build!:thumbsup:


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## Th232 (Jun 14, 2010)

Very well done!

Nice and clean, you should be very proud of it.:thumbsup:


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## Icarus (Jun 14, 2010)

jamesmtl514 said:


> super nice and clean build!:thumbsup:


 
+1  :twothumbs


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## griff (Jun 14, 2010)

shizzle.........


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## HarryN (Jun 14, 2010)

As soon as I saw the words "High CRI Rebel", I knew who wrote this thread.  Nice work Jeff - very nice.

I was curious if you can enlighten me a bit more on the visual results, for the obvious reason that I am doing a build with the same rebel p/n in a flash light, but haven't tested it as far along as you have.

One of the criticisms often put on LED lights is that the light is "flat". I am still trying to define this, but suspect it is mostly an attempt to describe that the light lacks broad color spectal distribution. (in other words, a lot of missing spectra in the light).

It is also possible that it refers to how the light is somehow partially polarized, collimnated, or diffused, but I am not at all sure.

I was wondering if you could comment or help me visualize a few things:

a) How do photos actually come out with just the high CRI LEDs vs the additional red and cyan? Would this show up on a monitor, or just with a printed picture?

b) Any comments on the "flat" aspect, regarding if it is from spectral distribution or is this solved with the umbrella reflector / diffuser?

c) Are the added colors really for skin tones or do they affect other aspects?

Thanks for posting about the project. It is such a great photography tool.

HarryN


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## PCC (Jun 14, 2010)

Very nice build. I have a Sherline miniature mill in my garage and your results with a drill press make my best efforts look poor by comparison!

My only criticism is that you used a polycarbonate lens (polycarbonate is the class of material, Lexan is a trade-name polycarbonate product). Acrylic sheeting offers 95% light transmission compared to UCL (98%). Polycarbonate is less, at about 90%. Acrylic is cheaper, too.


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## jason 77 (Jun 14, 2010)

WOW very impressive!! I have been looking forward to your post on this light of yours.

I agree drill presses are so much nicer to use than just a drill for stuff like that. Can't tell if you used any or not but a little 3 in 1 oil I find it cuts much smoother and makes the bits last longer...

I have a few questions,

-How are you using the pot to adjust the LEDs output? 

-Are you using a version of Dan's constant current from instructables, or is the constant current entirely in the battery pack you are using?

-could you maybe go into a little more detail about using the comparator with the termistor and voltage sensor?


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## ^Gurthang (Jun 14, 2010)

That is one VERY impressive studio light. 

Very natural lighting results. Thanks for showing us your work.


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## wquiles (Jun 14, 2010)

Amazing project, and great job on the pictorial documentation - that was awesome :thumbsup:


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## jeffosborne (Jun 14, 2010)

Thanks for the many words of encouragement about my light! 
At PCC's suggestion I will want to fashion a new front lens from the acrylic material. 5% more light sounds good. I've seen it available at the framing shop. Thanks for the tip.
HarryN posed a few questions:
a) How do photos actually come out with just the high CRI LEDs vs the additional red and cyan? Would this show up on a monitor, or just with a printed picture?
It is very content driven. Photo subjects that include pink or red or brown are noticably enhanced. Subjects without these hues are not so affected. I'll post a couple of samples soon and let you tell me if you notice.

b) Any comments on the "flat" aspect, regarding if it is from spectral distribution or is this solved with the umbrella reflector / diffuser?
LED lights can make very hard shadows - like a camera flash does. The umbrella softens the light. But I sometimes think of 'flat' as being flat color. No pop to the saturation of color in the photo. LEDs can give that reult due to the uneven spectral distribution. Higher CRI can correct that.

c) Are the added colors really for skin tones or do they affect other aspects?
I don't notice the cyan helping much, but the warmer tones are more prevalent in the subjects I shoot, so I see the effect of the red in more of the images.

Hey jason77, thanks for the tip on the drill bit lube - I had not considered that. And you asked:
-How are you using the pot to adjust the LEDs output? 

The pot controls a voltage level given to one input on the comparator:




The brightness control is the 10K pot near the center. 

-Are you using a version of Dan's constant current from instructables, or is the constant current entirely in the battery pack you are using?
Dan's circuit is shown above, and my variation. My new circuit is present in both the battery pack and the high-CRI light.

-could you maybe go into a little more detail about using the comparator with the termistor and voltage sensor? 
I will strive to make that portion of the schematic available soon!

