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UV LED Exposure Box

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How to build an Ultra Violet Exposure box using LED's.

Your last Veroboard project!

A UV exposure box is an extremely useful piece of kit. It can be used to make proper PCB's. It can also be used to make other things such as intricate photo etched parts (a subject for another instructable). The trouble is they can be a little pricey for the hobby enthusiast especially if you want the double side type.
This instructable outlines the construction of a double sided UV exposure box using the recent generation of high brightness UV LEDs.

Why use LEDs?
LEDs are far more energy efficient than either incandescent or fluorescent lamps offering between 5-10 times more efficiency making them cheaper to run and kinder to the environment. They also (unlike fluorescent tubes) do not contain mercury. LEDs have a far greater life span than the other types of lamp measured in decades rather than months. The frequencies being emitted are also in a tighter band making UV LEDs safer than the traditional UV tubes. There's also just something cool about LEDs, I can't put my finger on it, but ever since I was a kid I've found them to be one of the more fascinating electronic components.

Is there a disadvantage to using LEDs?
Not really, however the UV exposure box I have detailed here is a little less powerful than the commercially available ones. This means that your exposure times will be around 2 ~ 3 minutes as opposed to 30 ~ 40 seconds, but come on, do you really need your PCB's to be produced that quickly? Anyway sometimes having a slightly slower exposure time can be an advantage allowing you a little more control.

This UV Exposure box will consist of 2 UV panels; each having 84 LEDs a total of 168 LEDs. Each panel will draw about 700mA at 12v. This makes each panel 8.4watts a total of 16.8 watts for the whole thing.
 
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Great project mate, followed as best I could but am having problems getting all of the LEDs to light up despite checking the circuit and replacing components. I am using .5w 75r with 3.2-3.8 V, 75ma peak, 20ma forward LEDs. And a 12v 7.5a power supply to test. Could it be a problem with this combination? Any advice appreciated!
Bertz1 month ago

I just completed my own UV exposure box. I used 96 UV LEDs in a 12 x 8 matrix (good for 4" x 6" boards). LEDs were approx. 3" from exposure window. The LEDs were 1200 mcd with 160 degree viewing angle. Exposure time was 2 minutes.

jmatharu4 months ago
hey! very useful post!
I was wondering instead making a new circuit board can we use the LED light strip?
I am a graphic student and want to use it to expose my screen printing screens will it work on them?
Many thanks.
Dillon123 4 months ago
Parts express has UV led kits of 100 for $9.99
jbromley8 months ago
This is a very nice instructible. It is the most complete UV box I've seen. I do have one question. Is there a reason you put the jumpers and resistors on the copper side of the board and not on the component side? Was it just an aesthetic decision? Or will the UV cause component degradation of some type?
agodinhost2 years ago
Hi, please, were did you bought your leds? I did looked in ebay but I can't say for sure that those leds there really have 20º view angle. All leds that I found there are kinda suspicious, chinese cheap fake stuff, even tough they say they are 20º I can't trust. I'm in Brazil and at here ebay is the more easy way to get electronics by a decent price but not in this case.
I found these in Germany. They are more expensive, but because of the angle of 120° they are more suitable for illuminating flat surfaces. This is exactly what we need. http://www.ebay.nl/itm/50-UV-Leds-4-8mm-120-200mcd-Led-STRAWHEAD-schwarzlicht-ultraviolett-/370543226494?pt=LH_DefaultDomain_77&hash=item564618ee7e#ht_1926wt_1139
Isnt 200mcd WAAAY too little?
jeddar2 years ago
I have a question, would the output from this UV box be enough for it to be used as a sterilizer box? I do my own beer/wine brewing and I'd love to be able to fit a rig I could use to sterilize equipment in between batches..
brsullivan jeddar11 months ago
Sterilization also takes place at 365nm, however, to get the type of power needed would require a much more robust, i.e. $$$, set of LEDs that can output some real power. You'd also have to do some extensive testing to make sure sterilization is taking place, the research articles I've seen were using 15 to 30 minutes exposures to achieve complete sterilization of bacteria.

