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Old 20th October 2012, 04:22 PM   #1
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Default noob: blue LED polarity

Dual Mono 3886 kit ordered recently


alright i think i installed the blue LED backwards.


the long leg against the ">" arrow

from what i understood of the guide:
"arrow ">" this points towards where the long leg is soldered in.

multimeter gives reading(1.960) only when reversing the testing leads and dont light up LED.

when i was soldering the LED lit up briefly lol

is this a problem? is there another way to test it?

thanks
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Old 20th October 2012, 04:38 PM   #2
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A good way of testing LEDs is to use a lithium coin cell such as a 2016. I have dozens around here as I bought a batch of 50 keyring torches from dealextreme.com to use as doglights.

You can connect the LED momentarily directly across the coin cell. They have ~3.6V out which is sufficient to cause any LED to strike, but so little current drive capacity that destruction of the LED is unlikely unless it has a VERY low power rating, I've never blown one up doing this.
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Old 20th October 2012, 05:21 PM   #3
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hey counter culture,

nice tricks!(they are avaliable everywhere) i will try them for sure, but i'm more inclined to just cancel the led.

can i install a jumper wire instead?

here you can see a picture of the led i removed just now

the long leg supposed to go on the left against the arrow?
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Old 20th October 2012, 05:29 PM   #4
Mooly is offline Mooly  United Kingdom
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If the LED is just purely cosmetic then you can just omit it with no further action.
You say the LED lit while soldering ? You soldered with it powered up or with the caps fully charged ? If the iron was earthed then you could have a low resistance path via the iron that zapped the LED.
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Old 20th October 2012, 05:34 PM   #5
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the board is not in a condition to be powered up still

no the boards still not wired to ac.

i just saw briefly a blue flash reflected off the worktable

the iron is 2 wire so maybe its not earth? strangest thing... anyhow yeah i will check if it is cosmetic then i dont need this.

maybe the tip is somehow carrying voltage? (no idea about this making sense or no)
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Old 20th October 2012, 05:35 PM   #6
Marra is offline Marra  United Kingdom
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Leave it out and whatever you do do not install a jumper wire unless you mean to connect to an led on the front panel.
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Old 20th October 2012, 05:39 PM   #7
AndrewT is offline AndrewT  Scotland
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That little white arrow seems to be non standard.
A LED is a diode. We usually find diode symbols and orientation to be quite unambiguous.

The LED symbol with a part circle with one flat edge is used. That flat edge corresponds to the bar of the diode.
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Old 20th October 2012, 05:40 PM   #8
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alright thanks guys, i will not install jumper and just cut it off and if the boards not working(after ac on) i'll update so we find a solution.
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Old 20th October 2012, 06:08 PM   #9
pdul is offline pdul  Denmark
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Looking at the closeup it seems that you just have to turn the led around to get + on the long leg.
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Old 20th October 2012, 06:09 PM   #10
pdul is offline pdul  Denmark
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It is very clear that you have + on the series resistor
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