How many LED Resistors

@Trainmstr posted:

I’m planning on adding LED headlights and bi-color classification lights to an engine.

If a headlight (assumed white LED) on the front end is tied in parallel to red classification lights on the rear end, can one resistor near the power source be used or are resistors required for each LED? No, I cannot recommend a single resistor. This is because the LEDs are not the same color and exact type so they will have different forward voltages and thus with a single resistor solution- likely only one color of light would light (red being first, being the lowest forward voltage).

Does the resistor/s go on the cathode or anode leg of the LEDs? Normally doesn't matter. Resistors resist current and are not directional devices. The current is the same in the circuit with the resistor before the LED or after the LED in series.That said, you mentioned bi-color LEDs- which changes the scenario. They are typically 3 pin or wire devices, with either common cathode or common anode. The trick here is, then do not put a resistor on the common leg- put the individual resistors on the color legs- and being different colors, they would be different resistors to achieve the same level of current and thus brightness.

Again, here is the fundamental problem. Resistor "sharing" (one large resistor in series and several  LEDs in parallel) only works if the LEDs are the exact same color and batch, have the exact same forward voltage, and then are expected to share the same voltage and current equally. If you violate this rule of not using matched LEDs (color, size, and other specs) then one LED will likely conduct first and steal all the current from the other LEDs. Worst case scenario- this burns out the first LED to light and then even worse a cascade event happens killing all the LEDs. Again, this occurs because the single resistor is sized for the combined current of all the LEDs, but because the LEDs aren't matched, one color or one LED lights first, taking the bulk of the current- until it blows, then the next LED takes the brunt of again all that combined current until it dies.

Your question is compounded by the fact not only did you have a white headlight, but also bi-color LEDs, most likely (you didn't say it) red and green LEDs in the bi-color ones. Being that red, green and white all have different forward voltages, they need individual LED resistors to limit the current when put in parallel scenarios.

First, what kind of engine, old school or modern?  Does it already have a steady DC source to power LED's?  Will you have to rectify and filter your AC track volts picked up by the loco to run your DC LED's?  If the latter, this opens up several issues as to rectification and voltage regulation that are not particularly difficult to deal with, but you will be doing more than putting resistors inline with an LED.

One resistor per LED cathode.  Avoids multiple issues.  If the power source is track voltage directly, you should use a constant-current rectifier & regulator circuit card such as those from JWA Enhancements, Royz Trains or Rod Stewart.  Blunami, TMCC, and ERR command controllers output DC voltage for lighting - the regulators are helpful in the latter two cases to avoid flickering when encountering dirty track or center rail gaps.

@Trainmstr posted:

I’m planning on adding LED headlights and bi-color classification lights to an engine. If a headlight on the front end is tied in parallel to red classification lights on the rear end, can one resistor near the power source be used or are resistors required for each LED? Does the resistor/s go on the cathode or anode leg of the LEDs?

It's always helpful to give details on the environment you're working with, TMCC, Legacy, DCS, DCC, conventional, etc.

@Trainmstr posted:

I’m planning on adding LED headlights and bi-color classification lights to an engine. If a headlight on the front end is tied in parallel to red classification lights on the rear end, can one resistor near the power source be used or are resistors required for each LED? Does the resistor/s go on the cathode or anode leg of the LEDs?

This is being over thought you can go online (ebay) and buy any color you want with the resistor already added. This way you just tie together from the source to light what you want. The LED’s are all 12v.

20pcs Pre Wired 1.8mm Bright White Led Lamp Light Set 12V ~ 18V for Train Layout

eBay item number:295858759663

20 pcs prewired 2mm Red LEDs pre-wired 1k resistors for 12V to 18V DC use

eBay item number:402379561557

Vernon thanks for simplifying this subject, learned a lot from your response.

This is going to be a conversion of a Williams scale GE 44 Ton to DCC via Blunami using 180 bricks for track power. To date, Karl has been a real help answering my many questions (Thanks Karl).

Based on Vernon’s response, I will attach the proper value resistor/s to each LED and it doesn’t matter if the bi-color LEDs are single cathode or single anode. Correct?

Final question, does dc+ go to the cathode or anode of an LED?

Thanks to all who have responded!

@Trainmstr posted:

Final question, does dc+ go to the cathode or anode of an LED?

Doesn't matter, the current has to flow through the whole circuit.  The resistor can be anywhere in the circuit.