Is a LED fast enough to strobe and is any color faster - Page 1
Forum section: electronics / projects

author: Seekonk
author_rank: Super Contributor
timestamp: January 09, 2015, 07:56:12 pm
content: I'm helping a friend rebuild a couple SUN Distributor machines. They have a spinning neop flash tube that spins in an assembly that lights up the dial 0-360 when the plug would fire. I just got the first machine to work acceptably, but it is likely that the other machines will need new flash tubes. These are custom made and cost $250. A 10W LED would fit in the space just fine. The dial would be about 2 1/2 inches from the LED. Most any color would be acceptable. Is a LED fast enough, it would have to be better than 3 uS. White and some colors have dyes that might cause persistence problems. I have a number of power LED but they are all at my camp. Anyone with any similar experience. I'm incorporating a mini 328 to get real numbers out of this antique too. I want to borrow this machine for Halloween. Wear a white lab coat, some welding goggles and this spinning orange disk will scare the hell out of kids.

author: DJ
author_rank: Unknown
timestamp: January 09, 2015, 08:05:25 pm
content: Use an RGB LED, and a position sensor. Have the LED switch from blue to red as it changes direction relative to the viewer.Claim you have developed a quantum distributor which operates at the speed of light and apply for government funding.

author: ConKbot
author_rank: Super Contributor
timestamp: January 09, 2015, 08:09:06 pm
content: LEDs would definitely be fast enough, and you're correct about phosphors being slower than a non-phosphor led. I'd look at a cree XR-E, in red or green, or a luxeon rebel color, in red or green also.

author: Leadfootin
author_rank: Contributor
timestamp: January 09, 2015, 08:33:36 pm
content: Very easy to test led's. Set up a modulated driver circuit for the led's in question and then use a fast photo transistor and a scope to see the output. IE a sig gen with a 10% duty cycle square wave to drive, bias receiver diode, photo trans or use a small silicon photo cell.FWIW I have a similar unit and am contemplating such a project as well. The coil and power supply just don't seem to cut it any more. Note that for a point system you should use at least 100ma for a clean signal. Don't want to switch on the dirt on the contacts. It is easy to create a mag pick up circuit as well with only a few parts.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 09, 2015, 08:56:04 pm
content: I don't think phosphors would have any appreciable delay - how could they store enough energy to emit light anywhere near what they do when lit?

author: ConKbot
author_rank: Super Contributor
timestamp: January 09, 2015, 09:15:06 pm
content: Quote from: mikeselectricstuff on January 09, 2015, 08:56:04 pmSimilar to the phosphors used in a CRT. Though not meant to have persistance as a CRT would, there still is some.Grab an LED fashlight, a dark room, let your eyes adjust, cover the end of the flashlight, blink it on and off, and quickly remove your hand. You can see a fading afterglow.Would it make a difference? In this application? probably not, but avoiding white LEDs and using a color LED removes any doubt that it could affect it, and doesn't add any negatives for a 1-off situation like this.

author: Seekonk
author_rank: Super Contributor
timestamp: January 09, 2015, 09:49:23 pm
content: Leadfootin, Have you replaced ALL the capacitors? Besides the one at the flash tube there are four more under the power supply that are switched in and out as the speed changes. Actually it is one big rectangular capacitor with four .3uF 600V packed in it. Supply runs at about 520V. I used X2 capacitors since I had a lot of them. I modified a Harbor Freight freebie flashlight to strobe the inductor lobes but never got to try it. Spent all the time getting the unit to just work in a cold shop. It's 20 miles away and my flu has come back after being in the cold. Be another week before I can retest.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 09, 2015, 10:04:43 pm
content: Quote from: ConKbot on January 09, 2015, 09:15:06 pmSome of that could be capacitance in the driver. White LEDs are so efficient that they will emit a visible glow on a microamp or two. Any afterglow will be orders of magnitude below the normal "on" level.This LED flash claims durations down to 500nShttps://www.kickstarter.com/projects/vela/vela-one-the-worlds-first-high-speed-led-flash

author: Zero999
author_rank: Super Contributor
timestamp: January 09, 2015, 10:55:22 pm
content: Quote from: mikeselectricstuff on January 09, 2015, 10:04:43 pmThere is a significant afterglow and it isn't due to the leakage current.I have seen it before too. The afterglow is normally yellow or orange but it may appear white as human vision is monochrome below a certain illumination level.The LED filament bulb I have is the worst offender. The glow lasts a few seconds and is still faint after a minute. Again I'm pretty sure it isn't the capacitors because it's an orangish colour, as the blue LED dies turn of instantly.