Jeff


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## jtr1962 (Jun 15, 2010)

Very professional job, and I'm glad my ideas of mixing emitters were of some help! I pretty much figured the bulk of the color rendering improvement would be due to the red emitters, with the cyan adding a small amount perhaps only apparent when viewing certain colors. I'll also add that since you started off with emitters with relatively good CRI to begin with, the relative improvement by adding the reds and cyans is likely not quite as much it would have been starting off with standard CRI emitters. I'm still pleased however that it was a readily noticeable improvement even with CRI 85 as a starting point.


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## PCC (Jun 15, 2010)

jeffosborne said:


> At PCC's suggestion I will want to fashion a new front lens from the acrylic material. 5% more light sounds good. I've seen it available at the framing shop. Thanks for the tip.


No problem. There's a store local to me that sells that stuff and it's pretty cheap if you look in their scrap bin. I bought a sheet for $0.50 that measures 5X7" (IIRC) at 0.108" thick. When I was at the store they had a 2" thick sheet of the stuff on display and it was amazingly clear.

Did you use star mounted Rebels or did you solder directly to the contact pads underneath the emitters?


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## deadrx7conv (Jun 16, 2010)

Excellent build.


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## Data (Jun 16, 2010)

Wow, I love it. Very nice work.


Cheers
Dave


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## Packhorse (Jun 17, 2010)

Great job!

Any chance of a few pics of a subject lit using just the white LED's then with the red and cyan too?


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## neoseikan (Jun 26, 2010)

An awesome device it is.
How do you feel about high -cri?
Pure-white, or high-cri, which one helps more?


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## jason 77 (Jun 27, 2010)

jeffosborne said:


> Hey jason77, thanks for the tip on the drill bit lube - I had not considered that. And you asked:
> -How are you using the pot to adjust the LEDs output?
> 
> The pot controls a voltage level given to one input on the comparator:
> ...



So what is the new formula for calculating what the CC will be with your new circuit, since if I use Dan's original one .5 / .05 = 10 amps!? Or am I missing something?


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## Ra (Jun 27, 2010)

Exellent work !!! Very cleanly organised and decent build also..

But there is one thing I want to bring up:

The fans start a 140 degrees.. and shutdown is at 210 degrees ?? That's degrees Fahrenheit I hope !!??

With so many international members, maybe it's a good idea to mention that...

Keep up the exellent work !!


Regards,

Ra.


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## toby_pra (Jun 28, 2010)

Very very clean work! :naughty:


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## MikeAusC (Jul 10, 2010)

jason 77 said:


> So what is the new formula for calculating what the CC will be with your new circuit, since if I use Dan's original one .5 / .05 = 10 amps!? Or am I missing something?


 
The answer is already in the post above with the photos - 
- " My previous lights have used a small signal transistor across the sense resistor to regulate the MOSFET, which requires a .5 volt drop across the sense resistor – not very efficient. Here the small signal transistor is replaced with a comparator, so my sense resistor can be very low ohm - .05 ohm 1% is used. The voltage drop across this resistor is only .045 volts with the maximum 850ma current flow – much more efficient."

Mike


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## jason 77 (Jul 11, 2010)

MikeAusC said:


> The answer is already in the post above with the photos -
> - " My previous lights have used a small signal transistor across the sense resistor to regulate the MOSFET, which requires a .5 volt drop across the sense resistor – not very efficient. Here the small signal transistor is replaced with a comparator, so my sense resistor can be very low ohm - .05 ohm 1% is used. The voltage drop across this resistor is only .045 volts with the maximum 850ma current flow – much more efficient."
> 
> Mike



Ah so I was "missing" something LOL thanks Mike! :twothumbs


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## moviles (Jul 12, 2010)

jeffosborne said:


>



really nice the low-ohm sense resistor version


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## Bimmerboy (Jul 12, 2010)

WOW, Jeff! Your previous builds were already great, and they just keep getting better and better. Awesome power supply as well!

