Author Topic: DIY LED Exposure Unit  (Read 11960 times)

Offline Pangea

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DIY LED Exposure Unit
« on: January 20, 2018, 09:49:44 AM »
I'm in the process of rebuilding my LED exposure unit and figured I would document it here.

Through looking at some tech sheets it looked like most emulsion crosslinks at about 395 nm wavelength. I was surprised to find this wavelength was available in inexpensive strips and 2 density layouts. I opted to get the densest at 120/meter rather than 60/meter because more = better, right?

I ended up using 11 34" strips mounted on a piece of coroplast that was then stapled to a box made of 1"x4"s. It looks pretty janky and like a fire hazard which is the reason it's getting rebuilt.


The power supply is 120v AC in and has two 12v DC outputs that are rated at 10 amps. The time controller was purchased on eBay and pretty much interrupts the 12V DC circuit when the time is up, shutting off the LEDs. The switches on the laser cut control panel turn all the power on, start the timer and turn on safe lights (yellow LEDs). I intended to purchase glass at some point but it was never needed because I use a space saver vacuum bag for exposure. For the new build I'll probably end up adding glass and a proper vacuum lid in phase 2.

I plan on reusing the time controller/switches and the power supply but not the LED panel or box. This time around I ordered 4200 LEDs because I want the strips to be much closer together like the M&R Starlight/Workhorse Lumitron. The strips will be soldered this time instead of using the crappy snap connectors and will be done in parallel to avoid differences in brightness. If they're wired in series the end strip has voltage drop and the whole exposure would be uneven.

So now for the before pictures:

Wiring Diagram


Turned on


Janky Wiring


Control Panel Mock Up


Control Panel Finished


First Tests (WBP Emulsion)


Switched to Cryocoat with 18 second exposure




Final exposure time using a Stouffer 21 step is ~80 seconds. I'm curious (and I'm sure someone here can answer) when companies are advertising their exposure units can expose in 10 seconds, are they using the 21 step guide?


Anyway, I'll update this thread when I start the rebuild and make sure to add any updated documentation (wiring diagram). Until then, ask any questions if you have them!


UPDATE:

Redrew the schematic during my morning coffee


« Last Edit: January 20, 2018, 10:45:33 AM by Pangea »


Offline Prince Art

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Re: DIY LED Exposure Unit
« Reply #1 on: January 22, 2018, 10:27:15 AM »
Thanks for sharing. I built one similar to to this a couple of years ago. I need to go back & solder the connections - I do have a few places where brightness isn't consistent. The clips were a simple way to get the thing put together, but aren't great quality.
Nice guys laugh last.

Offline IntegrityShirts

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Re: DIY LED Exposure Unit
« Reply #2 on: January 22, 2018, 11:17:53 AM »
Looks good. I went down the same path a few years ago. Final results were ok, but finding a reliable quality LED source was the variable I ran into. The M&R LED's are not the same as the ones we can get off AliExpress or from Amazon. I built mine as an experiment, but it also can serve as a back up if my 5k Olec decides to die on me. I hope as technology progresses we might find a single point source LED in the 1000W range that is reasonably priced with enough wattage to REALLY replace a 5k traditional source.

Offline Pangea

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Re: DIY LED Exposure Unit
« Reply #3 on: January 22, 2018, 06:38:28 PM »
Just finished putting together my updated version:




There are a few dead LEDs because I had to pull them from my old unit due to a math error on my part. Soldering took about 3 hours so I'll probably post a proper update tomorrow.

Online blue moon

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Re: DIY LED Exposure Unit
« Reply #4 on: January 23, 2018, 11:56:00 AM »
try to eliminate the gap between the sections. It causes the light field intensity drop in that area and potentially weaker exposure.

Also, see if you can get a current regulating power supply. This will stabilize your light output and prevent overheating.

there's more to think about, but all together looks like a pretty good start. Looking at the exposed screen images, they look pretty good!

pierre
Yes, we've won our share of awards, and yes, I've tested stuff and read the scientific papers, but ultimately take everything I say with more than just a grain of salt! So if you are looking for trouble, just do as I say or even better, do something I said years ago!

Offline mimosatexas

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Re: DIY LED Exposure Unit
« Reply #5 on: January 23, 2018, 12:19:00 PM »
Based on the last image, isnt the glass pretty far from the bulbs?  Arent most of the LED units closer to like an inch or two from the glass vs 4 to 6 like in the image?

Online blue moon

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Re: DIY LED Exposure Unit
« Reply #6 on: January 23, 2018, 12:31:39 PM »
Based on the last image, isnt the glass pretty far from the bulbs?  Arent most of the LED units closer to like an inch or two from the glass vs 4 to 6 like in the image?

glass distance has to be calculated based on the cone angle. It should be set up so there is a correct overlap  from each LED without undercutting from the side bulbs. This is part of the reason why some of the manufacturers are just making really crappy units. They have field intensity variation of over 20% (in one case almost 40%!!!). Imagine having a stencil with weak spots throughout the image from being underexposed by 40%!

pierre
Yes, we've won our share of awards, and yes, I've tested stuff and read the scientific papers, but ultimately take everything I say with more than just a grain of salt! So if you are looking for trouble, just do as I say or even better, do something I said years ago!

