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Old 27th Jan 2020, 4:23 pm   #1
dave_n_t
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Default UV leds - wrong polarity

I'm in the process of making a UV exposure box (for home-made PCBs), and have just finished soldering 96 5mm UV leds on a veroboard panel in a nice regular arrangement (32 chains of 3 in series, with a 75 ohm resistor for each chain).

Having checked for miswiring, solder bridges, etc., I was somewhat disappointed that 4 of the 32 chains were not working on powering up. Extensive testing found 4 (out of a pack of 100) of the LEDs with the 'flat' side on the wrong wire. Turning them the 'wrong' way round fixed it + the board's now ready for 'calibration' testing.

(With hindsight I should have tested each LED for polarity before soldering )


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Old 27th Jan 2020, 5:50 pm   #2
G6Tanuki
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Default Re: UV leds - wrong polarity

Heh! These things are sent by the Gremlins in order to keep us on our toes! I once had to fault-find on a 'first production' run that foxed other people until I discovered that several hundred of the tiny light-blue-with-bands signal-diodes installed had reverse-polarity.

Good to hear you tracked it down in the end though!
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Old 27th Jan 2020, 7:33 pm   #3
David G4EBT
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Default Re: UV leds - wrong polarity

Not wishing to hijack the thread, but I guess your exposure box might possibly be this one, by Joe Farr, or something similar:

https://www.hobbyelectronics.net/con...sure-unit.html

Super bright UV LEDs are now quite cheap when bought in quantity - even from UK suppliers.

This UK supplier for example supplies 100 either 3mm or 5mm UV LEDs for £6.18p post free (2 x 50 at £3.09):

https://www.ebay.co.uk/itm/Super-Bri...934bda2cac78ba

I don't think any LED suppliers quote the UV wavelength but it's reasonable to assume that they fall in the desired UVA band of 315 - 400 nM, which is what we need for PCB exposure, and is the same band as insect killer UV fluorescent tubes. This video about converting a UV insect killer into a PCB exposure box, has an interesting explanation at the beginning about UV bandwidths:

https://www.youtube.com/watch?v=REnWbrRgZOQ

Every success with your UV PCB endeavours Dave.
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Old 27th Jan 2020, 8:12 pm   #4
Al (astral highway)
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Default Re: UV leds - wrong polarity

Hi Dave !

What an experience and you show great resolve and tenacity in sleuthing the problem and fixing it!

Can we have a photo of the array please ?
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Old 27th Jan 2020, 8:26 pm   #5
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Default Re: UV leds - wrong polarity

I thought it was just me.
Over the years I've had 3 LEDs with the flat on the anode lead and some no flat at all ...
I now tend to look for the larger metal form inside the LED (and short leg if new) to identify the cathode.
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Old 27th Jan 2020, 8:43 pm   #6
dave_n_t
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Default Re: UV leds - wrong polarity

Quote:
Originally Posted by David G4EBT View Post
but I guess your exposure box might possibly be this one, by Joe Farr, or something similar:

https://www.hobbyelectronics.net/con...sure-unit.html

David, it isn't based that one, but another one I found on t'internet. I've reworked the layout based on what I had to hand. But I was inspired to go the UV route after reading your answers to Wendy's questions a short while ago. I've tinkered with PCBs over many years (at least 50) - letraset, 'paint', Dalo pens, etc; until now have stayed clear of any photographic technique...




Quote:
Can we have a photo of the array please ?
Al,
Here's the rear and front views of the completed board:


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Old 27th Jan 2020, 9:28 pm   #7
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Default Re: UV leds - wrong polarity

That's similar to the one I put together. They are damn bright and work really well. Cheap ebay UV LEDs were used. Fortunately no problems with polarity!

Only have a half complete photo. It is resigned to the cupboard now thanks to JLCPCB
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Old 28th Jan 2020, 3:15 pm   #8
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Default Re: UV leds - wrong polarity

Did you find, by any chance, the difference between blacklight and black light?

I'll get my coat...
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Old 28th Jan 2020, 7:09 pm   #9
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Default Re: UV leds - wrong polarity

In my experience the 'physical' polarity even of visible-light high-brightness LEDs is the opposite of that of normal ones. If you look through the half-opaque lens of a conventional 5mm or 3mm LED you can usually see a 'big bit' and a 'small bit'.

In conventional (normal brightness) LEDs the 'big bit' is invariably the negative supply lead and the 'small bit' is usually the positive supply, but in an identical looking 'high brightness' LED the 'big bit' is usually the positive and the 'small bit' the negative.

I found this out because I got into the habit of working out which was the positive lead by looking inside the LED, and that worked for years until the first time I tried to use a high-brightness LED.
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