Power supply for LED light box (for photography).

[lookimback]

I bought one of those soft boxes from eBay (for photography) and I don't like it. The incandescent lights which came with it get really hot and stink, and they don't do much more than light the canvas it's made from. So, I want to make my own. I've been thinking about doing this for a while and I have lots of diffuser sheets and LED strips from LCD TV's I've parted out, which I think I can use to make the box. The real question comes down to powering the LEDs. I know the ones I have are 3vdc per LED, So I can do the math and figure out that I need 21v for each strip of 7. I think 7 strips on the top will work, but I don't know if I'll need to light more than just the top, so I'll probably want to design something capable of handling more load than that. If I had to light 3 sides and the top, that would be 28 strips. I don't know how much current these draw, so I'll need to know how to figure that out. My main question is how good does this power supply need to be? Can I just go with a bridge rectifier, a voltage regulator, and some caps to smooth out the ripples? Can I just go with something cheap and simple like a buck converter? I have some LM338Ts I bought a while back for a bench power supply I haven't gotten to yet, I'll probably want use them.

[budm]

Re: Power supply for LED light box (for photography).

Do you have bench power supply that can be set to constant current mode?
This is good magazine:
http://www.ledsmagazine.com/articles...-magazine.html
http://www.edn.com/design/led/442453...el-LED-strings
http://www.designingwithleds.com/con...iver-circuits/

[lookimback]

Re: Power supply for LED light box (for photography).

I don't have a bench power supply yet. I'd planned on building my own someday.

[redwire]

Re: Power supply for LED light box (for photography).

In photography, I have mixed results using white LED's. Usually not the greatest. Their spectrum is "peaky" compared to incandescent lamps and correcting white-balance in Photoshop doesn't work for me, it's impossible.

Try any white LED lamps first with your camera and editor before spending the big bucks. Warm white seems better but pure-white looks to the eye better.

[lookimback]

Re: Power supply for LED light box (for photography).

I'm only looking for better quality pictures of TV parts, not really anything too important. I just want to be able to get some clear pics without shadows or washed out images. I don't necessarily have to use the LEDs from the scrap TVs, I just like to repurpose as much as possible.

[kc8adu]

Re: Power supply for LED light box (for photography).

tv leds are on the blueish/purplish side.
fine for a reef tank but probably suck for photogaphy.

[lookimback]

Re: Power supply for LED light box (for photography).

Are there LEDs which would be OK for photography? I can buy new if necessary. I'm just looking for something cheap which can cover a 24" x 24" box evenly.

[stj]

Re: Power supply for LED light box (for photography).

you can get high CRI leds in surface-mount package pretty easy,
i'm not sure how much you would have to space them though to get the brightness and coverage your after.

[lookimback]

Re: Power supply for LED light box (for photography).

I can experiment with spacing. Even through hole LEDs would be fine, then I can just daisy chain them. Not sure just how I'll hold them in place though. Maybe I could get some perf board and make a bunch of modules with a specific number of LEDs in series (however many it takes to equal the voltage I go with), and then parallel the modules.

[stj]

Re: Power supply for LED light box (for photography).

well through hole is a no-go because modern led's use a phosphor coat.
i would put smd leds in series on stripboard.

something like these:
http://www.rapidonline.com/electroni...-series-502663

or these:
http://www.rapidonline.com/electroni...te-5050-538670

lots of company's make high-cri leds now.
CRI btw is how well it reproduces colours.

[lookimback]

Re: Power supply for LED light box (for photography).

CRI btw is how well it reproduces colours.

That was going to be my next question.

[kaboom]

Re: Power supply for LED light box (for photography).

That was going to be my next question.

CRI btw is how well it reproduces colours.

Then there is correlated color temperature (CCT), the "color" the light appears to be. It refers to the temperature of an ideal black-body radiator.

Please note, you can have high CCT and abysmal CRI- those disgusting "full spectrum" and "daylight" fluorescent lamps of years past are shining examples.

You'll want a CRI of at least 90, and CCT from 3k to 4k. I would actually use several CRI grades of LED; you'll be using the peaks of one grade to fill the valleys of the others.

So maybe one 2700, 3000, and 4000; with two 3500, per 5-LED array.

You'll want an even number of such arrays, each pair arranged like this:
3.5k-4k-3k-2.7k-3.5k
3.5k-2.7k-3k-4k-3.5k

If multiple pairs:
3.5k-4k-3k-2.7k-3.5k
3.5k-2.7k-3k-4k-3.5k
3.5k-4k-3k-2.7k-3.5k
3.5k-2.7k-3k-4k-3.5k

etc...

This evens out the different CCTs, so you don't end up with areas illuminated by mostly one color.

You can also explore the option of the modern fluorescent lamp, with it's high frequency (no flicker) ballast.

Certain ballasts can have their outputs paralleled to "overdrive" the lamps. Two two-foot strips, each with a 3k and a 4k lamp, should do nicely. But you'll have to pay attention to ballast factor, to avoid having high expectations of lumen output.

If you make your own striplights, or replace the ballasts in existing ones, you'll need a four-lamp ballast for each pair of tubes if you overdrive.

Modern instant-start ballasts are SMPSes, with high OCV. Most of the ballasing action happens by small capacitors in series with the lamps, just as in LCD TVs/monitors. Once the lamp strikes, the cap limits the current thru the lamp; the lamp itself determines its own voltage across it.

A PFC booster or doubler/rectifier provides a DC bus. The DC is switched at high freq into an inductor or transformer primary. Usually one inductor or transformer per pair of lamps. The other end of the inductor, or secondary of the transformer (whichever is used), is connected to one end of two capacitors. These are AC-rated caps capable of high reactive power, like polypros. The free end of those two caps each feeds a lamp. The lamp filaments are shunted (instant start). The other end of each lamp returns to the bus common.

When you parallel two outputs, you are putting two of those ballasting caps in parallel for a given lamp. This almost doubles the lamp current and just about increases the luminous output by 1.4 for fully rated outputs.But remember ballast factor!

F17T8 and a .88 BF= ~15W. Putting two outputs in parallel- F17T8, .88BF x2= ~30W.

[lookimback]

Re: Power supply for LED light box (for photography).

Thanks. I like the idea of using the different cri grades.

[kaboom]

Re: Power supply for LED light box (for photography).

Thanks. I like the idea of using the different cri grades.

I would actually use several CRI grades of LED; you'll be using the peaks of one grade to fill the valleys of the others.

I meant CCT grades. I knew I'd eventually mix the two up!
That's what happens when posting late...


CRI should be close to 90, or higher. Some of those colors may be around 88, but with the other ones "helping," that won't be a problem.

[lookimback]

Re: Power supply for LED light box (for photography).

I meant CCT grades. I knew I'd eventually mix the two up!
That's what happens when posting late...

Nothing to CRI about