author: Psi
author_rank: Super Contributor
timestamp: January 09, 2015, 11:00:44 pm
content: Should be easy to test.Shine a LED torch directly into another LED torch in a dark room then switch off and have a look at it.

author: Yansi
author_rank: Super Contributor
timestamp: January 09, 2015, 11:24:19 pm
content: Quote from: mikeselectricstuff on January 09, 2015, 08:56:04 pmExactly. My friend tested the "delay myth" of white LEDs an has come to a conclusion, no light decay is present. As the power is cut off, so is the light.Hero999: Of course that effect is due to capacitors in the LED bulbs. How could you think it is not? :-O

author: grumpydoc
author_rank: Super Contributor
timestamp: January 09, 2015, 11:25:05 pm
content: QuoteIt is but it actually depends on ambient lighting.Cone cells respond to colour but don't work in low light.Rods actually "see" green-blue light but only work at all in low light. This is because the pigment which absorbs the light (rhodopsin) is destroyed in the process and has to be re-synthesised. This is relatively slow and the cell can't keep up during daylight.So in daylight you see in colour but anything emitting below the threshold for detection will just appear black. At night you can see in low light but only in "black and white"

author: dannyf
author_rank: Super Contributor
timestamp: January 10, 2015, 12:02:41 am
content: QuoteOpto-couplers are typically spec'd to be within 10us (emitter output), and many times well within that limit. So I would say that LEDs are fast enough for your application.Light tensity is another matter.

author: Marco
author_rank: Super Contributor
timestamp: January 10, 2015, 12:04:46 am
content: Led phosphors (except for remote phosphor) have to be designed for fast decay to avoid saturation. Papers on white LED optical communication state a bandwidth of a couple of MHz which would put the 90/10 decay time far below a microsecond.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 10, 2015, 05:22:26 am
content: Quote from: Yansi on January 09, 2015, 11:24:19 pmevery matter in the universe are capacitive. how do your friend conduct/specify the test and the what is the test setup limitation? because there is no such thing as absolute "instantaneous", just human perception percepting beyound there capability.

author: Zero999
author_rank: Super Contributor
timestamp: January 10, 2015, 09:26:26 am
content: Quote from: Yansi on January 09, 2015, 11:24:19 pmI know it's the phosphor. I've excited it externally with a black light and there is significant afterglow. It isn't just the LED filament. I've tested a bare power LED with the same result. I haven't tested standard 5mm LEDs. I don't know how transparent the epoxy is to UV.

author: tggzzz
author_rank: Super Contributor
timestamp: January 10, 2015, 09:44:36 am
content: Quote from: Yansi on January 09, 2015, 11:24:19 pmNot true.Phosphors work by having electrons stimulated into a different energy state by absorbing photons at one frequency. Then,at some random time laterthe electron changes to a different energy state and a photon is emitted, optionally at a different frequency.The question then becomes what are the statistics of the delay before the second photon is emitted. Depending on the specific phosphor and the frequency required in this application, the delay might or might not be significant.So, whatmeasurementsdid you make to justify your assertions?Apart from that, LEDs are typically extremely fast, much more so than is implied by the speed of optocouplers, since there the speed is limited by the capacitances in the phototransistor.

author: tggzzz
author_rank: Super Contributor
timestamp: January 10, 2015, 05:19:35 pm
content: Quote from: ConKbot on January 09, 2015, 09:15:06 pmPeople have developed different phosphors with persistences up toseveral seconds- classically for the radar displays, the first computer memory, and more recently for storage scopes.http://en.wikipedia.org/wiki/Cathode_ray_tube#Phosphor_persistenceCRTs for TVs had phosphors with a persistence time chosen to minimise flicker, i.e. milliseconds not microseconds.For this application, don't guess. Either measure the persistence, or use LEDs without phosphors.

author: HKJ
author_rank: Super Contributor
timestamp: January 10, 2015, 05:35:31 pm
content: I do sometimes measure PWM on flashlights, here is a curve from a XM-L2 led where I measure light with a photodiode and voltage across the diode:There is not any afterglow in that curveGreen is voltage across the diode.Blue is voltage from photodiode, i.e. brightness.

