Author Topic: High power LED flash  (Read 2713 times)

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Offline beeedy

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High power LED flash
« on: July 22, 2014, 10:47:17 am »
Hello!

For a school project, I am currently working on a high speed camera flash implementing some high power LED’s! I have worked on and finished the variable pulse generation circuit to create a 5V pulse to trigger the LED, but am hitting a brick wall when it actually comes to powering the sucker.

The LED I am currently using is 37VDC, 2.2A super bright, and I am struggling a lot with how to actually power and control this guy. I found this simple little demo for a circuit that appears to be able to be controlled with my 5V pulse, but I just want to verify this will actually work without harming the LED’s in any way. http://www.instructables.com/id/Circuits-for-using-High-Power-LED-s/step8/a-little-micro-makes-all-the-difference/

I am also having difficulties sourcing a 37VDC power supply cheaply. Sure I can use my bench top power supply, but the idea would be to make the whole circuit as portable as possible, even a simple wall-wart would suffice. I have surfed digikey and mouser and although they have power supplies that look like they could handle 37V on the output, they don’t get in the range of 2.2A that this LED needs.

Any point in the correct direction of how to take my 5V pulse and control the high power LED would be greatly appreciated!
 

Offline richard.cs

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Re: High power LED flash
« Reply #1 on: July 22, 2014, 10:50:57 am »
What do you mean by "high speed"? Because they're phosphor-based normal white LEDs are pretty poor for this - even though the blue pump diode is really fast the yellow phosphor isn't.
 

Offline beeedy

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Re: High power LED flash
« Reply #2 on: July 22, 2014, 11:35:06 am »
As fast as possible. I don't have the LED's in front of me at the moment however so can't tell you exactly what type they are
 

Offline richard.cs

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Re: High power LED flash
« Reply #3 on: July 22, 2014, 06:39:33 pm »
As fast as possible. I don't have the LED's in front of me at the moment however so can't tell you exactly what type they are

Well white LEDs are "fast enough" for many common photographic applications but still very slow compared to a xenon flash, especially one operated at lower currents or with an arc-quenching SCR. You need to quantify this because if you really need "as fast as possible" then the answer will be "don't use [this type of] LEDs".
 

Offline beeedy

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Re: High power LED flash
« Reply #4 on: July 22, 2014, 10:09:00 pm »
Apologies for any confusion, White LED's are "fast enough", but like you said they are not the fastest around. I am completely ok with this! And want to try and get the fastest possible with the LED on hand.

I have found driver chips like so: http://www.clare.com/home/pdfs.nsf/0/713EA18902B59CCF85257566004FAD55/$file/MXHV9910.pdf but am wondering exactly what these offer me, as they seem to still rely on an external fet to actually do the on/off of the LED.

The LED I am using can be found here: http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&keywords=976-1147-nd
So Vf of 38.6V and 2.1A. Simply the easiest and cheapest way to make this 'flash' is all I am looking for  |O
 

Online sleemanj

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Re: High power LED flash
« Reply #5 on: July 23, 2014, 12:45:43 am »
So Vf of 38.6V and 2.1A. Simply the easiest and cheapest way to make this 'flash' is all I am looking for  |O

I'm assuming the flash is short, and the recovery time is comparatively long, so maybe you could just get away with...

A rough-and-ready boost converter charging a large capacitor to approx your LED forward voltage, clock generated from your microcontroller (or whatever), it doesn't need to be very accurate.

Either monitor the capacitor voltage using your mcu (through appropriate divider of course) and stop the clock when you are at the necessary level, or as in the diagram below you could short the clock out with a zener and a transistor (taking care of not drawing too much current from that clock - put a limiting resistor on the clock from the mcu).

NPN BJT with appropriate base resistor to do the flash.

A bit like this, ignore part numbers and values, I used a 15v clock signal because that particular software doesn't have logic level fets, but you'd use a logic level fet and a 5v clock signal.


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Offline beeedy

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Re: High power LED flash
« Reply #6 on: July 23, 2014, 10:46:02 am »
That does not seem too bad! I am wondering however why you have C2 and C3 in parrallel, as well as C1 and C4 in parallel. Is there something you are gaining from this besides an effective capacitance of 101uf?

So if I am understanding correctly, the clock is being used to slowly pulse current through the cap, charging it until it is at the voltage required by the LED. Once charged, the switch SW1 can be thrown to discharge the cap through the LED. Is this correct thinking? and if so- could SW1 be replaced with a digital signal to automate that as well?

I am still a little fuzzy on the theory behind these circuits, but if I have a source voltage of 5V how is it possible to get the cap up to 30+V?

I notice that in the C1+C4 and LED loop there does not seem to be any resistance to limit current flow through the LED. Is a resistor not needed here? and if not, why is that?

Thank you for all the help!
« Last Edit: July 23, 2014, 10:48:08 am by beeedy »
 


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