Flicker Free 50w LED for high speed video - Page 1
Forum section: electronics / beginners

author: kastnerd
author_rank: Newbie
timestamp: April 15, 2014, 02:56:41 pm
content: Hi, I am trying to make some flicker free lighting for high speed video. 5,000 - 10,000fpsIv got some 50w LED floodlight that have a 36v transformer in them. But it even at 2000fps it flickers.I tried adding a 4700uf Capacitor to it and that did help but did not completely remove the flicker.I am not sure if I just need a larger or multiple Capacitors or maybe a different type that works at a faster speed.Youtube video of the flicker.http://youtu.be/V9t1uzNEKQIThanks for any suggestions.

author: bookaboo
author_rank: Frequent Contributor
timestamp: April 15, 2014, 03:35:50 pm
content: The floodlight will probably have a constant current driver circuit inside. So no matter how smooth you get the input voltage there will always be flicker because the driver is switching very fast to control the current and therefore the brightness.You *might* be able to use a linear bench power supply set to the correct current limit (that might flicker also depending on the type).Or you could work out the exact forward voltage and use a linear power supply, though you would have to be careful there too as some power LEDs require different Vf at different temperatures.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 15, 2014, 04:31:12 pm
content: Yes, as bookaboo said, it seems likely that your LED driver is a high-frequency switch-mode power supply (SMPS). This is much more efficient than the old-school linear method and likely all new equipment uses SMPS. Of course, the high-frequency "flicker" is typically not a problem because nobody can see it. But you have created the exception to the rule where you CAN see it.So two possible solutions may be:Use a linear supply instead of a SMPS. NOTE: It is VERY important to maintain the proper power through the LEDs without overshoot or you will toast your LEDs. This is NOT a trivial thing.Just switch to old-school incandescent lighting.

author: mariush
author_rank: Super Contributor
timestamp: April 15, 2014, 05:13:11 pm
content: Have a look at led drivers like this one:http://uk.farnell.com/on-semiconductor/nsi50150adt4g/led-driver-pwm-50v-to-252-3/dp/2382343(digikey alternative:http://www.digikey.com/product-detail/en/NSI50150ADT4G/NSI50150ADT4GOSCT-ND/4499781)The above defaults to 150mA but it can raised up to about 350mA, depending on how well it's cooled and how much power it dissipates through it.Alternatively, you can just parallel several of them to get higher current.So grab about 10 of those and put them on a pcb and slap a heatsink on top of them and you have 1.5-3.5A constant current driver for the led and it shouldn't flicker that much.But note that you'll need a few volts above the forward voltage of the leds on the input for these to work right..

author: Jarrod Roberson
author_rank: Regular Contributor
timestamp: April 15, 2014, 05:17:31 pm
content: The way professional video gear gets around this is to sync with the frequency of the light.In the old days of NTSC for example, you had a way to sync a camera to the 60hz of the mains that they lights were running on, that way you didn't see the flicker, same with recording CRT monitors.Granted this might take some trail and error with a light that isn't set up to give access to the pulse driving the lights, but it is doable, that or use a light with a linear regulator and prepare to cool the thing.Another thing you might try that is super low-tech is multiple lights that are flickering at multiple frequencies that don't have any harmonic overlap. Then you should have light all the time, or most of the time and the flicker would become less pronounced and more like smooth pulsing modulation at worst.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 15, 2014, 05:18:29 pm
content: @mariush, it is not clear how substituting one SMPS for another will solve kastnerd's problem?That ultrasonic switching function is the CAUSE of the problem.

author: mariush
author_rank: Super Contributor
timestamp: April 15, 2014, 05:22:47 pm
content: It's not smps as far as I'm aware (I admit I didn't try them personally), it's basically just a transistor.From the datasheet:The CCR is based on Self-Biased Transistor (SBT) technology and regulates current over a wide voltage range. It is designed with a negative temperaturecoefficient to protect LEDs from thermal runaway at extreme voltages and currents.The CCR turns on immediately and is at 14% of regulation with only 0.5 V Vak. The Radj pin allows Ireg(SS) to be adjusted to higher currents by attaching a resistor between Radj (Pin 3) and the Cathode (Pin 4). The Radj pin can also be left open (No Connect) if no adjustment is required. It requires no external components allowing it to be designed as a high or low?side regulator.The URL mentions PWM because the datasheet mentions that it's possible to pwm the leds by placing a transistor in series with this component (see page 7) - the part itself doesn't do any pwm.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: April 15, 2014, 05:35:34 pm
content: You could try combining capacitors and inductors to create an LC filter. This will be more effective than just a capacitor.Alternatively you could use passive ballast, basically just a power resistor... But it will be hot, and expensive, and may require a heatsink.Or use an old fashioned flash bulb!

