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Most decoder outputs are track voltage minus about 1.5 volts (due to the voltage drop of the bridge rectifier). Some decoders do have either a regulated output or a pre-installed resistor on some outputs, so you really need to check the particular decoder's spec sheet to be sure.
Most decoder outputs are track voltage minus about 1.5 volts (due to the voltage drop of the bridge rectifier). Some decoders do have either a regulated output or a pre-installed resistor on some outputs, so you really need to check the particular decoder's spec sheet to be sure.
The only reason you need to know the voltage is so you can use ohms law and calculate the size of the needed resistor. Diodes are not devices driven by voltage, they only care about the current (hence the resistor). Even calculating the resistor size is often an unnecessary exercise. I have run across very few instances in model railroad applications where a 1000 ohm resistor doesn't work great. The only reason one would want to calculate the exact lowest value needed for a resistor is if you want the LED running at maximum brightness. I like running things well under their max specifications so they last longer. Besides, brighter isn't always better.
Even if the voltage output on the decoder is 1.5V, you need to check to make certain that the decoder manufacturer intended that to be used for an LED.
Things advertised as "12 V LEDs" are really an integrated circuit that has an LED and a resistor on one set of leads. The resistor has a value that has been calculated to limit the current to what the diode can handle at 12V.