Confused about LED circuits and IC drivers and diodes

@Craftech posted:

I noticed that one can make a stay alive LED lighting circuit with a bridge rectifier, capacitor, and either a variable or fixed resistor for the light intensity and an optional inrush resistor on the input side of the bridge for DCC so the DCC controller doesn't see the lighting circuit as a short when it turns on.

But some circuits have an IC driver and a diode.  Tried looking it up, but not sure of the practical difference.   Why the IC driver and why the diode?

Thanks,

John

John,

Unlike most other electrical devices, such as lamps, solenoids and relays, all of which usually prefer or at least tolerate a constant voltage feed, LED's prefer a constant current source feeding them.

The IC delivers much better control of current, to make sure that it's indeed constant.  LED drive circuits consisting of resistors and capacitors alone are approximations at best.  Imprecise or fluctuating current over the operating voltage range, especially on the high side, is what leads to early failure with LEDs.  The IC version is better, but as a result is unfortunately also more expensive.

Mike

If you use a bridge rectifier and a capacitor, the LED's will be incredibly bright with 18V on the track as you'll be driving with far more than 12 volts!

I have posted a complete build kit for a passenger car lighting module that does the whole job for you. If offers DCS compatibility, flicker-free operation, and adjustable intensity.  Many people here have build them and use them.  The circuit design is exactly the same as my commercial modules that are sold on Henning's Trains.  My commercial modules just use surface mount components for a more compact footprint.

If you order the components overseas, the cost is probably less than $2. The schematic, Bill-Of-Material, and PCB files for fabrication are all available below.

Pass_Car_Lighting thru-hole (top adjust).zip

Thanks guys,

I understand about the IC now, but what about the diode?  And thanks for the PCB file and parts list John.

Right now I am working on a set of HO passenger cars for my older son that will operate on DCC and also DC layouts.  That PCB and parts list won't fit.   There is also a variable resistor in the circuit for the intensity.  Both ends of the floor of the cars are taken up with a strange lever system that makes the couplers swivel when the trucks turn.  That limits the number of parts for this project.

For the O Gauge that PCB circuit looks super for when I get to it.  I usually spend my workbench time doing things for family and never seem to quit get to it though.  Buy hey, that's what it's all about for me. 

John

For HO, I suspect you really need to consider some surface mount ideas, thru-hole components and construction are just too big for those jobs.