4 X Cree 36V 175mA light fixture - Page 1
Forum section: electronics / projects

author: Pappy137
author_rank: Contributor
timestamp: March 05, 2017, 05:43:11 am
content: Well,I picked a doozy for my first go. I have several Cree XLamp MHB-B LEDs (http://cree.com/~/media/Files/Cree/LED-Components-and-Modules/XLamp/Data-and-Binning/ds-MHBB.pdf) on the way. The first project and the reason I bought them is a 4 LED array, hopefully 2 x 2 that will be replace an over-the-counter microwave task light. The switch that controls the light is an electronic switch that supplies 120VAC on high and about 68 volts on low. I want all 4 leds lit on "Hi" and 2 of them on "Low", as opposed to all of them dimmed on low (I might decide to light only one on the low setting depending on the actual light output). The attached pic shows kinda what I envision. I know if I run all 4 units in series 36V x 4 = 144 volts. That's probably a little much to try to drive with 120V (until you consider RMS, AC to DC rectification and capacitor smoothing) and it doesn't answer the high/low issue. I've looked as several SMPS designs and it looks like a a lot of the components can't live with 36 volt input, much less 72. I figure I'm looking at a buck/boost current regulator, well, really 2, one that cuts off below 100V input to control 2 of the LEDs and another that will only cut off under low available current conditions. Since the 175mA is the max rating, I'll be using around 120mA x 40V (max voltage at 120mA from datasheet) x 4 = 19.2W. At 100% efficiency, that's some less than 20W. The supply circuit is rated for at least 40W@ line voltage. I don't know if current is truly the limiting factor or if heat dissipation is. Either way, if I can achieve 60% efficiency, I'll be making less heat, using less current, and making LOTS more light.I need a magical black box that can determine:If I have enough power to light 2 LEDs (or maybe just one),If I want "Low" or "Hi" (one or two lit as opposed to four),How to apply the correct voltage - to the right circuit - to ensure 120mA current is flowing through the desired LEDsThis is far beyond my expertise of going to Fry's and getting a 120 to 12V center tapped transformer, 4 diodes, and heat shrink, then correctly combining that with an On Semiconductor NSI45020 CCR and six 2V 20mA LEDs in series, parallel arrayed several deep.Thoughts, concerns, can't be done, Qs?HELP!(Please don't tell me it can't be done. I'll likely prove you wrong.)One last thought - Money is always an issue. So what? Money is NOT "the" issue herein. Proof that I have a valid idea is sometimes worth a lot more than the cost of project which will never recoup its cost. I don't really want to spend $100 on this highly specialized driver. But, I gotta figure out how to use these LEDs which will soon be in my possession.

author: Pappy137
author_rank: Contributor
timestamp: March 07, 2017, 03:21:20 am
content: This thought just occurred to me. Is the "Low" voltage being attained by half wave rectification? I'll have to test this with an LED-diode-resistor rig I suppose.

author: Pappy137
author_rank: Contributor
timestamp: March 07, 2017, 04:58:11 am
content: The low voltage for the "Low" setting is indeed half wave rectified 120 VAC. Will a standard transformer live with pulsed DC? How bad will it's performance be degraded without the reverse voltage? I know some transformers work with pulsed DC because I used to be a mechanic and an ignition coil is a pulsed DC transformer. On a V-8 running at 2500 RPMs, the points and coil would fire more than 150 times a second. All that's changed with electronic ignition. This does indeed invoke some thought as to how I should proceed.

author: thm_w
author_rank: Super Contributor
timestamp: March 08, 2017, 10:18:40 pm
content: Transformer won't work well like that, you want a constant current LED driver.The problem is dimmable LED drivers afaik are based off the triac dimming edge, so a half wave rectified input might not be seen as a dimmed input.Simple fix would be to replace the switch. But its definitely possible to design a circuit that would detect if the voltage is full AC or half wave, and switch on additional dimming on an LED driver module.http://www.ledsupply.com/blog/understanding-led-drivers/You generally want to dim all lights instead of running one or two to increase efficiency. But if you find a solution that is easier to implement using that method, it would be OK.