author: kastnerd
author_rank: Newbie
timestamp: April 15, 2014, 05:39:57 pm
content: This LED Drive says 50/60hz DC36v 1500MA+/-5% PF>0.5Thats 1.5AI am going to try connecting 36v worth of batteries to see how that works.Normal Incandescent light flickers alot on high speed video and florescent is worse yet. Best light I have are some 750w Tungsten lights but they are so hot and not bright enough, I would need 3 or 4 of them. Need alot of light for high speed when running 1/10,000 shutter speedBut thanks for the feedback so far.

author: DmitryL
author_rank: Regular Contributor
timestamp: April 15, 2014, 05:49:26 pm
content: Quote from: kastnerd on April 15, 2014, 05:39:57 pmIMHO if you connect "36v worth of batteries" to the LED cluster instead of this PSU, you may end up with a burnt LEDs. This PSU is a _current source_, not a voltage one, which the batteries are. On the other hand, it will be a good experience for you and no more flickeringTry to take a look at the loaded PSU output with a scope....

author: Phaedrus
author_rank: Frequent Contributor
timestamp: April 15, 2014, 05:55:53 pm
content: What frequency is the flickering you see?

author: mariush
author_rank: Super Contributor
timestamp: April 15, 2014, 06:31:56 pm
content: 36v 1.5A is basically 55wGet a proper 28-32 Vac 80-100+ VA transformer, for example something like this:http://www.digikey.com/product-detail/en/VPT30-3330/237-1340-ND/2090078(30v AC 3.33A)Rectify this AC output to DC using a bridge rectifier, which will give you a peak dc voltage of 30v x 1.414 = 42.4v, minus about 2v, the voltage drop on two diodes in the bridge rectifier..so let's say 40v. The maximum current will be about 0.68x3.33 = ~ 2.25AAdd enough capacitance to have the input voltage sag as little as possible. A simple formula would be C = Current / (2 x Ac Frequency x Vripple) so Vripple = Current / ( 2 x Ac Frequency x C)Assuming you're in US, for 1.5 A of current , 10000 uF (0.01F) will give you Vripple = 1.5 / (2x60x0.01) = 1.5/1.2 = 1.25v so 120 times a second voltage will go up and down between about 38.75v and 40v. This should not affect you leds, because the led driver after it should further filter this and limit the current dynamically depending on input voltage and other factors. It's just stable enough for the led driver to not "sweat" so much.Now if you connect the leds, you'll probably burn them if they're only rated for 1.5A, because the transformer picked above can do 40V 2.25A so you still have to limit the current.A bunch of those chips I mentioned above should do it in the most smooth way, because those don't do pulse width modulation as most led drivers work.

author: kastnerd
author_rank: Newbie
timestamp: April 17, 2014, 07:28:45 pm
content: I am trying to build a few flicker free lights.I found adding 2 4700uF capacitors greatly reduce the flicker to a useable level. It still could have less.I am now thinking of maybe getting a 100w LED driver like this one.http://www.amazon.com/E-age-Power-Driver-85-265v-Floodlight/dp/B00E9P7W0S/ref=sr_1_3?ie=UTF8&qid=1397761688&sr=8-3It still would most likely need a some extra capacitors to remove the ripple and give flicker freelight.when testing the flight with 2 capacitors I tested shorting out a 3rd capacitor at 2000fps.If you look at this video, in the shadows you can see the ripple in the power of the light.http://youtu.be/pplfxXSvpGIThanks for the feedback.

author: jlmoon
author_rank: Unknown
timestamp: April 17, 2014, 07:44:53 pm
content: I have been following this thread as best I can.. am I missing something.. "flicker free LEDs for high speed video"? Don't really high level pumped LEDs operate in the switched mode? Doing this to provide more lumen and dissipate less heat.Just curiousJLM

author: Marco
author_rank: Super Contributor
timestamp: April 17, 2014, 08:45:27 pm
content: How about using a capacitance multiplier?