Fantastic work! :thumbsup:


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## jason 77 (Jul 21, 2010)

jeffosborne said:


>



Ok I just tried to emulate the circuit above, but must have messed up something. I used a 10K ohm trim pot as that is the only 10K ohm pot I had on hand and instead of a lm239 I used a lm393 which is a dual comparator instead of the lm239's quad comparator. I was not able to get consistant results though. I couldn't get the current to go above about 600mA and the pot didn't really act very consistently. I have gone my wiring over and over but can't find an error "I will take a break and go over it again, as I have a habit of screwing stuff up and not seeing why". Has anyone tried to build Jeff's circuit and gotten it to work? Also Jeff I see in the diagram, above the voltage divider it says 2.5v.... I had just assumed that was a note about what the voltage divider was putting out into the + input of the comparator, is this correct, or am I again missing something?

Thanks!


EDIT

ok I just rebuilt the circuit using a lm2901 and a 5K ohm normal sized pot... this time the dimming seems to work much more linearly, except that it now instead of the current stopping at 600mA it goes all the way up to my power supplies limit of like 1.4 amps? Also everytime I touch the pot with my bare hands the led dims? LOL I should just stop for today and go watch some TV.


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## jeffosborne (Jul 21, 2010)

Hey jason, it is good to hear you are trying out the circuit! My partial schematic's 2.5 volt reference is the output of a LM336-2.5 precision reference diode, that I include in most every design that requires a comparator. 

As the supply voltage varies, whether that is a battery's declining voltage, or some ripple or noise in a AC-to-DC power supply, the voltage divider that your potentiometer is a part of is delivering that variance to the comparator, causing the drift and error you are seeing.

You need to include the super-simple LM336-2.5. It is a 3-pin device that looks like a small signal transistor. It connects to ground, and to the supply voltage through a 5K ohm resistor, and provides a very stable 2.5 volt output. Your potentiometer and resistor then provides a stable reference voltage to the comparator. The part is 31 cents each at Future Electronics. 

The thermal portion of the circuit was asked about also. Really, I must get the whole schematic drawn out and share it. It is on my to-do list!

Cheers,
Jeff


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## Paul Baldwin (Jul 22, 2010)

Very neat and tidy build that looks like it works well too  I've another suggestion instead for drill bit lube with aluminium http://www.glasswarepro.com/2219300...t--+CRL+Tube+Wax.aspx?sgd=330d316d316d319d309
Using wax is far less messy when trying to do nice clean builds to the standard you are, it really helps stop clogging of your tools and extends life. You can also get your ally sheet with a very thin plastic film too if you pay a bit extra, simply pull it off when you have finished working on it and theres no need to burnish it afterwards 
I'd also be very interested in a whole schematic of your circuit!


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## jason 77 (Jul 22, 2010)

jeffosborne said:


> Hey jason, it is good to hear you are trying out the circuit! My partial schematic's 2.5 volt reference is the output of a LM336-2.5 precision reference diode, that I include in most every design that requires a comparator.
> 
> As the supply voltage varies, whether that is a battery's declining voltage, or some ripple or noise in a AC-to-DC power supply, the voltage divider that your potentiometer is a part of is delivering that variance to the comparator, causing the drift and error you are seeing.
> 
> ...



Ah I see, I will have to buy some LM336's then. I look forward to seeing whole schematics! Thanks!



Paul Baldwin said:


> Very neat and tidy build that looks like it works well too  I've another suggestion instead for drill bit lube with aluminium http://www.glasswarepro.com/2219300...t--+CRL+Tube+Wax.aspx?sgd=330d316d316d319d309
> Using wax is far less messy when trying to do nice clean builds to the standard you are, it really helps stop clogging of your tools and extends life. You can also get your ally sheet with a very thin plastic film too if you pay a bit extra, simply pull it off when you have finished working on it and theres no need to burnish it afterwards
> I'd also be very interested in a whole schematic of your circuit!



Cool I will have to get some of this wax. I was using the 3 in 1 oil because I had it lying around as it is very useful for a lot of things.. the wax sounds less messy though!


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## gewoodfo (Jul 27, 2010)

Hi: This was an amazing piece of work!!

I have a queton for you and the rest of the forumn: I am looking for a small LED light for a portable Tabletop studio thta will be used for taking high resolution photos of coins. I would wnat the lights to have a High CRI of >90 and color temperature of 5500 - 5600 Kelvins. I would like for the lights to be fully controlable via computer, just like the camera is going to be.

I envisoin these lights being on a gooseneck tht will allow flexible placement but rigid enough to support th elighht weights. I would prever to have the power source to be from a centralized UPS/ Battery pack that will be in the transportable case that the table top studionwill be in.

Any ideas and suggestions would be greatly appreciated.