author: james_s
author_rank: Super Contributor
timestamp: March 08, 2017, 10:27:31 pm
content: Why don't you just buy an off the shelf LED bulb and take it apart to fit the components into your application? A few years ago I converted a formerly fluorescent torchier lamp that blew up the ballast to LED. I bought 6 cheap LED bulbs, cracked them open and removed the PCBs with the LEDs and mounted those to an aluminum plate heatsink in the lamp. Stuck the driver boards underneath.I'm all for DIY, but good quality LED bulbs are well under $10 each now, many of them are dimmable with standard triac dimmers.

author: Kjelt
author_rank: Super Contributor
timestamp: March 08, 2017, 10:56:15 pm
content: Put the led on a bench psu with cc on 175mA and look at the voltage, my experience is that they are often only 32V and raise over lifetime, if so:Buy a decent 36V smps 1A from pulse or tdk lambda.Then run all four leds parallel and use a standard lm317t as current limiter in the downside of each led to ground (so four lm317s)Use that a lot since the leds take 32V the lm317 only has 4V to consume at 175mA which is less then one Watt. If you want the two on a different current use a different resistor to limit the current further. If you want to switch you can use fets and logic to bypass some resistors, i am on an ipad but can post a schematic if you cant figure it out.

author: Pappy137
author_rank: Contributor
timestamp: March 09, 2017, 02:58:49 am
content: Here's a simplified schematic of the design I'm currently considering.When the switch is on high, the relay will be energized by a dc coil closing the NO contacts and turning on the "high" circuit. When the switch is on low, the diode will prevent current to the relay, leaving the NC contacts closed, turning the "low" circuit.I'm using On Semiconductor CCRs - 2 NSIC2050JB & 1 20mA version (http://www.onsemi.com/PowerSolutions/product.do?id=NSIC2050JB). Page 6 of this document verifies PMW dimming is possible with the CCRs (http://www.onsemi.com/pub/Collateral/NSIC2050JB-D.PDF) and this one gives an example of a half wave rectified circuit (https://www.onsemi.com/pub/Collateral/AND8146-D.PDF).On high, all 4 LED will be powered by 120mA (at whatever voltage is required) each. I'll use a full wave rectified circuit to take the voltage down to around 45V capable of 1 or 2 amps.On low, exactly one LED will be driven by a pulse width modulated 120mA to produce about 50 to 150 lumens (I'll decide how bright when I get the LEDs here). I will also experiment with reducing the current by altering the combination of CCRs.

author: Audioguru
author_rank: Super Contributor
timestamp: March 09, 2017, 03:25:19 am
content: The LEDs are tiny. How will you cool them? How will you connect wires to them? How will you fasten them?

author: Pappy137
author_rank: Contributor
timestamp: March 09, 2017, 03:55:15 am
content: The LEDs will be mounted on these after I have silver soldered the stars to a copper heatsink. The center pad of the star is monolithic with the substrate. As I understand, the copper substrate is milled down around the center pad, and the pcb center is cut out. Then both parts are bonded together, this allows direct heat transfer to the star. I have some heavy copper flashing which I'll form into a "running W", then silver solder the stars to that. Since I'm running 120mA, only about 70% of the 170mA max, heat buildup shouldn't be a problem.

author: Pappy137
author_rank: Contributor
timestamp: March 09, 2017, 08:01:31 am
content: According to this document by On Semiconductor, (http://www.onsemi.com/pub/Collateral/AND8433-D.PDF) a series of LEDs can successfully be powered directly from full wave rectified line voltage with no transformer if the total voltage needed by the string is within 45 volts of peak rectified line voltage (170V in the good ole U.S. of A.). Cree says the typical Vf @120mA is 35V. 35V x 4 = 140V. Even if you figure minus 10% Vf that's still 126V. 170 - 126 = 44V - Still within (BARELY) specs.This is an equation taken from the above mentioned .pdf.120 VAC, 38 LEDs)The power dissipation of the CCR for Figure 3 is determined by:(Vak rms) x (IREG * Duty Cycle )Vak rms = Vbridge rms - LED string VF rms(120 Vbr rms-(38 x 3.3 V LED x 0.707 ) x (30 mA x 50%) = 31 V rms x 15 mA = 465 mWWhen I plug my numbers in (using Crees max Vf @120ma instead of typical and figuring current through one 50mA CCR)(120 Vbr rms-(4 x 40 V LED x 0.707 ) x (50 mA x 50%) = 4 V rms x 25 mA = 101.3 mW power dissipation.Using 33V here's how it looks(120 Vbr rms-(4 x 33 V LED x 0.707 ) x (50 mA x 50%) = 27 V rms x 25 mA = 675 mWUsing 35V (Cree's typ Vf @120mA)(120 Vbr rms-(4 x 35 V LED x 0.707 ) x (50 mA x 50%) = 21 V rms x 25 mA = 525 mWI think I'm rethinking again.

