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Topic: LED photostrobe Alpha test (Read 6352 times) previous topic - next topic


Apr 10, 2012, 11:18 pm Last Edit: Apr 10, 2012, 11:57 pm by focalist Reason: 1
Just a video showing the Arduino switching 24 watts of LED's via Darlington pairs made of pn2222 and TIP41c.

The final project will have just over 100 watts of white LED's, as well as 20 each RGB.  This current test is with 24 watts of warm white LED's, somewhere around 2000 lumens total light output, about one and a half times the light output of a 100w incandescent bulb, or about the same as a 100w quartz halogen bulb.

The flashes in this test are 1, 5, and 10msec in length (delay function), though the end project I want to be able to drive with pulses in the tens of microseconds.  These units actually have current limiting onboard, 500ma.  The remaining LED's will be driven from LM317 current regulators, I am considering running the LED's over their basic rating, say 150%, due to the short pulse lengths.  At least initially, I plan to stay somewhat within spec.



Looks Good... Drive um up there! ... 150, 200, 300%... LED's are cheap, and getting cheaper all the time.
As long as the pulses give time between for thermal dissipation, you can probably pull it off. All the LED's I have burned out, took at least a second to self destruct, but that was continuous current +200%.


Added my ten watt cool white (12v @ 800mA), bringing it up to 2.8A @ 12v, a bit over 33 watts, yielding something like 3000 lumens.  Going to set camera up later, and test it out to see how it does as macro flash, illuminating at a close distance.  I'll use the arduino to run different pulse lengths to get sufficient exposure at close range.  Reports will be made.


Nice and bright :-)
But why Darlingtons instead of MOSFETs? AFAIK, Darlingtons have higher loss/get warmer.


Apr 11, 2012, 05:43 pm Last Edit: Apr 12, 2012, 07:32 am by focalist Reason: 1
I'm planning on picking up some logic level MOSFET's as soon as I see a batch going for cheap.  I've seen a few N-Channel models, logic level input, with max voltages around 60v and a current capacity of 10A.. those would be the right answer here I think.

Right or wrong, cleaned up the wiring, and up to 34 watts now..


EDIT>>Went up on ebay and got a batch of 20 N-Channel Logic Level MOSFETs for seven bucks shipped.  11A, 60v maximum.  Time to get my feet in the waters of MOSFET... these will switch a LOT faster, and since it's Darlington, I'm never reaching full saturation on the output stage, right?  Also a lot more headroom power-wise.  Seems like I've been spooked by the scary looking symbol too long..



Reducing the pulse length from 10ms to 10us means 1/1000 of the light output all other things staying the same. Or with other words: 100W for 10ms is 1J while 100W for 10us is only 1mJ. If you want to get 1J during 10us this translates to 100kW. This is the reason why high speed photography is usually not approached with LEDs.

You might want to consider some focusing lenses to compensate for that. Or consider xenon flashes.
Check out my experiments http://blog.blinkenlight.net


Apr 15, 2012, 09:13 pm Last Edit: Apr 15, 2012, 11:08 pm by focalist Reason: 1

The core emitter has arrived... 3500 lumens of 3000K, a fifty watt warm white LED array.  Eighteen bucks.

Guys, understand.. I know the physics I am facing.. But I am also depending upon the inverse square of the distance from the source.  I am not lighting a room.. At best a field of a few square feet... And I already have the Fresnel lens for focusing this behemoth.  This is an exercise in "let's see if I can make this work somehow".... And I do have a few tricks in reserve, including having harvested a dozen xenon flash units from disposable cameras.  They are perfect for some things when triggered through a simple optoisolator.  It is important to keep in mind that I am not trying to make something that's a normal use flash.. It'd be cool as hell for milkdrop photography, or a feeding hummingbird for example.. Very close, macro even.  The power decreases by the SQUARE of the distance, so I believe that I have a fighting chance at getting at least some limited success with over 100 watts of LED's.  Note that stage professional PAR cans are now 100w LED arrays already...that's replacing carbon arc in some cases.


That is a professinal stage flood, LED... Five hundred bucks.  I am cautiously hopeful seeing these products.

I really am caught up with the 'If I can get this working at any level, it would be a very cool tool' thing.  If it doesn't pan out as a strobe, it's still a solid state floodlight that is a consistent color every time.. That alone is worth the cost of the parts.. The total on this now.. Hmm.. Like thirty bucks.

It will probably be several days until I can hook it up, in any case.. Gotta now look at the best way to feed that LED.  It'll be PWM'd with MOSFETS, but I will be taking a little time looking for a good current regulator setup for it... Though I understand there is a variation on LM317 thing with a feedback loop, that easily can do. 

It's got to limit at 2.5-3amps at 16-18v, anyone got a particulary cheap and easy solution (I love LM317 regulators.. and I have a bunch...but current sources couldn't run tandem due to tbe feedback circuit, right?  If they can, easy enough to tandem the little beasts.. Three would be plenty, an amp each...)


