Bridge rectifier diodes

[cedarwoodron]

In a constant lighting circuit, using either 3mm LEDs or smaller 0603 micro LEDs, is there a preferred voltage value, such as 100, 400, 1000v ? Or, is there a one- value- fits-all that would be best for these led sizes? I am trying to buy efficiently, as I see the possibility of making the type of circuit for a number of uses in both engines and caboose lighting. Any advice very welcome ! Cedarwoodron

[gregc]

light emitting diodes provide maximum light intensity at a specific current (and hence voltage).   Exceeding this current/voltage damages the LEDs.

With constant voltage source, such as with DCC, a series resistor can be used to meet these requirements.  See LED Circuit.

If the voltage varies, a current regulator circuit, using an lm317 can be used to limit the maximum current through the LED.

If the voltage is sufficiently large, multiple LEDs can be wired in series and either a single resistor or regulator circuit used to limit the current.   In other words, the same current through one LED can flow through additional LEDs.   A different resistor value would be need for one or multiple LEDs, but the same current regulator circuit can be used regardless of the number of LEDs as long as the voltage is sufficient.

since dcc changes polarity, an LED would not be active constantly, i believe a higher peak current would be possible, as long as the average current is within spec.

[MisterBeasley]

Are you looking for the rating for the diodes?  You will likely be using no more than 15 volts as your input, so anything that can handle that is fine.  Chances are, the lower voltage ratings will be the most economical.

FYI, you can get a "bridge rectifier" which is the same 4 diodes in a single package.

[cedarwoodron]

OP- I should have been more specific in my question- sorry. I was looking at bridge rectifier diodes on the Radio Shack website, and a number of different voltage (capacity/rating?) values were given for various ones listed. I am looking to improve some kitbash models, older DC Athearn BB diesels and I want to light a number of cabooses (Athearn BB's). I use Intermountain wheels, where one side is insulated. As long as I am re-posting, another question: some modelers use delicate magnet wire in these lighting circuits. I have a bunch of 28 and 32 AWG computer cable, where I can peel off an insulated wire as I need to- I have used these with 3 and 5mm LEDS- are they good for the micro LEDs I referenced in my prior post? Cedarwoodron

[hon30critter]

28 and 32 AWG will be fine with the 0603 LEDs.

Dave

[rrinker]

 Rectifier diodes are rated by peak inverse voltage - that is the most voltage they can take in the reverse direction (no current flowing) without breaking down. Since the absolute maximum allowed for DCC, for larger scales, is under 25V, then a common 50 or 100V PIV diode is just fine. Up to a certain point, for a given current capacity, they are all the same size. Diode size increases more rapidly for an amp increase than for voltage. It is always OK to use something above the expected rating, so if you have some 400V ones on hand, they will work just fine.

 High current is not needed for lighting a car, even a 1 amp diode or bridge is overkill for a half dozen or so LEDs in a coach.

 Capacitors are also rated by voltage, 25V is a common one, but since it is possible for DCC to get that high, it is usually recommended to use 35V (the next step) capacitors for filters and keep alives, since electrolytic capcitors tend to fail in a  spectacular fashion when overvolted or connected backwards.

                   --Randy

 

[cedarwoodron]

Thanks, Randy- that is enough to guide my purchasing plans. I have worked with 3 and 5 mm LEDs, but am now planning to start using these 0603 type micro LEDs for smaller lighting tasks, a number of which will involve constant bidirectional lighting. Cedarwoodron

[MisterBeasley]

I recently lit up a caboose.  I added an interior, while I was at it.  I also put a pair of Tomar lanterns on the back.  The Tomar lights, despite their miniscule size in HO, are incandescents, not LEDs.  I put an incandescent bulb inside, too.

I use Intermountain wheelsets, so I've only got 2 wheels on each side to provide pickup.  It' OK, and since it's only lighting an occasional dropout doesn't matter.  But, I put a decent-sized capacitor across the DC output of the bridge rectifier, of course being sensitive to polarity.  This lets the caboose coast over dead spots a bit.

This particular caboose is a Walthers bay-window model.  It already has a rudimentary "interior" of molded plastic - a couple of seats and a table.  To this, I added a tile floor and wood paneling, printed on the computer and glued in place, and a couple of figures.  To be honest, you can't see any of this stuff, except maybe the outline of one figure in the window.  But, the "wood paneling" pasted on the walls eliminates glow-through from the interior light.  It's something you might want to think about.  One day, I'll have to take the caboose apart and put something like this on the inside of the caboose roof to eliminate glow-through there. 

[cedarwoodron]

My thinking with the micro sized LEDs is that you can get a more "scale appropriate" lighting effect, as the 3 and 5 mm LEDs really do broadcast more light that is needed inside a caboose. I generally primer gray all my rolling stock interiors, as well as structures, but I picked up a can of dark gray auto primer that I like better, having used it on the last two projects, as it absolutely cuts the light bleed effect thru the walls of the car. Hopefully, as I get some experience with these smaller LEDs, I can develop a lighting strategy that incorporates variously-sized LEDs (and appropriate resistors) on future car and diesel projects. Note- I was about to order some of these from a Hong Kong supplier on EBay when I heard about that monster storm, but it hit mainland China to the north, so I guess I am good to go on putting that order in. Cedarwoodron