Offline mimosatexas

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Re: DIY LED Exposure Unit
« Reply #7 on: January 23, 2018, 05:19:28 PM »
I know it requires calculation, but I thought one of the major benefits of using these LED strips is the dense clustering of the LED meant the sweet spot was significantly closer than something like a flo bulb unit.  I cant find any useful info on the angle of the cone from these kinds of strips, but I would assume its somewhere around 20 to 30 degrees, which should result in something between 2 and 3 inches away max.  All speculation of course without actually testing.

Online blue moon

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Re: DIY LED Exposure Unit
« Reply #8 on: January 23, 2018, 05:31:38 PM »
I know it requires calculation, but I thought one of the major benefits of using these LED strips is the dense clustering of the LED meant the sweet spot was significantly closer than something like a flo bulb unit.  I cant find any useful info on the angle of the cone from these kinds of strips, but I would assume its somewhere around 20 to 30 degrees, which should result in something between 2 and 3 inches away max.  All speculation of course without actually testing.
I’m guessing you might be able to get something as low as 45, but almost all are around 120. That makes for a lot of undercutting if the distance is too far.

Pierre
Yes, we've won our share of awards, and yes, I've tested stuff and read the scientific papers, but ultimately take everything I say with more than just a grain of salt! So if you are looking for trouble, just do as I say or even better, do something I said years ago!

Offline mimosatexas

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Re: DIY LED Exposure Unit
« Reply #9 on: January 24, 2018, 10:26:38 AM »
if the angle is 120, that would mean the ideal distance sits closer to an inch away max unless im missing something

Online blue moon

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Re: DIY LED Exposure Unit
« Reply #10 on: January 24, 2018, 10:29:47 AM »
if the angle is 120, that would mean the ideal distance sits closer to an inch away max unless im missing something

it would depend on the LED spacing. The further apart they are, the further the glass has to be . . . Vastex unit still has the best field I've measured (data is kinda old now though) and their unit is about 6" away.

pierre
Yes, we've won our share of awards, and yes, I've tested stuff and read the scientific papers, but ultimately take everything I say with more than just a grain of salt! So if you are looking for trouble, just do as I say or even better, do something I said years ago!

Offline Northland

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Re: DIY LED Exposure Unit
« Reply #11 on: January 26, 2018, 08:59:00 PM »
This post caused me to look a little closer at my exposure unit. I'm using a DIY LED that is about 300 watts of 5050-SMD diodes (1500 total)... that are mounted 1.75" below the glass. The spacing between diodes and rows of diodes is 16mm (5/8") and covers about 18" X 32" in total area, so it's a pretty tight pattern. I don't know what angle the diodes broadcast at... but I think the 1.75" distance between the diode and glass is a good compromise between even exposure, without too much undercutting.

The nature of my work is predominantly simple spot color stuff... so I've never really investigated it's maximum capabilities.
For a long time I burned S-mesh screens at 15 seconds.... but changed that to 45 seconds to increase durability (I catalog a lot of my screens for re-use and the removal of tape was pulling emulsion from the edges of the screens). The longer (45 second) exposure time fixed that problem.

I've just burned a 180S, using a Chromaline test film, and I would say the dots between 10%-90% we're really good... and a 6 point font looks good too.
The 90% to 100% range looks the same (fully Open) and the 0% to 10% range looks the same (fully Closed).
Maybe I could squeeze a couple percent better by adjusting the exposure time, but that's a capability that I haven't needed, yet.

So.... I'm guessing that a decent DIY LED will require about 75 watts of power per square foot.
And, the spacing between diode rows should be the same as the spacing between the diodes on the strip.

The quality of LED strip is questionable.... I needed 15 meters of strip to get 10 meters of uniform product, but it's cheap so that isn't a deal-breaker.



 
« Last Edit: January 26, 2018, 09:01:11 PM by Northland »

Offline Pangea

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Re: DIY LED Exposure Unit
« Reply #12 on: January 30, 2018, 05:23:44 PM »
Based on the last image, isnt the glass pretty far from the bulbs?  Arent most of the LED units closer to like an inch or two from the glass vs 4 to 6 like in the image?

Yes, I had the panel about 4 inches from where the screen rests for exposure. I've never seen an LED exposure unit other than photos and those weren't too helpful in figuring out the proper distance. I exposed a new test screen this morning and raised the panel up 1.5" higher to see what would happen. The Stouffer test came out at a pretty solid 9 with 25 seconds exposure on a 305 with Cryocoat emulsion so I could probably cut it in half and hit a decent 7.

Here are some photos:






Online blue moon

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Re: DIY LED Exposure Unit
« Reply #13 on: January 30, 2018, 05:27:23 PM »
going to 7 from a 9 requires doubling the time rather than cutting it in half. Or am I missing something?

pierre
Yes, we've won our share of awards, and yes, I've tested stuff and read the scientific papers, but ultimately take everything I say with more than just a grain of salt! So if you are looking for trouble, just do as I say or even better, do something I said years ago!

Offline Pangea

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Re: DIY LED Exposure Unit
« Reply #14 on: January 30, 2018, 05:37:43 PM »
Other way around. I'm aiming for a 7 but I'm at a 9 (which is slightly over exposed).