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 17, 2014, 09:14:06 pm
content: Quote from: jlmoon on April 17, 2014, 07:44:53 pmYes, you are correct. Which is why I suggested back at the beggining of this discussion that using old-school, continuous incandescent lighting may be a prime alternative.I must say that all the discussion of "filtering", etc. completely mis-understands the whole nature of switch-mode current regulation (which high-output LEDs require, as jlmoon observes.). This discussion has gone very wide off the mark.

author: electr_peter
author_rank: Unknown
timestamp: April 17, 2014, 09:23:54 pm
content: See Bob Pease videos "Driving LEDs without Getting Burned"Part IPart IIThese smart guys are basically saying, among other things, that:1) LED should be driven with constant current (LED is current driven)2) LED "apparent" brightness is controlled via PWM (constant current mode is switched on off with PVM)3) add heat sink for >3W LED4) switching frequencies are in 1kHz region. You cannot get away from this switching easily.So, if you want flicker free LED lighting, you could build/buy special constant current LED drive with very high switching frequency (discussed briefly in videos posted above).Other than high frequency, you could try DC with resistor for LED/incandescent (with significant derating of current) - derating should guarantee stable output. Running at full brightness with resistor only is not recommended.Syncronisation would be an option, but it is harder to implement - all sides of hardware have to have it(how to sync CC drive???), phase shift can cause issues, etc.

author: Marco
author_rank: Super Contributor
timestamp: April 17, 2014, 09:37:58 pm
content: The thermal mass of an incandescent is just a filter too, not a very good one at 60 Hz either.PWM is not necessary to vary the intensity of LEDs, it's just convenient because it gives you linear control (regulating the continuous current can create colour shift, but that's not too relevant for a white LED ... there is no efficiency advantage of PWM over using a lower constant current, the opposite in fact).IMO the capacitance multiplier would be the easiest active filter to add to a switching constant current LED supply.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 17, 2014, 09:39:09 pm
content: Quote1kHz would cause audible artifacts, and require large inductors.Most modern switching regulators run in the 100s of kHz to low MHz region.Some devices targeted at LED drive cut corners by relaxing the ripple requirement compared to standard power supply circuits.Pretty much any modern switchmode LED driver should be able to drive without flicker in the low Khz framerates you're looking at.This is not a hard problem - there are plenty of solutions out there.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 17, 2014, 09:43:08 pm
content: Quote from: Marco on April 17, 2014, 09:37:58 pmWhile that is true, I fear it ignores the main point here. Switching is used to increase the total current (and therefore the total brightness) without exceeding the total dissipation of the device. This is a very common practice in high-power LEDsAttempting any kind of filtering of the pulse-mode drive current will result in either reduced brightness, or overheaded LEDs. Neither of those seems desirable, IMHO.

author: Marco
author_rank: Super Contributor
timestamp: April 17, 2014, 09:46:49 pm
content: Quote from: mikeselectricstuff on April 17, 2014, 09:39:09 pmWith a short enough shutter time he can capture the full amplitude of the luminance variation regardless of framerate, just at beat frequencies.

author: turbo!
author_rank: Regular Contributor
timestamp: April 17, 2014, 09:48:15 pm
content: Use a hybrid power supply. You use a SMPS to overshoot it a bit, then smooth it out with a linear regulator. You're probably seeing a beat flicker, because LED drivers and ballasts are usually a pretty good 120Hz AM transmitter. The carrier wave might be in 100s of KHz, but there's a good amount of 100 or 120Hz riding along.Hybrid power supply is like building a block just a tad bit bigger than you need, so you can sand and polish it to precise dimensions instead of having to machine a very large size difference.If the camera has "AC line synch" like oscilloscope does, use it. This avoids the frame rate and 2x line frequency from swimming against each other and hold them rock steady with respect to each other.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 17, 2014, 09:53:29 pm
content: The problem of illumination for high-speed photography has been with us since before some of you were born. Google is your friend. There are abundant discussions of various solutions for high-rate illumination. Re-inventing the wheel doesn't seem like a valuable activity.

author: Marco
author_rank: Super Contributor
timestamp: April 17, 2014, 09:56:56 pm
content: Quote from: Richard Crowley on April 17, 2014, 09:43:08 pmCitation needed, sounds strange to me. Unless you need pulsed light you are just going to get less light for the same amount of power compared to constant current.

author: electr_peter
author_rank: Unknown
timestamp: April 17, 2014, 09:58:18 pm
content: Quote from: mikeselectricstuff on April 17, 2014, 09:39:09 pmI stand to be corrected.mikeselectricstuf suggestion to use modern high freq CC drivers should be good enough for 2-5kFPS (few orders of magnitude difference in freq).

