# led's and 470 ohm resistors



## Cobra GT (May 25, 2014)

Is it necessary to add a resistor to a led I am hooking up to a backup light in a gp9 m trainline with tsunami decoder. Tried it with and with and they both work on the tsunami and on a 12v power supply. I think the tsunami puts out 5v if not mistaken.


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## rkenney (Aug 10, 2013)

LED's operate at a specific voltage and current. The resistor in series drops the voltage to the required amount and limits total current through the led/resistor circuit. Hence the name current limiting resistor.

Supply voltage minus desired (led) voltage (Vf), divided by desired current flow (led current If) equals series resistor in ohms.

Just round up or down to the nearest standard resistor value.

In your case the output from the decoder might already be suitable. Many chips specify exactly what current and voltage they can provide for an output.


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## Ray Haddad (Feb 19, 2014)

rkenney said:


> LED's operate at a specific voltage and current.


Only if there is a resistor inside of the LED. All diode junctions drop around 1.5v to 3v (LEDs are diodes, after all) when they are forward biased (LED lit) no matter what the voltage coming into it is. It could be 1000v, for that matter, if the current is limited to no more than 15ma. The voltage will indeed determine the value of the resistor in the circuit which is often referred to as a current limiting resistor. There is a formula to use.

Some LEDs operate differently so you need to know the full spec on the one you are using. What I cited above is for the common, garden variety LED. This link below explains it in detail but without too much complexity.

http://www.microbuilder.eu/Tutorials/Fundamentals/OhmsLaw.aspx


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## DonR (Oct 18, 2012)

Here's an easy to use calculator
to determine correct resistor for
LEDs (or any bulb).

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

Don


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## gunrunnerjohn (Nov 10, 2010)

The decoder may have current limiting for the LED lighting. If it doesn't, for a 5V DC voltage supply, you need at least a 100 ohm resistor for a white LED, and a 180 ohm resistor for red or green LED's.

White (and blue) LED's operate nominally on 3V, and red/yellow/green LED's operate nominally on 1.5 volts. A larger resistor isn't a problem, but a smaller one will allow too much current to the LED and greatly shorten it's normally long life.


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## golfermd (Apr 19, 2013)

You will either burn up the LED, or it will be extremely bright without the resistor. The former is more likely.


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## rkenney (Aug 10, 2013)

As GRJ and I already pointed out it is quite possible that the output on your decoder already provides the necessary current limiting. Refer to the spec sheet!


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## Cobra GT (May 25, 2014)

Thanks guys for the help the spec sheet I think it said jusy not to exceed a certain ma for all 4 light wires thanks yep nothing about a resistors just not exceed 100ma with all 4 combined as well as motor stall curent


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## Hutch (Dec 19, 2012)

You will need a resistor for each LED to keep it below 100mA. I have a Tsunami decoder and I used resistors. It's just normal procedure. I don't remember what the instructions specified though.


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## Hutch (Dec 19, 2012)

I just looked and the spec sheet and it definitely calls for a resistor when you are using LEDs. No resistors are required if you are using a 12V incandescent bulb.


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## Ray Haddad (Feb 19, 2014)

gunrunnerjohn said:


> The decoder may have current limiting for the LED lighting. If it doesn't, for a 5V DC voltage supply, you need at least a 100 ohm resistor for a white LED, and a 180 ohm resistor for red or green LED's.
> 
> White (and blue) LED's operate nominally on 3V, and red/yellow/green LED's operate nominally on 1.5 volts. A larger resistor isn't a problem, but a smaller one will allow too much current to the LED and greatly shorten it's normally long life.


Sorry, John, but a 5v circuit requires a 330 ohm resistor at a minimum, not 100 ohm or 180 ohm.


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## Hutch (Dec 19, 2012)

Hmm.. I just did the math. 5V - 3V White LED = 2V across the resistor. 2V /.02A = 100ohms. I think John had it right.

I didn't look up the current rating of a white LED so it may very well be different.


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## gunrunnerjohn (Nov 10, 2010)

Ray Haddad said:


> Sorry, John, but a 5v circuit requires a 330 ohm resistor at a minimum, not 100 ohm or 180 ohm.


Sorry Ray, but you're wrong. Almost all the common LED's you'll encounter are rated at 20ma maximum current. You need to drop two volts for a white LED, 50 ohms per volt, unless my math has totally failed me, that's 100 ohms.

Since someone nicely posted a link to an LED calculator, I computed it for you.


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## golfermd (Apr 19, 2013)

Ohm's law still can't be broken. :laugh:


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## Cobra GT (May 25, 2014)

I thought since I know so much I would tweek Mr Ohm, s law just a little bit.lol I really dont care ir I burn up a trainline I am worried about my $100 decoder.


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## Ray Haddad (Feb 19, 2014)

John, the LED is typically 15ma, not 20ma. Sorry. 330 Ohm for 5v still stands. But hey, you go ahead and do what you want. Besides, you're always right - or so I'm told.


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## Cobra GT (May 25, 2014)

Guys guys I hav some old stock radio shack one and ylows are 20 ma red are 28 ma and greens are 25 ma the party pack I just bought are all 20 ma red yellow green clear white


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## gunrunnerjohn (Nov 10, 2010)

Ray, where does this "typically" come from? You can run an LED on any current up to it's maximum specification, but that doesn't make it a 15ma LED.

I went to Digikey and did a search on thru-hole LED's, I picked white. Then I scrolled down the column, and guess what? All of them are rated at 20ma. A couple at the end said 10ma test. However, when you check the data sheet, they are rated at 20ma continuous forward current. I did the same thing with red. Again, most were tested at 20ma, but the ones that said 10ma test are actually rated at 20ma. I did find some rated at 30ma as well. However, until you get down to the really small SMT LED's, you don't find less than 20ma ratings.

I don't know about always right, but in this case I'm on solid ground. Why is it that whenever I disagree with you that you get defensive? Could it be you don't like being corrected when you're wrong? Clearly, in this case, you're wrong. I suggest you just live with it and move on.


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## Ray Haddad (Feb 19, 2014)

I rest my case, John. Well done. You are right. As I was told, you are never wrong.


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