author: dannyf
author_rank: Super Contributor
timestamp: January 10, 2015, 06:07:13 pm
content: QuoteBut lots of afterglow in the human eyes,

author: Marco
author_rank: Super Contributor
timestamp: January 10, 2015, 07:08:05 pm
content: Quote from: 3roomlab on January 10, 2015, 09:51:52 amDoubt it, it's a pain to get nice variable width pulse (well you could use a PFN, but I don't think they have 1500 meter of coax in there). Probably a couple smaller (ie. faster switching) MOSFETs in parallel.I wonder how much lumen they really get out of those LEDs ... if the phosphor is saturating the efficiency will drop like a brick.

author: Seekonk
author_rank: Super Contributor
timestamp: January 10, 2015, 10:14:12 pm
content: I did a quick mock up with a 4W line operation (120VAC) white LED that I have. Normally it is a full wave bridge with a 560 ohm resistor. I removed their board wired it to 18V DC and 82 ohm resistor,all switched by a FET. It is easy to over clock it and stop a computer fans motion at 1.6khz in a modestly lit room with the LED about 2 inches away. With those short pulses you can stare directly into the LED and easily see the points of light. It was a stretch for me to come up with 175V from parts I have. I like the idea of the higher voltage LED as this has to be fed by brushes that aren't that substantial. That article appears to have the yellow dye LED like I have and they allude to them being commercial varieties. That inspires me to try voltages 50% higher once a micro closely controlls the on time. Willing to destroy a couple for science.Since we are into an esoteric discussion, I'm wondering why they used neon. Did the orange give a better contrast to the white arrow in room light? Does the eye have better persistence with that color? I really want to try it in red. As a manager once told me....The optics of a project are everything.Or was it just cheap and had a long life

author: tom66
author_rank: Super Contributor
timestamp: January 10, 2015, 10:50:37 pm
content: Quote from: mikeselectricstuff on January 09, 2015, 08:56:04 pmYou'd be surprised. Typically red and green phosphors have long delays. Blue phosphors are much faster. Delay can be on the order of 5ms for some phosphors used in plasma displays (which operate by a different principle, UV to light rather than blue to light, but the general operation is similar.)

author: tggzzz
author_rank: Super Contributor
timestamp: January 11, 2015, 01:39:38 am
content: Quote from: tom66 on January 10, 2015, 10:50:37 pmRadar displays have phosphors with a persistance 1000 times longer than that; several seconds (not ms) in fact!The delay is/was very useful with long-range primary radar, so that you could see how far the target had moved between successive sweeps.

author: eliocor
author_rank: Unknown
timestamp: January 11, 2015, 02:20:17 am
content: According to thePhosphor Handbook, the typical YAG:Ce phosphor used on white LEDs (combined with blue light chips) has a typical luminescence lifetime of about 10÷100ns (chapter 5.9)

author: dannyf
author_rank: Super Contributor
timestamp: January 11, 2015, 12:58:46 pm
content: QuoteWhy do you have to bring facts and reasons to a discussion that have long been settled by our "experts" here?Our experts have spoken. The time for discussion is over. The experts have done their part. Now, it is your time to do your part: believe in them.

author: Zero999
author_rank: Super Contributor
timestamp: January 11, 2015, 07:53:17 pm
content: I don't care what it says in that book. I've seen it with my own eyes. I would film it but I don't have a camera which is good in low light.I've just tested a 5mm white LED and there is some afterglow. I didn't have much luck exiting the phosphor with a black light because most of the UV was absorbed by the epoxy. I sat in a dark room for 10 minutes in order for my eyes to adapt to the dark, closed my eyes, switched the LED on for a few seconds, then switched it off and opened my eyes and I could see a faint afterglow. No capacitors or SMPS were used, just a white LED, some button cells and a switch.What does that book say about the type of phosphor used in CRTs? The initial decay may be very rapid (if it wasn't then moving images on a CRT wouldn't be possible) but there is also a long after glow, as many people here notice the phosphor on their old 'scopes and TV sets glowing dimly in the dark.

author: SeanB
author_rank: Super Contributor
timestamp: January 11, 2015, 08:10:18 pm
content: Depending on the blend there will be an afterglow from phosphor. Best is to just use some high power RGB led units and drive all 3 with a current ( varied for each chip inside the unit) to get a white light. That at least will operate to the MHz range, well above what you will need in a timing light.

