Resistor for 0402 LED

[Mike C]

  I'm wireing an LED into my new loco . Have all the wires run and have a 1000 ohm 1/2 watt resistor , but is this enough ? It's a steam loco so I don't want it too bright , but do want to see it . BTW it's hooked to a Tsunami 2 2200 sound decoder . According to the users manual I can adjust the brightness of the LED I think .....Mike

[Point353]

The resistor value depends, in part, upon the specs for the particular space modulator LED you're using.
A typical white LED has a forward voltage drop V_F of about 3V.
The Tsunami 2 outputs about 12V.
So, the net voltage across the resistor will be about 9V.
A typical white LED is specified to operate at a forward current I_F of about 5mA.
9 V / 5 mA = 1800 ohms .
You could try starting out with a 1000 ohm resistor, but you probably don't want to go to a lower value.
If the light is too bright, then keep increasing the resistor value until you get the desired result.
 

[RBrodzinsky]

You don’t really need 1/2, but 1000 ohms is very normal. Doesn’t hurt anything, but takes up room. I tend to use 0603 SMD resistors.

[Mike C]

  Thanks ! Space (modulator ) isn't a problem as it's going into an HOn3 loco .

[tehachapifan]

You certainly don't want an earth-shattering kaboom!

[reinhardtjh]

You certainly don't want an earth-shattering kaboom!

Unless it's obstructing your view of Venus. 

[craigolio1]

I use 10k resistors on my 0402s if the light is to be viewed directly. They are like suns with only a 1k.... in my opinion.

[Mike C]

I use 10k resistors on my 0402s if the light is to be viewed directly. They are like suns with only a 1k.... in my opinion.

 I seem to remember reading somewhere here that a 5K was suggested  for that reason , but I couldn't find the thread . The decoder does have a CV to adjust the brightness , so I think I'll try to combine a bigger resistor with the dimming CV . Now to find the 1 store in town that sells resistors .....Mike

[eja]

This might be useful  ... from Ngineering

https://www.ngineering.com/led_calculators.htm

[Mike C]

 Thank You ...Mike

[soo]

Might want to use the X19 model.
Y-it

[mmagliaro]

Another vote for a higher value.  A white 0402 is blindingly bright on a 12v source until you get up to a 2.7k ohm resistor.  You should buy an assortment of values from 1k up to 10k and see what you like.  Also, you don't need anything bigger than 1/8th watt for an LED (which will be a lot easier to find room for inside a loco than a 1/2 watt device).  You could go smaller with a surface mount resistor, but I assume you want something with leads.

[Mike C]

  So far I've wired it up twice and I get nothing . Resistor on the blue wire , with black LED wire to blue wire with Red wire to white decoder wire . Can't tell which wire is cathode so I am assuming red is hot and black is ground . Resistor is 1/4 watt 5K ....Mike

[wvgca]

easy way to tell ....
positive is longer of the two wires [if they haven't been trimmed]
or the negative has a 'flat spot' on the lower part of the led ..
or the positive is the internal 'stick' part, and the negative is the much larger 'flag' part  ...
the blue wire is positive all the time ...

just for standard 3mm / 5mm type leds .... smd type easiest way is to use a multimeter under 'diode' or read the spec sheet

[RBrodzinsky]

  So far I've wired it up twice and I get nothing . Resistor on the blue wire , with black LED wire to blue wire with Red wire to white decoder wire . Can't tell which wire is cathode so I am assuming red is hot and black is ground . Resistor is 1/4 watt 5K ....Mike

The blue wire from a decoder is + and the white is -. Reverse your wires