I think using a non-toxic spray method would be a lot quicker and cheaper in the end.
CODIY jeddar2 years ago
No, the wavelength for sterilization is much shorter than this type of LED can emit (<300 nm where these are likely closer to 400 nm). UV LEDs for sterilization are expensive and difficult to find in small batches.
MFXPYRO11 months ago
I built a double sided UV exposure box with vacuum pulldown around 10 years ago using tubes and it is still going strong without replacing the tubes. Also my box will do boards up to A3 size, How many LED's and what would they cost for an A3 sized double sided unit? Yes they're more energy efficient but no one is going to have a unit running for more than a few minutes at a time so cost is negligible. You've made a nice looking unit but I just can't see any real benefit of LED's over tubes both in cost and construction time. TBH. Hopefully I'll post an indestructible of my unit soon but as I built it so long ago I'll need to dismantle it for photos.
Aquafuz12 months ago
I'm a bit lazy to research this so I guess I'll just ask,
Which will give better and faster results; total watts, total mcd or both?
lsilva141 year ago
I managed to finish this project, but with some mods of my own :p

Instead of painting the wood white, I covered it with metallic duck tape which gives me extra brightness.

I also tested some pcb that usually take 120s in a retail exposure box with UV lamps and they took the same 120s in my box with the LED.

Thanks for the idea and the design, it is excellent!
Peterflick1 year ago
So i found some LEDs but they peak at 400-405nm, everything else checks out. Will they still work?
Jake3131 year ago
Awesome project! Are those Metal Film Resisters?
Jakob28031 year ago
Looks very nice, I might copy your project and make a scaled-down version! ;)
rlorenzo2 years ago
can i just use a normal LED.....?
no because a normal coloured LED does not operate at the right frequency to be used for this purpose due to the wavelengths being different this diagram i found on google images shows the uses of the wavelength. :)
Gallery_Image_8935.jpeg
rlorenzo2 years ago
can i just use normal LED's ??? this parts are hard to find here back in Pilippines.

UV LED is available at e-gizmo (taft ave, along DLSU). 3.50 PHP a piece.
dwah2 years ago
this glass too thick for LED UV going pass thur glass..
Vogavt3 years ago
I built the box, but I seem to keep blowing LED's. I'm using the supplier that you recommeded and everything appears to be okay. However, I noticed that when I turn off the wall-wart type 12v power supply, I have some latent voltage residing on the board. I can see some residual glowing of some of the LED's immediately after turning the unit off. It does eventually fade out, but if I turn the unit back on too quickly, I have a bright flash of the LED's. It's as if there is a capacitor somewhere that's holding current in the 12v transformer. Any ideas or suggestions to bleed off the current? I'm a newbie at these sort of things.
Vogavt Vogavt3 years ago
Okay. After surfing around the web I've found out what a "Bleed Resistor" is, but I haven't been able to find out definitively how to calculate the amount of resistance needed or exactly where to put it in the circuitry. Any suggestions would be welcomed.
Why guess resistors? Calculate:

Most efficient way is to use make strings of few LEDs in series with resistor (and you can connect many such strings in parallel)

R=(V-n*Vf)/If

P=R*(If)^2

where

R= resistance (Ohms)
V= power supply voltage (Volts)
Vf= LED forward voltage drop (usually about 2.2-3.8V for most LED, read specs)
If= LED forward current (usually 20mA, read datasheet)
n=number of LEDs in series (in one string)
P = resistor power (Watts, minimum rating)

note: you should aim to get the voltage difference to be about 2V:
(V-n*Vd) ~ 2V

reason:
if the difference is greater, the more energy is wasted into heat by resistor (you need more powerful resistor and power supply).
if the difference is too small you loose current regulation because LEDs are non-linear device.

calculation example, assuming you are using

V=12V
Vf=3.2V
If=0.020A (that is 20mA)
n=3

R=(12-3*3.2)/0.020=120 Ohm
P=120*0.02^2=0.048W (you can use 1/8W or 1/4W without problem)

note how P is small. that is because we have (12-3*3.2)=2.4V

using large resistors makes no sense because to get current to match spec, you would need high voltage (this is not just dangerous but also means that most of the energy is wasted in heat so those resistors would have to be for high power).

for example using same scenario
Vf=3.2V
If=0.020A (that is 20mA)
n=3
R=47k

we would need resistor to have power rating of at least
P=47000*0.020^2=18.8W

(and that was just power three LEDs)

Also DC power supply would need to provide much higher voltage which is
V=n*Vf + R*If
V=3*3.2 + 47000*0.020
V=9.6 + 940
V=949.6V

That is way too much just for poweering few LEDs.