author: eliocor
author_rank: Unknown
timestamp: January 11, 2015, 08:44:10 pm
content: Quote from: Hero999 onTodayat 20:53:17>What does that book say about the type of phosphor used in CRTs? The initial decay may be very rapid (if it wasn't then moving images on a CRT wouldn't be possible) but there is also a long after glow, as many people here notice the phosphor on their old 'scopes and TV sets glowing dimly in the dark.Not all LEDs are made equal: maybe yours are using a different type of phosphor.BTW, if you are really interested in*, you can buy the handbook: you'll learn that CRT phosphors (more than 70 pages dedicated to this argument) are MORE than slightly different from the LED ones...I also suggest you to not trust your eyes, but to make a very simple kludge (a photodiode connected to an oscilloscope) to check what you are affirming. This is the way to make a SCIENTIFIC experiment. I'm waiting for a screenshot of your oscilloscope which shows the afterglow of your LED. I also suggest you to try different type of white LEDs made by different producers. Maybe you'll discover several variances in the afterglow.*) this are the contents of some of its chapters:Part III: Practical phosphorsChapter 4 Methods of phosphor synthesis and related technology4.1 General technology of synthesis4.2 Inorganic nanoparticles and nanostructures for phosphor applications4.3 Preparation of phosphors by the sol–gel technology4.4 Surface treatment4.5 Coating methods4.6 Fluorescent lamps4.7 Mercury lamps4.8 Intensifying screens (Doctor Blade Method)4.9 Dispersive properties and adhesion strengthChapter 5 Phosphors for lamps5.1 Construction and energy conversion principle of various lamps5.2 Classification of fluorescent lamps by chromaticity and color rendering properties5.3 High-pressure mercury lamps5.4 Other lamps using phosphors5.5 Characteristics required for lamp phosphors5.6 Practical lamp phosphors5.7 Phosphors for high-pressure mercury lamps5.8 Quantum-cutting phosphors5.9 Phosphors for white light-emitting diodesChapter 6 Phosphors for cathode-ray tubes6.1 Cathode-ray tubes6.2 Phosphors for picture and display tubes6.3 Phosphors for projection and beam index tubes6.4 Phosphors for observation tubes6.5 Phosphors for special tubes6.6 Listing of practical phosphors for cathode-ray tubeChapter 7 Phosphors for X-ray and ionizing radiation7.1 Phosphors for X-ray intensifying screens and X-ray fluorescent screens7.2 Phosphors for thermoluminescent dosimetry7.3 Scintillators7.4 Phosphors for X-ray image intensifiers7.5 Photostimulable phosphors for radiographic imagingChapter 8 Phosphors for vacuum fluorescent displays and field emission displays8.1 Vacuum fluorescent displays8.2 Field emission displaysChapter 9 Electroluminescence materials9.1 Inorganic electroluminescence materials9.2 Inorganic electroluminescence9.3 Organic electroluminescenceChapter 10 Phosphors for plasma display10.1 Plasma display panels10.2 Discharge gases10.3 Vacuum-ultraviolet phosphors and their characteristics10.4 Characteristics of full-color plasma displays10.5 Plasma displays and phosphorsChapter 11 Organic fluorescent pigments11.1 Daylight fluorescence and fluorescent pigments11.2 Manufacturing methods of fluorescent pigments11.3 Use of fluorescent pigmentsChapter 12 Other phosphors12.1 Infrared up-conversion phosphors12.2 Luminous paints12.3 Long persistent phosphors12.4 Phosphors for marking12.5 Stamps printed with phosphor-containing ink12.6 Application of near-infrared phosphors for marking

author: dannyf
author_rank: Super Contributor
timestamp: January 11, 2015, 09:28:50 pm
content: -very simple kludge (a photodiode connected to an oscilloscope) -You can find all of that in an optocoupler.

author: dannyf
author_rank: Super Contributor
timestamp: January 11, 2015, 09:30:04 pm
content: That's why looking at an opto datasheet will be illuminating on this.

author: eliocor
author_rank: Unknown
timestamp: January 11, 2015, 09:33:03 pm
content: Quote from: dannyf onTodayat 22:28:50>-very simple kludge (a photodiode connected to an oscilloscope) -You can find all of that in an optocoupler.You are right, but he was referring to white LEDs: to my knowledge, the most LEDs used in optocouplers are IR ones.