author: Pappy137
author_rank: Contributor
timestamp: March 09, 2017, 08:44:16 pm
content: Will an AC solid state relay switch half wave rectified DC?

author: Pappy137
author_rank: Contributor
timestamp: March 16, 2017, 07:19:29 pm
content: Well, here's where I am right now.The single pole 4 throw switch is a touch pad that cycles through "High - Low - Off". I will have the On Semiconductor CCRs in series with the LEDs to limit current to 120mA.As you can see, when the switch is on High, DC current through D7 energizes the coil to the relay and closes the NO contacts, bypassing the PWM circuit. When the switch goes to Low, D7 stops the current to the relay, opening the NO contacts and closing the NC circuit. This in turn enegrizes the PWM dimmer.Now, I just have to learn how to put together a PWM circuit capable of dealing with 170VDC @ 170mA. How hard can that be?

author: Pappy137
author_rank: Contributor
timestamp: March 17, 2017, 03:44:53 am
content: Question for the experts? Since I'm not looking to vary the "dimmer", is PWM the way to go? I've got a situation more like a three way bulb than an old school rheostat dimmer switch. I have a choice of High, Low, or Off. I've read that PWM dimming preserves the optical characteristic output of LEDs because they are driven at full current for short bursts, rather than at lower currents (which can change the color temperature and CRI). Are there cons to PWM, other than the complexity? I could simply further restrict the current with a second set of the NSIC20XXJB CCRs.Can I drive something like this transistorhttp://www.onsemi.com/pub/Collateral/EN2101-D.PDFwith an LM555 (or some other IC)?The PWM circuits I've seen so far are generally limited to 24V max, (tho some claim you could successfully push it to about 40V). Is it simply a matter of providing low control voltage to an IC which drives high voltage transistors?I've learned way more than I intended to when I started this.

author: Kjelt
author_rank: Super Contributor
timestamp: March 17, 2017, 08:56:22 am
content: Quote from: Pappy137 on March 17, 2017, 03:44:53 amYes if the frequency is too low you will see this in the light (flicker).When dimmed very low (<15%) there is another effect that the light off period icw the short light on period can also be perceived as flicker.So the solution is as for instance the TI and Maxim chips do, go to 500kHz or 1,2 MHz but then you have another problem: EMI, use good groundplanes and beads to reduce the EMI and check with a spectrum analyzer.If you only need full on, one dimposition and off, I would just create a simple current source (lm317) and bypass it for full on.

author: Pappy137
author_rank: Contributor
timestamp: March 19, 2017, 02:53:13 am
content: I think MAYBE,,, I've decided how I'm gonna do it.If the light quality is acceptable at lowered current levels, the low circuit will draw less than 100mA (so maybe heat dissipation from the resistors won't be a problem) with no flicker due to the two smoothing caps while the resistors lower the voltage. Since there's only one LED to dim on Low, it will not need to be as dim as if all four were lit. If I don't like what dimming does to the light quality, I'll investigate a PWM solution.I'll incorporate the 120mA current control regulators in the LED2 part of the circuit, so it will be present at High and Low settings.I looked at an LM317 circuit for the Low circuit, but 36 volts is at the upper limit of its output and I don't know off hand how to build that circuit.Suggestions, comments concerns?

author: Pappy137
author_rank: Contributor
timestamp: March 19, 2017, 04:26:53 am
content: Here's a representation of my heat sink.It's roughly 5 x 2.5 x .375" and has about 20 sq. in. of 16 oz. copper (20 mil) in it. I'll probably run a strip down the middle of it so I can have a good surface to silver solder the copper mount stars on. That'll add another 5 sq. in. of copper heat sink.

author: Kjelt
author_rank: Super Contributor
timestamp: March 19, 2017, 08:55:12 am
content: QuoteIf you want help then start interacting.The lm317 is 36V yes but that is why you put it in the lower part of the circuit behind the leds.If you have for instance a 75V Psu and two strings of 35V leds in series then you have 5V left for the lm317 to take care off. I will look for my circuit and post it for you but it is in every lm317 datasheet a current regulator, the resistor from output to ground is the shunt, the voltage across that should be the Vref around 1,25V.