Also, i am not a DIYer, so I need to find a commerical product or get very detailed instructions on how to build these lights. I envsion have four of 5 of the lights in the case. One a base/backlight and the other four on each side of the base. The area that the lights would need to cover would be no more that a4" x 4" area.

Thanks,

G.E.


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## MikeAusC (Jul 27, 2010)

jeffosborne said:


> You need to include the super-simple LM336-2.5. It is a 3-pin device that looks like a small signal transistor. It connects to ground, and to the supply voltage through a 5K ohm resistor, and provides a very stable 2.5 volt output.


 
If you need a 2.4 volt Zener Diode and you don't have anything else, you can use a Red 3mm or 5mm LED !

They can also be used as a voltage-generating light detector !


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## tnathletics2b (Jul 28, 2010)

Man, the craftmanship of that light is extraordinary!

Where did you get your red LED's? I am looking for some to make a night vision light of sorts. Thanks!


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## DM51 (Jul 28, 2010)

That is such a great idea, beautifully made!  The design is fantastic - it looks so completely professional. I bet you could sell plenty of these to professional studio photographers.


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## jason 77 (Sep 7, 2010)

jeffosborne said:


> Hey jason, it is good to hear you are trying out the circuit! My partial schematic's 2.5 volt reference is the output of a LM336-2.5 precision reference diode, that I include in most every design that requires a comparator.
> 
> As the supply voltage varies, whether that is a battery's declining voltage, or some ripple or noise in a AC-to-DC power supply, the voltage divider that your potentiometer is a part of is delivering that variance to the comparator, causing the drift and error you are seeing.
> 
> ...



ok I now have the LM336 and hooked it up so that it is supplying the 2.5 volts to the 220K ohm resistor, instead of the lm239 I used a lm339 dual comparator. I am still getting a dimming/brightening when I touch the POT or the heatsink on the led, or any of the other parts of the circuit? Am I doomed to not be able to use this cool circuit for myself? LOL


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## MikeAusC (Sep 8, 2010)

jason 77 said:


> I am still getting a dimming/brightening when I touch the POT or the heatsink on the led, or any of the other parts of the circuit?


 
If you're getting dimming/brightening then yoru circuit is oscillating and your body capacitance is causing it to change.

Try connecting a 0.001uF (1000pF) between the output of the 339 and the - input. 

Try shortening wiring and keeping the opamp input wires away from those carrying high current.

You need a single-earthpoint to stop oscillations.


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## Walterk (Sep 8, 2010)

Very nice!


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## jason 77 (Sep 8, 2010)

MikeAusC said:


> If you're getting dimming/brightening then yoru circuit is oscillating and your body capacitance is causing it to change.
> 
> Try connecting a 0.001uF (1000pF) between the output of the 339 and the - input.
> 
> ...



ok I re-did the circuit the best I could to use shorter wires and also to seperate the high power mosfet part from the comparator as well as use a .001uF cap like Mike suggested. I also messed up my previous post as I am using a LM393N dual comparator and not a LM339...

Still having oscillation issues I guess as it still dimms and brightens when I touch or get near the circuit. Also the max amperage the led gets is like 1.3 amps and not the 900mA that I had calculated for this circuit....


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## MikeAusC (Sep 8, 2010)

Comparators have very high gain, so you really need the capacitor across input and output, as well as a 10uF from +ve battery to earth (-ve).

Especially if you have open wiring, rather than a Circuit Board.


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## jeffosborne (Sep 8, 2010)

Sorry to leave the schematic yet undone, so many things to do it seems.

Anyway thanks MikeAusC for your insightful observations. I did go over the spec sheet for the LM393 part, and saw that it does have an open-collector output, which is required here. It also operates well with low voltage inputs, which is also needed. So the part should be fine for this regulator. Jason, I wonder about the second comparator on this IC. If it is not being used, you should tie it's inputs to ground, so they are not open to making an oscillator. The output of the unused comparator can remain unconnected. Also, what is the value of the sense resistor you are using? And, do you have the 100K pull-up resistor on the output of the comparator?

Thanks again for all the encouraging words regarding my light! It is a handy tool. It was used to shoot some baby photos lately, Noah is 6 months old:




Jeff O.