author: paulie
author_rank: Frequent Contributor
timestamp: January 11, 2015, 10:44:43 pm
content: Actually OP was referring to all colors not just white.

author: paulie
author_rank: Frequent Contributor
timestamp: January 11, 2015, 10:45:47 pm
content: So Dannyf's suggestion was not totally useless.

author: paulie
author_rank: Frequent Contributor
timestamp: January 11, 2015, 10:46:38 pm
content: Although many are annoyed at his tendency toward pointless consecutive posting.

author: paulie
author_rank: Frequent Contributor
timestamp: January 11, 2015, 10:47:41 pm
content: Some say it's to boost his post count so newcomers will think he's the cats meow.

author: paulie
author_rank: Frequent Contributor
timestamp: January 11, 2015, 10:49:23 pm
content: My theory is it's a play to ferret out the Post Police and get them to cry TROLL!

author: paulie
author_rank: Frequent Contributor
timestamp: January 11, 2015, 10:50:45 pm
content: Or maybe actually is a personallity defect and he really can't help it.

author: paulie
author_rank: Frequent Contributor
timestamp: January 11, 2015, 10:52:10 pm
content: Anyway my theory that it was to boost his own thread view count is blown because apparently he does it in others too.

author: paulie
author_rank: Frequent Contributor
timestamp: January 11, 2015, 10:52:28 pm
content: LOL.

author: paulie
author_rank: Frequent Contributor
timestamp: January 12, 2015, 02:09:17 pm
content: Haha. Anyway I apologize for derailing an excellent topic. Specially since this question has always been high on my list.I followed dannyf suggestion which made a lot of sense and wired up a TCRT5000 opto detector to an AVR. It was immediately obvious both the LED and sensor were capable of operation in the microsecond range. The actual limiting factor turns out to be the collector resistor. 1k allowed rise/fall times in microseconds. 10m only milliseconds. Just what might be expected but no way was the LED a limiting factor.

author: Seekonk
author_rank: Super Contributor
timestamp: January 12, 2015, 03:27:48 pm
content: The only question left is how bright can I get it. Currently it is running at 196 volts and normal voltage is about 140V. Need to find a photodiode to see at what point additional voltage it doesn't give more light. From that article it would seem that another 70V is possible. This is a picture was taken in ambient light. The red tinge is due to my placing my finger over the flash. Notice the pink arrow is at an angle from the white arrow of the fan indicating a time difference, so motion was stopped.

author: LaserSteve
author_rank: Super Contributor
timestamp: January 12, 2015, 11:39:04 pm
content: With a 1 Ghz 7405 Storage Scope and a 110 Mhz rated Thorlabs Photodiode I got 150 Nano Seconds FWHM on a 350 mA Royal Blue Luxeon. This was five years or so ago. Drive was an ancient HP pulser rated at 10 NanoSecond rise and fall times. Limiting factor was LC product of the Die C and the L of the Clip leads on the Coax plus the bond wires. I did not have a matching network on it, just a 10R0 series resistor. Most LEDs can do better then that.We later did some experiments with RF pulses as drive and saw far faster then 150 nS.Note the Phototransistors in older Optocouplers are notoriously slow.Our application was a laboratory strobe. Warm White Luxeons showed a phosphor decay time. I forget how long, but decreasing to 30% of peak took a while.Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: January 13, 2015, 01:25:34 pm
content: @3Droomlab, Phosphors used 5 years ago were different then today's batch. Also I'd be glad to show you phosphors on LED devices exhibiting phosphorescent decay. I have quite a few commercial white led lamps around that house that exhibit SECONDS of significant deep violet phosphorescence.Nice kitty zero JPG, but don't assume all LED phosphors exhibit zero decay. Just the fact that you can't measure any on your present device suggests to me that you should 1. Read Yen and Weber... 2. Make some measurements across a broad range of devices. 3. Document your test setup and prove it, EEV style.Steve

author: paulie
author_rank: Frequent Contributor
timestamp: January 13, 2015, 01:30:47 pm
content: Or simply use regular LEDs instead of white.Speaking of lasers it would be interesting to know how they perform int his respect. Particularly the blue/violet. I'm having problems with my xy(z) PCB exposer and not sure if it's modulation issues or the oddball radiation distribution.