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 17, 2014, 10:09:06 pm
content: Quote from: Marco on April 17, 2014, 09:56:56 pmGoogle returned "About 2,070,000 results (0.89 seconds)". How many would you like?Here is one to get you started:http://www.cree.com/~/media/Files/Cree/LED%20Components%20and%20Modules/XLamp/XLamp%20Application%20Notes/XLampPulsedCurrent.pdf

author: Marco
author_rank: Super Contributor
timestamp: April 17, 2014, 10:23:03 pm
content: Quote from: Richard Crowley on April 17, 2014, 10:09:06 pm1 would be ok.QuoteThat is non sequitur, nowhere does it suggest you can get more light out of the LED for normal lighting (ie. not visually pulsed) by running it pulsed rather than with constant current. In fact it goes out of it's way to tell you that it's not the standard way to run them and you will lose all their product guidance when you do ...LEDs are driven with PWM mostly as a matter of convenience, secondary to preserve colour temperature ... but not because it's a good way to get more light out of the LED.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 18, 2014, 12:49:03 am
content: Questions:1) WHY do you think Cree wrote that document?2) WHY do you think the Cree's customers WANT to pulse high currents through their LEDs?3) WHY do you think that it is worth creating high-current pulses to drive LEDs?(Hint: The answer to all of those is because you get higher light output using pulsed high-current drive.)Sometimes the trees block the view of the forest.

author: Marco
author_rank: Super Contributor
timestamp: April 18, 2014, 05:32:49 am
content: Impulse flashes, stroboscopic lighting and flashing warning indicators are common uses for LEDs, that's why ...

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 18, 2014, 08:46:41 am
content: Quote from: Marco on April 17, 2014, 10:23:03 pmYou won't get more light out by pulsing, and if anything you reduce efficiency as the light vs. current curve typically starts to flatten as you approach the maximum current.The only benefit of pulsing in this application would be if you wanted to have an effective exposure time lower than the frame interval, e.g. to reduce blurring.Otherwise the only reason to PWM would be for dimming, which you won't be wanting here as you want max brightness.Halogens are still hard to beat in terms of brightness vs. cost, though getting smooth DC to avoid ripple gets harder at higher powers - 12V halogens on a car battery is probably the cheapest solution.

author: ovnr
author_rank: Frequent Contributor
timestamp: April 18, 2014, 09:41:50 am
content: Quote from: Richard Crowley on April 18, 2014, 12:49:03 amAnd sometimes people get lost in the forest. It happens to the best of us.Pulsing your LEDs willnotimprove their efficiency. In fact, the lower your drive current is the more efficient they tend to be (running a 10W 100 lumen per watt LED at 1W will probably get you 110-120 lm/w). Bridgeluxspectheir LEDs at up to 2x standard drive current, and the efficiency curve goes downward, as does the light quality.The reason Cree wrote that document is because people - sometimes misinformed people - ask them about it. Besides: High-power LEDs are used as camera flashes in cell phones and the like. People run them on low frequency PWM with shitty drivers that overshoot. Et cetera. Cree tells you how to do it "safely",notthat they recommend it.(edit: Fixed URL)

author: miguelvp
author_rank: Super Contributor
timestamp: April 18, 2014, 09:53:33 am
content: Pulsing will help on energy consumption if the target for the lighting are human eyeballs due to our persistence of vision.But the task at hand is not for human sight.BTW I hate fluorescent lighting at 60 Hz I can notice the interference between my eyesight and the light blinking at that rate.

author: ovnr
author_rank: Frequent Contributor
timestamp: April 18, 2014, 10:08:29 am
content: Quote from: miguelvp on April 18, 2014, 09:53:33 amPulsing will help efficiency if the vision system (be that eyeballs or cameras) uses a shutter and spends a significant amount of time not receiving light (and that the pulses coincide with the shutter being open). Fortunately, we don't go around blinking our eyes at 60 Hz all day long (that'd be tiring!), and as such we wouldn't receive any benefits from using pulsed illumination. The only place the persistence of vision effect can be helpful is low-power indicators (better to pulse at 1% duty cycle than burn a lot of power in a resistor - assuming you don't blow up your LED, obviously), but that's because it masks the flicker of an otherwise unsuitable drive method.Really, guys. Just get a power LED, hook it up to a constant-current PSU and a transistor you can PWM at various duty cycles, then try to see for yourself if you get any more light out of it running at 50% duty @ 1W vs 100% duty @ 0.5W. I recommend using a camera with full manual mode (fixed aperture, ISO and exposure). Take care to match the wattage (not just the current!), and also make sure your PSU doesn't overshoot on PWM turn-on, which can skew your results.