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## jason 77 (Sep 10, 2010)

jeffosborne said:


> Sorry to leave the schematic yet undone, so many things to do it seems.
> 
> Anyway thanks MikeAusC for your insightful observations. I did go over the spec sheet for the LM393 part, and saw that it does have an open-collector output, which is required here. It also operates well with low voltage inputs, which is also needed. So the part should be fine for this regulator. Jason, I wonder about the second comparator on this IC. If it is not being used, you should tie it's inputs to ground, so they are not open to making an oscillator. The output of the unused comparator can remain unconnected. Also, what is the value of the sense resistor you are using? And, do you have the 100K pull-up resistor on the output of the comparator?



Thanks for the advice Jeff and Mike, I rebuilt the circuit again and grounded the two inputs on the second comparator that isn't being used, as well as putting the .001uF cap across the output and - input of the comparator and the 10uF cap across the + and - of the power. Unfortunately the dimming/brightening problem persists, although much less so when touching the led heat sink than before... the pot still has the same amount of it though.

Below is a drawing I did of how I have the circuit hooked up.. Any thoughts?


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## brightnorm (Sep 10, 2010)

Aside from its functional considerations the unusually high quality of your work has created a device that is very aesthetically pleasing, even beautiful.

Brightnorm


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## CKOD (Sep 12, 2010)

jason 77 said:


> Thanks for the advice Jeff and Mike, I rebuilt the circuit again and* grounded the two inputs on the second comparator that isn't being used*, as well as putting the .001uF cap across the output and - input of the comparator and the 10uF cap across the + and - of the power. Unfortunately the dimming/brightening problem persists, although much less so when touching the led heat sink than before... the pot still has the same amount of it though.
> 
> Below is a drawing I did of how I have the circuit hooked up.. Any thoughts?


 
It would be better to tie one input to ground and one to something else besides ground, that way it stays in the same state. If you have them both the ground, then as the ground potential changes during switching etc... it can cause the unused comparitor to change states, and add noise to the power line, etc. All it takes is a tiny change for one pin to be above the other.


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## MikeAusC (Sep 12, 2010)

I would add another 10uF from the Comparator + input to ground.

It's standard practice to Earth all unused inputs - provided you have a good ground point.


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## jeffosborne (Sep 13, 2010)

Jason 77, your schematic looks fine EXCEPT - (1) I would get rid of the .001 capacitor at the sense resistor, and (2) check the voltage at the positive input of the comparator with the potentiometer set to max. .05 volts gives you 1.0 amp. I get .045 volts there at 850ma, for example. Also, I sent you a private message, not sure you check your forum messages... Jeff


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## Potato42 (Mar 28, 2011)

Jeff that is a great looking light you came up with! I'm wondering if you have any long term reports from using the light? How has it held up? Have you modified it any further? Have you made any newer versions? I'm working on a design myself and this light has been a huge help and inspiration. Thank you very much for detailing the build.


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## rokspydr (Mar 28, 2011)

WOW awesome build but where are the night time shot's?


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## Illum (Mar 29, 2011)

This is not a flashlight, this is an indoor studio light 

Jason 77, could it be that your issue lies in the use of a current sense resistor at 5% tolerance and an logarithmic pot as opposed to a linear taper?


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## jeffosborne (Mar 29, 2011)

Howdy Illum, Potato42 and rokspyder! 
I can report that the high-CRI light has been getting a workout. At Christmas the family group photos were better than ever. Baby Noah comes every 3 months for new pics and I really enjoy that, and I did some product photography recently for my daughter's etsy site. My son used the light for a college graphics design class project, titled the Cinderblock. rokspyder, I hope this will suffice for that ever-popular night time shot:





Here is the title page from the Cinderblock booklet he produced, also made using the high-CRI light:




The high-CRI light remains unmodified. But I have made a new light that helps to fill-in with my photo lighting setup, Hydra-3. It is a non-high CRI neutral white Rebel-ES 1000-lumen flashlight and 2,000 lumen floodlight. It does have a light-stand fitting like the high-CRI light:




I plan to post a complete write-up for the Hydra-3 soon.

Jeff O.


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## jason 77 (Mar 29, 2011)

Very cool Jeff, I look forward to seeing the Hydra-3 write up!


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## Potato42 (Mar 29, 2011)

Thanks for the follow up Jeff! That Hydra-3 looks interesting as well. I see you used reflectors in that one. I am thinking of something along the lines of both your designs combined, although a bit larger. I found a company that extrudes heat sinks in a number of different sizes to use as the base. Where did you get the aluminum and what kind is it? It doesn't appear to be annodized, but perhaps you clear coated it? Since aluminum oxidizes so readily, I'm wondering if you've noticed any changes in the finish of your light. Also if you'll indulge one more question, why did you opt to build your "driver" rather than use an off the shelf component?


