Question on LED Passenger car lighting in Conventional Operation

Most likely yes.  Especially if the LEDs stay on when you press Bell (if you have that).  At a given track voltage you might see the brightness change when you press Whistle.

You probably know you can "make" a bridge rectifier using 4 diodes?  That is, you probably bought the 1N4007 diodes in a multi-pack.

Yes I can make them from single diodes but I have some of the square type available. I'll give it a go. 

This topic raises some questions about LED lighting. Most of us use 12V strip lighting for pax cars, which is great for command operation where the track voltage is 18 VAC or so. But for conventional where the track voltage may only be in the 9-12 VAC range, the power to the LED strip might be marginal, or inadequate causing dim operation at times. (Though it is true that using a bridge diode instead of half wave, per above, will provide about a 20% "boost" in the DC over the rms AC value) It might be better to use 5V strip lighting, which would always get the necessary voltage to run at nice brightness. You would only need to set the reg output to 5VDC instead of 12VDC, or use an LM7805T reg chip, however you are doing it. And it would still work just fine at 18VAC track voltage, no problem. Of course if you are using gunrunnerjohn's current limiting module you don't need to worry, as it limits the current to the strip to 5-45 ma regardless.

I searched 5VDC LED strip lighting and found that most offerings are the brighter 5050 style, but I did find at least one site that offers 5VDC style 3528 lighting; ebay item 183008467037. 

Opinions?

Rod

Success! Installed bridge rectifier --> 470uf 35v cap --> 510 ohm resistor --> + pos LED. Lights up nicely and is very steady when the whistle function is used on the transformer.

jth877 posted:

Success! Installed bridge rectifier --> 470uf 35v cap --> 510 ohm resistor --> + pos LED. Lights up nicely and is very steady when the whistle fiction is used on the transformer.

"Better" transformers are designed so that pressing Whistle or Bell only triggers the Whistle or Bell.  That is, the engine doesn't speed up/slow down and/or passenger cars don't brighten/dim.

Rod Stewart posted:

...And it would still work just fine at 18VAC track voltage, no problem. Of course if you are using gunrunnerjohn's current limiting module you don't need to worry, as it limits the current to the strip to 5-45 ma regardless.

...

Opinions?

It should work - but on a case-by-case basis.  The issue is heating.

For the same brightness using the same LED (e.g., a 5050), the 5V strip uses 3 times the current as a 12V strip.  That is, total light output of an LED is exactly proportional to LED current.  

In GRJ's design, the limiting factor (in my opinion) is the ability of the IC current regulator chip to dissipate the heat as it drops the voltage from the full AC command voltage down to 12V DC or whatever.   If the LED strip is now 5V (instead of 12V), the IC chip must drop an additional 7V or so (at 3x the current).  So one can do-the-math but bottom line is for low-currents (nearer the 5 mA setting) things might be OK...and to be clear OGR guys are report GRJ's modules being set near the low-end for acceptable pax car brightness.

Another option is to modify the 12V LED strip by shorting-out components to make it into what amounts to a ~9V LED strip, or even a ~6V LED strip.  You would have fewer LEDs per unit-length, but remember that the "competition" is a passenger car that might have had only 2 incandescent bulbs for the entire car!  So the LED strip approach (even with every third LED disabled) will have smoother lighting.  This would lower the heating dissipation in the IC regulator chip.

And yet another option is to add a piece of scrap metal heatsink to the IC regulator chip to allow it to dissipate more heat.

A heatsink will allow you to drive a lot higher currents, but you will also have to change the 27 ohm resistor as well.  Even with a heatsink, I wouldn't count on more than around 100ma out of that regulator, so a 12 ohm replacement would be the ticket.

John, you lost me at least. What are the 27 and 12 ohm resistors you are refering to?

Rod

With existing 27 Ohm, adjustment range using trimpot is 5 - 45mA.

With a 12 Ohm, 5 - 100mA.

What he said.   Note that I don't necessarily recommend the mod, but it will increase the current.  However, the higher current means a shorter hold-up time for the capacitor.  That cap was really sized for a maximum of 45ma, drawing a lot more will cut the flicker resistance way down.

Stan and John; thanks for the clarification. I get it now. 

The best, Rod

If you wanted to have a high current version, here's some suggested changes.  You'd also need a heatsink on the LM317 to handle the load.  I've dropped the resistor to 12 ohms, the pot to 50 ohms, and upped the capacitance from 330uf to 1120uf.  This obviously makes the module bigger, no way around that, the bulk of the size increase is the capacitors.

Note that when running at higher track voltages, the LM317 has to dissipate all the current to the LED's across whatever voltage drop is required for the LED's. 

Let's say, for argument's sake, that you're running two LED's in series, so they need 6V to run.  You should be able to generate that from 6 VAC on the track, the caps will charge to the peak voltage of around 8 volts, give the LM317 a couple volts drop and you're there.  However, if your LED's draw 100ma, and you're running with, let's say, 14 VAC on the track, the caps will charge up to around 18 volts allowing for two diode drops across the bridge.  Now you're shedding 12 volts around the LM317 at 100ma, or 1.2 watts.  That will require a decent heatsink to keep the regulator from going into thermal shutdown.  It goes without saying that you need to keep the regulator away from plastic surfaces.

John, just wondering, can these parts be mounted on your same pcb?

Rod

Nope, you'd need a new PCB layout to mount the parts.

Kinda what I figured. Thanks John!

Rod

It would have been too easy the other way.