author: miguelvp
author_rank: Super Contributor
timestamp: April 18, 2014, 10:10:57 am
content: I think you are missing the:Quoteof the OP

author: pinkysbrein
author_rank: Contributor
timestamp: April 18, 2014, 10:20:10 am
content: Quote from: miguelvp on April 18, 2014, 09:53:33 amLook up Talbot Plateau...

author: miguelvp
author_rank: Super Contributor
timestamp: April 18, 2014, 10:32:14 am
content: Quote from: pinkysbrein on April 18, 2014, 10:20:10 amInteresting, makes me think my phobia against fluorescent lighting comes from interference between the light and the computer display or even the incandescent lights in the room, and that make sense. In any event, OP wants to film in between 5,000 - 10,000fps so it doesn't apply but still very interesting information.

author: ConKbot
author_rank: Super Contributor
timestamp: April 18, 2014, 06:01:12 pm
content: Long thread for what should be pretty simple.. TLDR, get a meanwell driver and your LED/thermal solution of choice and youre done. Used one with a cree CXA25xx (forget which one, ~60W) and a reflector I mangled to make it work with the LED for a prototype unit. The prototype worked great with a phantom miro running in the 10k fps range. That worked well so now I have some bridgelux vero29's, 120W meanwells, and ledil reflectors on the way.The meanwell led drivers have a constant current output, and adjusting the output with the trimpot on the screw varies that current, not a dutycycle, so there is minimal ripple on the output. I dont know if the dimming ones use pwm to do the 0-100% dimming, but chances are youre not going to be trying to dim them any when youre doing high speed video

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 18, 2014, 06:19:45 pm
content: Quote from: ConKbot on April 18, 2014, 06:01:12 pmmeanwell stuff is generally good, but avoid the CLG60 as it has a very poor ripple spec

author: ConKbot
author_rank: Super Contributor
timestamp: April 18, 2014, 06:29:09 pm
content: Quote from: mikeselectricstuff on April 18, 2014, 06:19:45 pmouch, 2v on a 12v output. Yeah thats pretty shit.I dont know the part numbers I used for the 60W or 120 W drivers, but the LPF-16D-54 works great on a CXA1507 for fiber illuminator for microscope lights. (Which tend to get stolen for the high speed video too if it something small and the camera is up close)

author: edavid
author_rank: Super Contributor
timestamp: April 18, 2014, 06:57:07 pm
content: Quote from: miguelvp on April 18, 2014, 10:32:14 amThat makes no sense. Things that just add can't interfere. You need multiplication.

author: kastnerd
author_rank: Newbie
timestamp: April 18, 2014, 07:03:45 pm
content: Thanks for all the feedback. Now is 2 good size capacitors its working good. But I will look at some high quality power supplies for my next light. I need quite a few lights to film high speed indoors. The bridgelux vero29 looks like a nice LED, Currently I have the square 50w LED with the LED area about 25mm in size.LED lights of this power require good sized heat sinks and some people use fan's.I have some large 750w Tungsten lights, and they have a tiny bit of flicker in them. But they are so hot sometime you can't keep them on with out cooking what ever your aiming them at. Its hard to find good lights for high speed.

author: jsquaredz
author_rank: Contributor
timestamp: April 18, 2014, 07:43:24 pm
content: What using a BJT to do the constant current drive? You setup the Base to emitter bias with an emitter resistor and a voltage divider on the base. This sets up a feedback loop to maintain a constant current from the Collector to emitter. As long as the load isn't changing, I don't think there would be switching per se. I will test it on the scope to see but here is a good read on it.http://www.pcbheaven.com/userpages/LED_driving_and_controlling_methods/?topic=worklog&p=2

author: jsquaredz
author_rank: Contributor
timestamp: April 19, 2014, 06:03:29 am
content: You're right. Probably just an output cap issue.quote author=AcHmed99 link=topic=29808.msg428225#msg428225 date=1397850921]Quote from: jsquaredz on April 18, 2014, 07:43:24 pmThen you would need a big ass heatsink for the pass transistor or a switching pre-regulator to keep just enough headroom anyway. Over temp the Vero10's I have can vary between 20V-31V. There shouldn't be any problem in getting or building a SMPS CC driver for the LED and the application.The driver he has states PF>0.95 so it's probably the 2fline ripple he is seeing. They probably cheaped out on the output caps ,it also looks like it's crapping out so thats probably not all they cheaped out on.

author: scientist
author_rank: Frequent Contributor
timestamp: April 19, 2014, 07:03:37 am
content: Wouldn't sunlight and a large Fresnel lens work?