# how to test LEDs



## txdyna65 (Aug 17, 2012)

I have a bunch of LEDs that I would like to test to see if they work. Is there a simple way of doing this?
Edit : Guess I should be a little more specific, these are 3mm and 5mm red and green single LEDs with the long and short metal leads coming off them.


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## NIMT (Jan 6, 2011)

Simple use any DC power supply 3volt to 12volt and a 1K ohm resistor LED's are polarity sensitive, if it doesn't work on direction chage the direction of the wires.


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## txdyna65 (Aug 17, 2012)

Figures I would need a resistor, guess I will go see if I can find some 1k resistors tomorrow. So looking at your drawing just use 1 resistor total, or one coming off the pos side and one coming off the neg side?


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## NIMT (Jan 6, 2011)

You just need one, it can just be on either one of the sides of the LED.


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## txdyna65 (Aug 17, 2012)

Ok got ya, Thanks Sean


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## coupman35 (Dec 9, 2012)

You can use a small flat 3v battery also no need for a resistor


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## gunrunnerjohn (Nov 10, 2010)

Actually, a 3V battery is a bit much for colored LED's, best to use a resistor.


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## T-Man (May 16, 2008)

I use the battery all the time. It works fine for checks. Best way to check them. Not the best quality but that is why they work. The dollar store three pack is a good buy. Maybe I will check the voltage.

The 1.8mm drive me nuts! I am constantly checking for the positive side.


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## gunrunnerjohn (Nov 10, 2010)

Measure the current through a red LED powered by 3V. I just tested it, exactly 3V, red LED, and the forward current was 65ma, or more than three times the maximum rating. Did it kill it? No. Is it a good idea to test them this way? Not IMO. I don't run components beyond their ratings if it can be avoided, and in this case it's easily avoided.


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## txdyna65 (Aug 17, 2012)

Well I didnt have to go buy resistors afterall. Awhile back I bought a decoder and some LEDs from Sean and he had put some small resistors in the package with the LEDs. I have no clue what ohm they are, they are tiny to me, but I know nothing about resistors. Anyways, I used one and found out which LEDs were good and which werent. Thanks again Sean


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## NIMT (Jan 6, 2011)

If they were from me they are most likely 1K's, Blue resistors with some colored stripes right?


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## txdyna65 (Aug 17, 2012)

yep those be the ones


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## T-Man (May 16, 2008)

gunrunnerjohn said:


> Measure the current through a red LED powered by 3V. I just tested it, exactly 3V, red LED, and the forward current was 65ma, or more than three times the maximum rating. Did it kill it? No. Is it a good idea to test them this way? Not IMO. I don't run components beyond their ratings if it can be avoided, and in this case it's easily avoided.


In that case, I may go with a 1.5 volt instead.


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## txdyna65 (Aug 17, 2012)

Sean while on the subject of this, if I wanted to put some LEDs in a building, could I use a 250ma cell phone charger plugged into the wall and use the one 1k resistor and wire them in series, say 3 or 4 LEDs this way?


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## NIMT (Jan 6, 2011)

Yes you can. You can also wire them in parallel, Just use one resistor per LED.


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## fred up (Apr 29, 2013)

1k, colour bands. Brown, Black, Red.

For cheap components try Bits Box.

http://www.bitsbox.co.uk/

No minimum order, if you only want 1 resistor that is all you need to order.(postal charge applies)

I have no connection with this shop other than a satisfied customer.

If you want to calculate the resistor value for single led's try here.

http://led.linear1.org/1led.wiz


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## gunrunnerjohn (Nov 10, 2010)

Shipping's a killer to the US for one resistor.  Fortunately, we have some excellent resources here as well, Digikey and Mouser are two of my favorites. Typically, shipping for small orders from Digikey is only about $2 or so. Mouser charges more for shipping, but many items are a bit cheaper, so for large orders they're very useful.


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## golfermd (Apr 19, 2013)

Might be able to find them at Radio Shack.


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## gunrunnerjohn (Nov 10, 2010)

My goodness! If you really want to overpay, RS is the place to go!


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## NIMT (Jan 6, 2011)

I order in bulk and have 10,000+ LED's and resistors on hand.
You can never have too many LED's.

I always try to keep a stock pile of Warm Whites which are perfect for headlights and building lighting.
I have SMD in 603, 805, 1210,and 3830's and 1.8mm, 2mm, and 3mm

I also just got in a bunch of 1.8mm red, yellow and green's that are perfect for signals in HO and N


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## gunrunnerjohn (Nov 10, 2010)

I keep a stock, but not quite like that. I probably have several thousand, but I don't do tons of other folks work either.


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