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Another ordering mistake

A project log for Portable LED Flash

As modern photography is revolutionized by LEDs, I'm applying my experience in this field to enhance my hobby

Yann Guidon / YGDESYann Guidon / YGDES 04/04/2017 at 17:578 Comments

The project started a few weeks ago, with a little misunderstanding.

The idea came from my unpractical 10"×10" 60W panels and I was looking at the new cheap 100W COB modules. My panels are nice because they mix both warm white and cold white, which makes them suitable for not-too-bad photography, but the 100W COB are single-temperature...

I mostly find cold white (8000K) or warm white (3000K), both of them are unsuitable : I need something inbetween. And on eBay I find 100w modules with "full spectrum" light : Great !


Of course, you suppose what is going to happen : I ordered 4 modules, got them after waiting, then tried them.

The result is totally unsuitable for photography because it creates a weird pink/fuchsia tone !

Then I understand why they are used and made in bulk : for growing plants. I don't. Anybody needs 400W of chlorophile-friendly light emitters ? I'm open to trades :-D (electronics-related)


The happy ending (of this episode) is that I have already found 4500K modules. I'm waiting for the delivery of 2 others...

(yes, the coating messed with my camera's white balance)

Discussions

C.Savage wrote 04/07/2017 at 16:48 point

You might actually be better off going with a Bridgelux COB LED. They come in various color ranges (3000k to 6500k) and sit around 36W power consumption (36V, 1A). They spit out a decent 4000 Lumens. You *might* need a bit of active cooling though. Ive used a smaller one for a bench light and Im pretty happy with it. 

They are around $10 each, and another $1.25 for the little connector. Add in a decent heatsink for another $12. The advantage of these LED's over the chinese ones is that you have a verified source of color temperature and you know what you are getting.

Take a look:

https://www.digikey.com/product-detail/en/bridgelux/BXRC-40E4000-C-73/976-1430-ND/6152440
I picked out a neutral color LED.

Here is the tiny connector

https://www.digikey.com/product-detail/en/molex-llc/0688014229/WM9599-ND/3880999

Side note: You must keep the temperature of these LED's below 85C or else it will loose brightness and longevity. To find out how hot they will get, use this formula:

t°=Ambient+(Pd*Θs-a)

Where
Pd is power dissipated in Watts

Θs-a is the thermal resistance of your heatsink (Measured in °C/W)
and Ambient is the Ambient temperature (°C)


So for example, this heatsink (which I think is the wrong one anyway)

https://www.digikey.com/product-detail/en/aavid-thermalloy/NX301117/1061-1149-ND/4215386

has a Thermal resistance of 1.9 C°/W

Our LED dissipates 36W of heat. Assuming a temperature of 25C, we just plug it in

=25+(36*1.9)
=93°C

Which is close to the absolute max of 125C. You will loose some brightness and longevity without active cooling. Your thermal resistance of the heatsink will go down as you blow on it. You could also use a big heatsink.

Compare to this heatsink (I am currently using this one myself)

https://www.digikey.com/product-detail/en/aavid-thermalloy/HSSLS-CALCL-019/1061-1157-ND/4215401

1.2 °C/W

=25+(36*1.2)
=68.2°C


Which gives us about 10°C worth of room if ambient gets higher.

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Yann Guidon / YGDES wrote 04/07/2017 at 19:24 point

Thank you :-)

my choice of LEDs is already made but your computations still hold :-)

however they don't consider the under- and over-driving aspects of the project. The heatsink is obviously too limited for a 400W rating, and I'll run the LEDs in under-current in the "continuous lighting mode", which only lasts several seconds. The idea is to watch the heatsink's temperature and adjust the current.

This allows the full power to be used for short pulses and the heatsink can absorb the spikes of heat.

With 1) active temperature regulation 2) short spikes, the thermal resistance formula is less relevant.

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Brad Buhrkuhl wrote 04/25/2017 at 21:33 point

I second getting a good, high quality Bridgelux.  You can get actually bin'ed parts from reputable sources, up to 95 CRI!  They are WELL worth it over the ebay junk!

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Thomas wrote 04/05/2017 at 17:57 point

For growing "plants" you say?

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Yann Guidon / YGDES wrote 04/05/2017 at 18:10 point

yes, that's what they say....

potatoes, strawberries, whatever is made of chlorophyl ;-)

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Thomas wrote 04/05/2017 at 18:41 point

Sure. Next thing up on HaD: a "refrigerator to potato-grow-box conversion" project.

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K.C. Lee wrote 04/05/2017 at 18:43 point

My potatoes grows pretty well in the bottom drawer in the fridge even without lights.  :(

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Yann Guidon / YGDES wrote 04/05/2017 at 18:54 point

muahahahaha that'd be a killer project and a potential winner :-P

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