author: LaserSteve
author_rank: Super Contributor
timestamp: January 13, 2015, 01:48:41 pm
content: Quote from: paulie on January 13, 2015, 01:30:47 pmFrom two years of making commercial LED flashlights, I can tell you this. LEDs are binned by both the manufacturer and the distributer. All those nice led signs in Times Square, with the perfectly matched parts, were the results of sub-binning. When we bought early batches of commercial 350 mA leds, we would receive boxes that we found were sub-binned.When you bought in batches of 16 or 24, all the LEDs came on a leadframe used for robotic handing that were part of the aluminum PCB. If a box had been sub-binned, 3 or 4 leds were tossed in the box that were NOT attached to the lead frame. It was clear that these had different characteristics, Visually, even before I measured the spectrum. We learned to keep track of the lot and grading codes inkjet printed on the back of the devices.While less of that occurs now, many manufacturers mix their phosphor batches to meet a data sheet goal, and lot to lot variations can be significant. Visually you would be hard pressed to tell the difference, but with a spectrograph, blend variations are apparent.We actually had clients who would pay more for a slightly yellower spectrum. Word quickly got out to enthusiasts that there were variations in performance. Some customers would ask us for yellow, others would ask for the devices with a violet tinge.A lot of money has been spent on optimizing phosphor production in the last three years. However batch variation does happen.This is dialed out prior by blending at the phosphor factory.Modern sign systems, say last four years, use factory calibration to dial out the variation. Prior to that, LEDs underwent significant binning.We sold significant amounts of Maglite retrofits, significant amounts of a 40 hour led flashlight to the military, and my other led lighting product was in Radio Shack, Sears Hardware, and Walmart.If a PCB etcher was failing, I'd find LEDs that did not use a phosphor, if your using blue or deep violet. If your using LEDs that make UV direct from the die, be aware that LED is broad spectrum often 20 nanometers or more, and the spectral distribution slides around a wide band.I'd also say start quality control measures on your resist. Some resists age like crazy once the bottle is opened. They do have a shelf life.Steve

author: paulie
author_rank: Frequent Contributor
timestamp: January 13, 2015, 01:53:44 pm
content: I suppose my question about laser diodes was a bit off topic here. Kinda like phosphors.

author: LaserSteve
author_rank: Super Contributor
timestamp: January 13, 2015, 01:58:37 pm
content: Quote from: paulie on January 13, 2015, 01:53:44 pmIs it? The entire index to Yen and Weber's Edition of thePhosphor Handbookwas copy/pasted about two pages back in the thread.The first poster was looking for any phosphor decay time issues as part of his question. He may not have know the correct term for the issue, but he knew it could be there. Shockingly, I'm the only one who even mentioned a valid measurement setup.Steve

author: paulie
author_rank: Frequent Contributor
timestamp: January 13, 2015, 02:06:30 pm
content: OT seems to be the rule rather than exception in these forums. Anyway I found lack of power and collimation a problem trying to use LEDs. Much better luck with cheap laser pointer guts.ps. I'm using film so bottle life less of an issue. 3 yr old film still behaves same as the new stuff. As long as it's kept in the dark.

author: LaserSteve
author_rank: Super Contributor
timestamp: January 13, 2015, 02:23:08 pm
content: Quote from: paulie on January 13, 2015, 02:06:30 pmLaser diodes typically ship within 3-5 nanometers FWHM of design wavelength. On average they change 0.2 nm per degree C, with increasing temperature making the wavelength longer. It is unlikely your seeing a significant issue with wavelength in your exposure setup with a laser diode. Unless the LD is on the ragged edge of the adsorption/activation spectrum of the resist, the LD will just about always do what you expect of it, provided it has enough heatsinking.PWM will not significantly affect the performance of the laser diode device, unless you are near the Knee in the LD Light Output Vs Current curve, where output starts to decrease with increasing current.So if the Laser Diode is well within the specified adsorption band of the resist, it is not hot or overdriven, and your exposure time and power density is correct, you probably need to look elsewhere for your problem.SteveSteve

author: paulie
author_rank: Frequent Contributor
timestamp: January 13, 2015, 02:57:12 pm
content: I have no issues with wavelength or power output. All that is solved and faster than the steppers. Like with the LED discussion the main thing with a PCB exposer is on/off time. There seems to be a disparity between specs and my observed results. Keeping it close to the "knee" does not seem to work because of pattern spread. Practical experience with all on or off times are my main concern.