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## griff (Mar 29, 2011)

Killer Hydra-3........


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## swa819 (Mar 31, 2011)

very nice! clean and pro!


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## Aepoc (Mar 31, 2011)

very inspiring!


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## onetrickpony (Apr 2, 2011)

Beautiful work, looks like an industrial piece of art. How many lumens is that putting out? Someone needs to build something like that with a bunch of reflectors or lenses and make a cannon of a search light with high cri (ha!).


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## HarryN (Apr 11, 2011)

Nice project work. I am getting the urge to built up a photo light for personal use. - I hope you don't mind if I borrow some ideas from yours.


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## jeffosborne (Apr 12, 2011)

HarryN, I would be flattered if my ideas are borrowed! Let me know if I can assist in any way. Here's a tidbit of info: my light could have supported 21 or 24 LED's and not just 18. The heatsink and fan operate rather leisurely, and could support more heat removal. More light is often good in the studio.

Potato42, the aluminum is type 30-06, purchased at a local sheet metal shop. The finish on the light has not changed much over time, but I will consider an enamel clear coat for a future light. I built my own driver because I can (electronic design is my occupation), and because I wanted functions that off-the-shelf varieties do not offer. No PWM was one design objective, another being a linear regulator, and not switching regulator.

Jeff


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## jeffosborne (Apr 12, 2011)

HarryN, I would be flattered if my ideas are borrowed! Let me know if I can assist in any way. Here's a tidbit of info: my light could have supported 21 or 24 LED's and not just 18. The heatsink and fan operate rather leisurely, and could support more heat removal. More light is often good in the studio.

Potato42, the aluminum is type 30-06, purchased at a local sheet metal shop. The finish on the light has not changed much over time, but I will consider an enamel clear coat for a future light. I built my own driver because I can (electronic design is my occupation), and because I wanted functions that off-the-shelf varieties do not offer. No PWM was one design objective, another being a linear regulator, and not switching regulator.

Jeff


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## jeffosborne (Apr 12, 2011)

Once was a pile of parts...




What fun it is to build!

Jeff


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## fnj (May 12, 2011)

jason 77 said:


> ok I re-did the circuit the best I could to use shorter wires and also to seperate the high power mosfet part from the comparator as well as use a .001uF cap like Mike suggested. I also messed up my previous post as I am using a LM393N dual comparator and not a LM339...
> 
> Still having oscillation issues I guess as it still dimms and brightens when I touch or get near the circuit. Also the max amperage the led gets is like 1.3 amps and not the 900mA that I had calculated for this circuit....


 
Jason 77, if you haven't given up, allow me a silly question. You do realize that you have to supply ground and V+ to the comparator, right? These connections are not shown in the schematics reproduced in this thread.


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## fnj (May 12, 2011)

jeffosborne said:


> ... The pot controls a voltage level given to one input on the comparator: ...
> 
> 
> 
> ...


 
I'm more than a little confused by the schematic as given.

1) At max pot setting, the + input is connected to a voltage divider of 6.667k / 226.667k, right (the parallel of 10k and 20k being 6.667k). The 2.5 volt reference, coming through this voltage divider is going to be (6667 / 226,667) * 2.5 = 0.0735 volts into the + input - right? The comparator is going to drive the MOSFET so as to make the - input approach the + input. So the voltage across the 0.05 ohm sense resistor is going to be 0.0735 volts - not 0.045 volts. That equates to a current through the sense resistor of 0.0735 / 0.05 = 1.47 amps - right? Not 0.9 amps, or 0.85 amps. So where am I going wrong?

2) What is the purpose of the 20k fixed resistor? Wouldn't the voltage divider be just fine with a single fixed resistor in series with the pot? I am guessing maybe the fixed resistor is to lessen wiper noise on the signal.

3) What is the purpose of the 100k resistor? Is the LM239 an open drain guy?