So that was introduction for current limiting resistors.
For bleeding resistors we calculate it from time constant.

t=RC

where
t = time (seconds)
C=capacitor in power supply
R= resistor (note this is combined resistance of load and bleed resistor)

R=t/C=Rb*Rc/(Rb+Rc)

Rc=V/(m*If) this is load resistance (for m LED strings drawing current If)
Rb=bleed resistor

Say you have simple 12V PSU with bridge rectifier and 10000uF capacitor and 6 LED strings drawing 20mA

C=0.010 F
m=6
Rc=12/(6*0.020)=100 Ohm

Without bleeding resistor t=RC=0.010 * 100= 1 seconds
which is shourt enough

adding bleed resstor in parallel, we can bleed the capacitor faster (total resistance is smaller and t=RC is smaller).

Say we use Rb=18 Ohm

R=18*100(18+100)=15.25 Ohm

t=RC=15.25 * 0.010 = 0.1525 sec

so turn off time was reduced by about 6x.

note that value of bleed resistor was quite small. usinglarge value such as 10k or 47k would not make any difference.

also keep in mind power disipated by bleed resistor P=12^2/18=8W
(which is more than used power for LED circuit in this example).

good luck
http://youtu.be/qPt0X5s-1JU
djhamer (author)  Vogavt3 years ago
You’ll need to put your bleed resistor across the whole circuit, i.e. between the + and – supply. Make sure its in the 10s of K ohms e.g. 47Kohms.
dedson12 years ago
By varying the types of LEDs to get the proper output you could also use this as a light box for seasonal affective disorder. Just a thought.
djhamer (author)  dedson12 years ago
I really wouldn't, UV is very bad for your eyes. The amount of UV energy coming out of this lamp is more than enough to do some damage. Even a few second is enough to cause discomfort. Since the consecration of light intensity is in a band invisible to the human eye, your eyes cannot adjust properly and you natural reaction to look away as you would if you looked at the also does not occur.
agodinhost2 years ago
There is a site that provides one wizard to help you with any different setup in mind: http://led.linear1.org/led.wiz

Just enter your number of leds and the wizard will help you to decide the resistors, the arrange of leds and the PSU that you will need.

My 5 cents, I hope it helps.

Cool instructable by the way, really cool stuff.
Love the link.
rykonen2 years ago
I love this instructable. How do I figure out how much power my LED's will need? I want to do four times the amount you have here... Could I just repeat the LED corn rows a few more times before I run it to the 12v ? (That idea doesn't seem right some how...)
Nazdro rykonen2 years ago
Hi,

I'll assume you're using the same kind of LEDs and resistors that are specified then for every veroboard you've got 84 LEDs in bundles of three connected in parallel. Thus for every bundle you've got a forward current of:

I_f = (12V - 3x3.4V)/75ohm = 1.8/75 =24mA

So one veroboard, with 84 LEDs, will draw a current of

I_vero = (84/3)x24mA = 672mA

So if I've understood you correctly you wish to have 8 veroboards of UV LEDs then after you've connected them in parallel the total power drawn from the source would become:

P_total = 12Vx(8x672mA) = 12Vx5.375mA = 64.5W

So you'd need a 12V 65+W power supply, though to be on the safe side I'd personally use a 75W. Though as I said, you might be using LEDs with a different forward voltage so here's a formula where V_f is the forward voltage of your diodes:

P_total = 2304(4-V_f)/25 W
rykonen Nazdro2 years ago
Looks like I will be able to use certain laptop chargers, as some of them have these exact specifications. Very cool. Thanks again.
Nazdro rykonen2 years ago
You're welcome, and I hope it works out for you :D. Though as with all such things, take care that everything is unplugged when you're working on it. 5 amperes are quite the punch if they ever decide you're the path of least resistance.

Just out of curiosity, what kind of setup needs that big an exposure surface?
rykonen Nazdro2 years ago
The box I have is big and I want to be able to make stencils for screen printing, as well as other things like guitar amps and synthesizers in the future... :) oh summer!
rykonen Nazdro2 years ago
Wow, thank you! That was incredibly helpful. It's nice to hear these formulas from a couple different places to get the application correct.
Pyroflea3 years ago
Would I bet better off having a 7 x 7 grid of 7000mcd LEDs, or a 10 x 10 grid of 3000mcd LEDs? I don't make very large boards, so both are of adequate size. I get get either configuration for the same price, so I'm just curious which to go with.
DeanM873 years ago
Hi, nice instructable! was just wondering, where you got your LEDs? well, ebay obviously! but what seller? ot Store? there are many but most dont list all the specs. some leave out hte mcd! some leave out the wavelength! one store i found was TopBright LED store. is that the place you got yours?

Regards,
Dean
loluengo3 years ago
Has anyone had experience using this UV lamp for curing UV solder mask? Is it enough powerful for this use?

Great instructable!
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