author: ebclr
author_rank: Super Contributor
timestamp: March 19, 2017, 10:09:44 am
content: Here is your magic boxhttp://www.ebay.com/itm/Potter-Brumfield-K10P-11D65-48-48V-DC-General-Purpose-Flange-Mount-Relay-10A-/261706094126

author: Kjelt
author_rank: Super Contributor
timestamp: March 19, 2017, 02:48:00 pm
content: So just to put my schematic here and explain a bit.My ledpanel is about 32V @250mA my PSU is 36V so this leaves 4V for the LM317 in the lowerpart to control the current.The lm317 will control the current such that the voltage across R5 (and R6 and R8 when active) is 1,25VI wanted 4 dimpositions so I added two resistors that could be added with the help of two fets which in turn are controlled by two transistors.If all inputs are low only R5 is active so the currrent will be controlled to 1,25/82 = around 15mAIf the economy_46mA input is active the current will be 1,25 / (82||27) = 1,25 / 20 = 61mAand so on.You only want full on (how many mA is that?) and one dimposition, so you only need one input for that.I would encourage a fuse before the led panel such that in event something does go wrong you have a controlled exit strategy.I have been using this for the last three years, works a charm and could well be expanded to higher voltages since the lm317 only "sees"the voltage that is left after the led panel which is very low voltages.On the other hand I choose a smps because I did not want my kid to die if he accidently was fiddling with the leds, I personally hate circuits that are not isolated from mains for that reason.

author: Pappy137
author_rank: Contributor
timestamp: March 19, 2017, 05:10:57 pm
content: QuoteI agree whole heartedly! This fixture will be inside a recess in the bottom of an over - counter microwave. There is a glass lens covering the opening, so in this case - isolation is not completely necessary. I do have a fuse included in the design, and I suspect there's an internal fuse in the microwave. Are the calculations for the 317 based on RMS or peak voltage? The power source I need to control is half wave rectified 120VAC, so peak is around 170V, RMS is a little less than half that.Thanks for the schematic too!!! I'll have to study that some. Since I know I can put the 317 at the tail of a higher voltage string, I looked at how simple it really is to use. I'll probably figure out how to get it into a project soon.

author: Kjelt
author_rank: Super Contributor
timestamp: March 19, 2017, 09:48:38 pm
content: The calculations are for dc voltage, not sure if the 317 likes or what will happen with ac or half wave rectified.Also remember that leds in contrast of old school halogen or wolfram bulbs are extremely fast in response of noise, so it will not be a surprise to me if you will experience a lot of flcker if you do not use somewhat larger capacitance but that will have to be determined in the experiment.

author: Pappy137
author_rank: Contributor
timestamp: March 20, 2017, 07:24:12 am
content: If I had AC, I'd probably just run it through a little transformer, rectify it and regulate the current, but I have half wave rectified, so a standard transformer is out. I'm planning in a couple of pretty big smoothing caps, which will bring the RMS up, but I'll still have 167 or 68 peak. I think if I can get the min voltage up to about 20 or 24 volts, flicker will be imperceptible. The LEDs come on at 30 or 31 volts and Cree calls "typical" 100 % at 35 volts, 120mA. 175mA is absolute max current and 40V is max voltage - according to Cree. So, if I get the min volts to 34 or 35, there should be zero flicker. AND, this is the Low circuit, so I want probably 150 or 200 lumens rather than 400 - 450. There ought to be an easy way to get from 120V VAC -> half wave rectified -> 40 or 48 smooth volts DC. This has always been the challenge. The high circuit from the switch is 120 VAC, the Low circuit - from the switch is 120 VAC,,, half wave rectified. I can't change the switch, it is part of the key pad assembly. I have considered using the OFF-HIGH-LOW switch for nothing more than a control signal to a different power supply, which would drive the new lamp. Too much trouble. I'd have to take the stinking microwave down from where it's hanging. I'm gonna screw in my nifty intermediate base (E17 - I think) converter, plug into that, punch a "button" on the nuker panel, and "Let there be light",,, in the appropriate amounts.I'm gonna say this again. I've already learned way more than I intended to when I started this project,,, and it has been FUN!