Sourcing UV integrated resistor leds

[dalpets]

I sincerely doubt those eBay leds are capable of 3000 mcd , but even if they do that, they're only 20 degrees angle.

...

I have used this instructable to build the exposure box (yeah, I am too lazy to calculate the angles etc.):
http://www.instructables.com/id/UV-LED-Exposure-Box/

It works a treat and the narrow angles are not really a big problem - you need many of those LEDs anyway to get a reasonably uniform illumination. Compared to the Farnell LEDs you have mentioned I could probably build several of those boxes using the much cheaper Chinese ones - and it will likely work the same for the intended purpose.

Of course, for critical work I wouldn't rely on a rig like this, but then I would go for a proper light box using UV CFL tubes and not bother with LEDs. Or I would send the the board to be manufactured in a board house.

@Melkor: The free resistors are a convenience, you don't have to use them I think they ship resistors calculated for 12V use, if I remember correctly, i.e. you buy 100 LEDs and they give you a roll of 100 matching resistors.

The eBay LEDs are perfectly fine for hobby work.

It's been a very long time since I've done trig, so I too would like to avoid it, if possible.

Before I dive further into this project it gives me some confidence to know that you were successful with it.

So, could you tell me what separation distance you used between the artwork & the led boards & what you consider to be the max size board you could expose with this arrangement?

Were you able to establish exposure times for different size boards?

Did you make any significant changes to the author's design?

I must say I don't like the author's power rails setup. Too easy to create short circuits. I used individual raised pins from a male header strip with heat sink inter stages.

Did you persist with the  author's recommended & finicky smd type installation of the resistors, or did you install them instead on the led side of the board? I found that if you don’t follow the authors smd style install, but install the resistors through hole with short raised leads they tend to short curcuit against adjacent solder joints with handling.

Thanks for any feedback.

Regards
dalpets

[dalpets]

No, they are not integrated. LEDs with built-in resistors is a specialty item. Not very common. If you combine that fact with UV LEDs not being very common either, the chance of finding what you want becomes even more slim.

Why don't you ask your source, "channelengineer" in the youtube video?

With normal LEDs, connect as many of them as you can in series and run the entire string at a higher voltage. Then you only need one resistor per string, which should be easier to manage.

Unfortunately many of these youtube authors don't monitor their sites, so more often than not you can't get an answer

[dalpets]

It would not be practical for them to have integrated resistors as the value of the resistor would be dependent on the Forward Voltage of the LEDs and the voltage of the source you use to power them up.

Well that doesn't seem to be the case if this engineer can do it, it can be done, mate.

  (@19min30s)

regards
dalpets

[Zero999]

As everyone mentioned above, it's better to jut buy the LEDs and select the appropriate resistor value or even better, use a switched mode constant current source.

As far as choosing the right LED is concerned. The mcd rating is meaningless with UV LEDS because it's the radiant intensity, corrected to allow for the sensitivity of human vision at different frequencies. Any pure UV source below 380nm will have an output of 0mcd because the human eye isn't sensitive to that wavelength.
http://en.wikipedia.org/wiki/Candela

Most of the LEDs discussed in this thread aren't really UV but visible violet i.e. peak emission at 405nm. This will work for exposing PCBs, according to what I can find about photoresists 300nm to 450nm will work to some degree so you could even try blue LEDs but peak sensitivity occurs in the 350nm to 380nm region. Unfortunately LEDs are less efficient and more expensive at these wavelengths so violet (400nm to 420nm) may be give the best compromise between the sensitivity of the photoresist and LED efficiency/cost.
http://www.dupont.com/content/dam/assets/products-and-services/electronic-electrical-materials/assets/DEC-Riston_GeneralProcessingGuide.pdf

[SeanB]

It would not be practical for them to have integrated resistors as the value of the resistor would be dependent on the Forward Voltage of the LEDs and the voltage of the source you use to power them up.

Well that doesn't seem to be the case if this engineer can do it, it can be done, mate.

  (@19min30s)

regards
dalpets

Those are lamp assemblies, designed as a drop in replacement for a standards automotive wedge base lamp. They are not LED's alone, but a 10mm packaged LED with 2 68R 0.125W resistors soldered onto the LED leads, the other end of the resistors then being fed out of the moulded base and bent over to emulate the form factor and leadout of the wedge lamp it replaces. Horribly inefficient, but lifetime and power use a lot better than the incandescent lamp it replaces, with the caveat though of it now being polarity sensitive so not a universal replacement for the lamp it is designed to emulate.

[janoc]

It's been a very long time since I've done trig, so I too would like to avoid it, if possible.

Before I dive further into this project it gives me some confidence to know that you were successful with it.

So, could you tell me what separation distance you used between the artwork & the led boards & what you consider to be the max size board you could expose with this arrangement?

I have built it on a eurocard-sized piece of stripboard, pretty much the same as the guy in the instructable.

Re max size of the board - that depends a lot on the distance of the LED array from the board and exposure time. I haven't played too much with it, but I wouldn't go beyond something like eurocard with it - the lighting wouldn't be uniform towards the edges anymore. Small boards are fine.

Were you able to establish exposure times for different size boards?

Did you make any significant changes to the author's design?

For me it took about a minute to expose my board, at cca 25cm distance. However, that may no work the same for you - different LEDs and I am using the dry film negative resist (the foil that you laminate on the board). You will need to experiment.

I must say I don't like the author's power rails setup. Too easy to create short circuits. I used individual raised pins from a male header strip with heat sink inter stages.

Yes, the shorts were a problem, all it takes is a little solder bridge or molten insulation on the wires. However, the wiring can be done in various ways, it is not all that critical.

Did you persist with the  author's recommended & finicky smd type installation of the resistors, or did you install them instead on the led side of the board? I found that if you don’t follow the authors smd style install, but install the resistors through hole with short raised leads they tend to short curcuit against adjacent solder joints with handling.

Thanks for any feedback.

I have put them "smd-style" as he did. I didn't have any shorts with them - just make sure you align the leads along the stripboard track before soldering & cutting, don't leave them "hanging over". Whenever I had a short somewhere, it was either a solder bridge between two strips on the strip board or a wire touching something it wasn't supposed to touch due to the insulation shrinking/melting somewhere.

In hindsight it would have been easier to just use SMD resistors instead, but I had a bag of regular 1/4W resistors that came with the LEDs on hand.

[dalpets]

Thanks Janoc for your reply.

The Channel Engineer has replied & here's the long & short of it.

Ultra Violet Wedge LED Indicator Lamp, 9mm Lamp, 10mm dia., 12V dc
available from http://uk.rs-online.com           item code 0336160 ( not exactly the one he used on youtube)   

This lamp has an integrated IN4148 diode for reverse voltage protection & the lamp has an integrated 450 ohm resistor for current limiting

Cheers