# LED lights



## Hamish (Apr 3, 2015)

Ok, I know this has been done to death but a couple of quick questions. Do all LED lights need a resistor? When does/doesn't an LED need one? 
Maybe there is a link to this answer?


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## gunrunnerjohn (Nov 10, 2010)

All LED's need either a precise voltage or current limiting of some type. Typically, the current limiting is a resistor for most uses.


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## wvgca (Jan 21, 2013)

most do, but not all leds, and not necessarily in all cases .... depends on the leds working voltage, 'most' are in the 2v to 3.2v range, so if your supply is higher voltage then you will need a resistor, and some leds have a built in resistor already, but those are few..
an old school easy formula is supply voltage minus working voltage mutliplied by 50 ... equals resistor value in ohms, and will give a bright light, increasing the resistor value will give a dimmer , more realistic light that usually looks better and the led will last noticeably longer ..


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## DonR (Oct 18, 2012)

Yes, most LEDs do need a current limiting device as per GRJ.

But if you have several, in a panel for example, you can use
one resistor in the lead from your power supply. It's not
necessary to have a resistor for each LED in such a situation.
You could use similar wiring to connect several LEDs to
light a building also.

The point is, you want to match the source voltage to that
of the LED. Most source voltage we use in model railroading
will be around 12 to 14 volts. You would typically use a 470
to 1,000 ohm resistor depending on how bright you want
the LEDs.

The Strip LEDs that many of us use for lighting passenger cars
and buildings have a built in resistor that serves 3 LEDs. They
do not require an external resistor if connected to 12 V dc.

Don


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## Hamish (Apr 3, 2015)

Thanks for the replies, so I have been given some red, green and clear led's and dont know any data how do I know if I need to fit a resistor?


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## T-Man (May 16, 2008)

Use the resistance to your advantage. If you are doing signals with different colors you may want to vary resistance to control brightness. That way you can have different colors with adjusted brightness to match. For 12 volts DC range 330 to 1000 ohms is ballpark. Higher is safe but lower is more apt to burn out over time. 330 is low depending if track voltage is used. In some cases it may be fatal to the LED. It all depends on how it is used. Bread boards are great to test circuits out. You can try 5 red LEDs and use 470 and step the resistance to 1000 with each LED and then compare the difference.

Going back I guess Don hit all the important points


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## T-Man (May 16, 2008)

I use a 3 volt slim battery to check LEDs The clears could be red or yellow. It comes in handy to check the polarity and ensure the LED works.


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## gunrunnerjohn (Nov 10, 2010)

My "test set" for LED's is a bench supply with a 1K resistor. I know that at 10 volts on the supply, there's no way to supply excessive current to the LED.

When you get to picking resistors to control intensity, you'll probably notice that different colored LEDs need different current levels to have similar brightness. My experience is that red are generally pretty bright on fairly low current, but green needs a bit more to match them most times.


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