author: LaserSteve
author_rank: Super Contributor
timestamp: January 13, 2015, 03:28:16 pm
content: If on/off times are the issue, check the diode driver for speed response. The less then 300 mW Laser Diode in most packages is good to 10 to 100 Mhz or more. It is the diode driver that affects device timing more then anything else. You may be hitting the rise/fall and bandwidth limitations of your driver. Most of them become a nasty integrator if you start to push the speed.This requires a good oscilloscope and a fast photodiode with proper termination at the scope input.We usually try to find photodiodes with a very small surface area, ie about a square millimeter or less. We reverse bias them to increase response speed, and we load them properly at the scope input. If the scope does not have a 50 Ohm input, we add a BNC TEE and a 50 Ohm terminator on the O'scope input.Take a look at Thorlabs DET10A and the schematic on its data sheet for an example of a moderately fast detector.Steve

author: eliocor
author_rank: Unknown
timestamp: January 13, 2015, 06:32:19 pm
content: I have thetoolsto capture an hi res spectrogram of a white led, but unfortunately I will not be able to do it until March

author: Seekonk
author_rank: Super Contributor
timestamp: January 13, 2015, 08:54:57 pm
content: "PWM will not significantly affect the performance of the laser diode device, unless you are near the Knee in the LD Light Output Vs Current curve, where output starts to decrease with increasing current."So how much current can you safely expect to to put into a LED. The high speed kickstarter project seems to say they are using commercial LED at very high currents. The LED I am using is 4W 35ma at about 150V. If I stay under a 0.2% duty cycle, how high a current could I pump into this. Would 5-10 times be reasonable. They were claiming 2,000% overdrive. Those were really short pulses.

author: paulie
author_rank: Frequent Contributor
timestamp: January 14, 2015, 12:28:08 am
content: From what I can tell the cheap pointers are using Sony SLD3236VF or knockoffs (knocked off the assy line) so driving them directly from an AVR pin at 40 or so ma should be about 30-35mw. Well below the 80mw pointer operation and 200mv CW spec. Low res tests show just right for my tiny motors that can't go any faster. Depending on the PCB file it may stay on for several ms so probably considered continuous. The problem is with 6-8mil traces where im getting smearing. It is turned completely on and off in a very short time.I will try to characterize with a high speed diode as suggested to see if that is the problem or if it's the queerly oblong TEM pattern just not focusable. Thanks for the input.

author: paulie
author_rank: Frequent Contributor
timestamp: January 14, 2015, 12:31:46 am
content: Quote from: 3roomlab on January 13, 2015, 06:14:15 pmI prefer the term "snarky". Sounds more like a 1950s cruise missile. Of course a little pussy never hurts anyone.

author: joeqsmith
author_rank: Super Contributor
timestamp: January 14, 2015, 03:36:52 pm
content: Quote from: Seekonk on January 09, 2015, 07:56:12 pmInteresting project. I am curious about how you came up with a 3 uS max requirement.If the old distributor type engine had a top end of 6,000 RPM. 6,000/60 = 100 rotations / second. So one turn of the crankshaft every 10 mS. The distributor runs from the camshaft. Assume it's a 4-stroke so 2 turns of the crank to run a cycle. 10 mS * 2 = 20 mS per rotation of the distributor. Let's say it can resolve to 1/10 of 1 degree. 20 ms/3600 = 5.556uS.Seems very high performance for that old system. Must be looking to do more than just scare the neighborhood kids.

author: Seekonk
author_rank: Super Contributor
timestamp: January 14, 2015, 10:09:34 pm
content: You are correct. The one I'm working on only had a tach that went up to 6,000 rpm. Another on of their lot only went up to 4,000 rpm. They race at Bonneville each year and want the machine to do 10,000 and the distributor only does half that. That will take a VFD and a flat belt in order to get that. All the bearings have been replaced. I am adding a micro and a once per revolution to the machine in order to get out hard numbers if it can be shown that the speed is consistent throughout the revolution. A Hantek 6022BE was also purchased. Looking at the ignition pulses and once per rev it should be able to perform the same calculations as the micro. With open software available, it could be a nice ready made package. There will also be a hand held strobe to stop motion in the distributor. The eye can be very perceptive in seeing issues. I have some other interests besides this machine. The number quoted is about twice as fast as needed in order to bring up any potential issues. It has been an interesting discussion. I am concerned about balance issues at those speeds.