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## jason 77 (May 17, 2011)

jeffosborne said:


> Jason 77, your schematic looks fine EXCEPT - (1) I would get rid of the .001 capacitor at the sense resistor, and (2) check the voltage at the positive input of the comparator with the potentiometer set to max. .05 volts gives you 1.0 amp. I get .045 volts there at 850ma, for example. Also, I sent you a private message, not sure you check your forum messages... Jeff


 
Ok I rebuilt the circuit using a fresh LM393 and took out the .001uF as you suggested. I actually went down and bought a 10kohm linear pot from frys as I wasn't 100% sure the ones I have are linear. The circuit now works fine I think, I can touch the pot and the heat sink for the mosfet and there is no change in the light output. I took some measurements and found that when the voltage at the positive input of the comparator is .05 volts the circuit is suppling the LED with 800mA of power, however at full tilt "pot up all the way" I get .074 volts and the LED is getting 1.24 Amps... The .05 ohm sense resistor is rated at 1% tolerance but when I measure it with my multimeter "fluke" I get .03 ohms, although I am not sure how accurate the meter is when measuring resistance under 1 ohm....



jeffosborne said:


> The high-CRI light remains unmodified. But I have made a new light that helps to fill-in with my photo lighting setup, Hydra-3. It is a non-high CRI neutral white Rebel-ES 1000-lumen flashlight and 2,000 lumen floodlight. It does have a light-stand fitting like the high-CRI light:
> Jeff O.



I am curious as to which Rebel ES LEDs you are using as the highest neutrals I have seen are rated at 310 lumens at 1 amp?



fnj said:


> Jason 77, if you haven't given up, allow me a silly question. You do realize that you have to supply ground and V+ to the comparator, right? These connections are not shown in the schematics reproduced in this thread.


 Yes I do realize that  thanks for the hint though...


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## jason 77 (May 18, 2011)

testing as my last reply isn't showing up.....


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## anuragwap (Mar 18, 2012)

deleted


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## jeffosborne (Mar 19, 2012)

Hi anuragwap! The output of the LM239 comparator is open collector, which means it will only sink a circuit to ground when the output is low. It will not pull the circuit high when the output is high, so a pull up resistor is required. The 100K value is a very mild pull up, as the input of the mosfet requires little to no current, only a voltage. A lower value resistor would work, for example, but it would needlessly drain current when pulled low.

Your 8 18650's would be 33.6 volts fully charged, so be sure to check the specs on the mosfet transistor you use. The maximum gate to source voltage for the mosfet I used is 20 volts as you mentioned, so a resistor and zeiner diode could be used to limit the gate voltage. Also, because this is a linear drop down regulator, it is most efficient to keep the supply voltage just above the total voltage required by the string of LED's you are driving in series. This makes for less heat at the mosfets, too.

I did find a hot new mosfet that has a super-low on resitance: 1.2 milliohm at 195 amps! It costs a little more, but super performance. It is an International Rectifier IRLB3034PbF. I think they were $1.67 each at Future Electronics.

So what do you have in mind for your light? 
Cheers,
Jeff


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## anuragwap (Mar 19, 2012)

deleted


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## Waldur (Dec 1, 2012)

Hi!

Sorry for bringing up the old topic, but it is too good for being forgotten 
As I am building a PSU for a bunch of LEDs (driven by 700mA each) and I am allowed to use only 5VDC voltage-line, a low-voltage-drop driver is more than perfect!
I'm not into inductors, and chips that should be soldered in the oven, I find this circuit with comparator very attractive.
If you can lead me to the final solution, I will be more than pleased 
This currently used FQP50N06L is very good for me, it is available and with good price.
I will power the driver up with 5VDC from the regular power supply. And use only one Cree XRE 3W LED. Voltage drop across the LED is approx. 3.5V.
There is no need to dim the brightness manually, it means no pot will be necessary. I will test the circuit and get the right values for the right current in the output.
But what do I need is to get the possibility to turn the circuit/LED on and off via uC output, and by that I mean the controlling voltage is 5VDC, but with very low current.
One way is to use ULN buffer, in parallel, and turn the whole circuit on and off.
Another way is to keep the circuit running and let the signal from uC to the right place in the circuit to set it on or off. And maybe not use additional components at all? OK, the uC will be Arduino UNO, but using just digital I/O's to switch on and off.
It would be like a PWM signal-input, but in my solution used only for 0% or 100%.
Is it possible to achieve that?
I believe that someone has this solution in his/her head, and if it possible to get it finally drawn down, the X-mas will be much whiter 

Thanks in the advance!!!!



All the best, 


Heldur


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