How about an LM350 for 3A? Still needs a fair bit of heat sinking. Or an LM338. Still needs heat sinking. If you want to get rid of that, you have to go with a switcher, which ends up requiring potentially bulky inductors, but perhaps that's not a problem.


Apr 16, 2012, 12:53 am Last Edit: Apr 16, 2012, 02:47 am by focalist Reason: 1
I'm not sure if the current rating applies when you are wiring it as a current source rather than voltage regulator... The current rating is load max when operating as a voltage regulator.  In the spec sheet for LM317, a circuit for a 1A current source is given, but I have no idea what kind of load is on the regulator when operating as a current source.  It could even be that in that configuration it can carry more current, even..

If they can be used in parallel (Kirchoffs apply here?), then I'd just run three and call it good... I believe it is possible to run them side by side as voltage regulators, but in current regulation 'mode' I'll either need to research (or just try it).


I think the current rating is the same. As far as I understand it, a "current regulator" really just is a voltage regulator with feedback, which makes it very similar to an adjustable voltage regulator.


Apr 16, 2012, 09:39 pm Last Edit: Apr 16, 2012, 09:48 pm by focalist Reason: 1
Hmm.. I was uncertain about that feedback loop, as there would of course be current from the other regulators.  If there is a diode or transistor in the output stage preventing current from coming from the other regulators, it will work I would think.  I even have some diodes that can handle the power level, though I still wonder if they would be unnecessary or just added drain.

Hmm.. Gonna need a test load too.... Maybe an incandescent bulb, 12v.. Close enough for government work... I may have a 55 watt headlight bulb spare to use as a nice load....

TheMOSFETs arrived in the mail today, in all honesty now that I have read about them, I am excited to give them a try.  No more Darlingtons for me on heavy loads... so much better at carrying current.. And so much cleaner and faster switching.. Guess it was time for me to move out of the 1940s...  makes it clear how far my formal electronics training went... basic theory, and I wasn't paying attention anyway.  On the other hand, I am extremely proficient in home firefighting now  :smiley-mr-green:


Apr 18, 2012, 08:29 pm Last Edit: Apr 18, 2012, 08:30 pm by focalist Reason: 1

Okay, so I have looked over the specs on typical adjustable regulators, and it still looks like a reasonable way to provide current limiting.  Since I have LM317T in hand, I want to make three of the current regulator circuits from the spec sheet (1.2 ohm between output and adjust makes for 1a regulator).

In thinking about this, and getting ready to wire up a test circuit, I was looking at the schematic in spec sheet for running the LM317 in tandem as a VOLTAGE regulator.. and it shows the setting resistor "shared" between the adjust connections on the regulators.. This makes sense to me, I think, as the resistor output voltage remains the same, even if the current is spread across the multiple terminals.. And as it only cares about that voltage, "sharing" the resistor ensures a single feedback loop to the regulators, keeping them balanced.

Given this is the case, does the same design consideration apply if I were connecting multiple LM317 current sources?  I am thinking that a single voltage split three ways will be better than three sorta close voltages...


Darlingtons for me on heavy loads... so much better at carrying current.. And so much cleaner and faster switching.. Guess it was time for me to move out of the 1940s...  makes it clear how far my formal electronics training went... basic theory, and I wasn't paying attention anyway.

This makes me wonder if you are driving the mosfets properly. Do you account for the gate capacitance?
Check out my experiments http://blog.blinkenlight.net


Apr 18, 2012, 09:51 pm Last Edit: Apr 18, 2012, 10:06 pm by focalist Reason: 1
How so?  I thought  darlingtons are inherently slower, in fact I was concerned if they were getting fully ON before going off again.  I thought that MOSFETS are inherently better until you are into MHz switching rates,
Isn't the gate charge dissipated when the pin is taken low?  I haven't even added a gate to source high value resistor yet, which I understand is typically how the gate charge is bled off.

Even without any additional components, no gate resistor, it seems to work great.. Though the gate resistor is a better idea so as to protect the AVR from the current draw of charging the gate. I have read conflicting rcommendations regarding the use of gate resistance, and even use of the gate to source resistance.  

Even if there is a charging delay, that only shortens the length of the emitted pulse from the LED, I would only have to compensate by adding that time interval to the turnoff delay.  If I have read properly, these should be able to switch in the tens of nanoseconds in their normal use.

Am I missing something?


Darlingtons for me on heavy loads... so much better at carrying current.. And so much cleaner and faster switching.. Guess it was time for me to move out of the 1940s...  makes it clear how far my formal electronics training went... basic theory, and I wasn't paying attention anyway.

This makes me wonder if you are driving the mosfets properly. Do you account for the gate capacitance?

I think that quote is out of context. The words right before that quote were "no more Darlingtons ..." and the "so much better at carrying current" sentence references MOSFETs, not Darlingtons.

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