Analog "keep alive" LED power helper question

[u18b]

A guy asked me to shift his old N scale  Arnold/Rapido Commuter car to modern LEDs.
Currently, it transfers power from the axles by springs.  The whole thing is subject to flickering. 
I told him to use a Kato car... But he has nostalgic feelings for this car.

It has two incandescent bulbs.  In one direction, the headlights light (I assume for when the train is backing).  In the other direction, the red lights come on (I assume for End of Train).

So since this is analog....

Can I do the following....
Truck left side =  diode -->  capacitor --> resistor --> LED pole
Truck right side  =  direct wire --> LED other pole

Will the capacitor act as a well when wired this way.  I assume  I will have to watch polarity.
I'm thinking the diode protects the capacitor for when the track polarity is reversed.

If I am correct, what size would be sufficient?  10 mF?

Any help appreciated.

[peteski]

Sounds like you are basically asking for Directional lighting of the front/back in DC, with some flicker prevention cap installed?

But it is not like a typical directional DC lighting since only the front has a headlight and the rear only has red lights?  When the model trvels backwards, only the red light is lit (and headlight on the other end is dark)?   That seems unusual. Usually the red rear markers are on when the train travels forward. Could you clarify?

[nkalanaga]

I think this is for a push-pull commuter train.  Going one way, the loco has it's headlight on, and the cab car would have markers on the cab (trailing) end.  Going the other way, the cab car is leading the train, so its headlight would be on.

You would still have to have both sides of the truck(s) connected to both LEDs for a complete circuit.  Call the two left and right sides.

Headlight:  Left side > diode conducts with cab leading > dropping resistor > capacitor > white LED > right side

Markers:  Left side > diode conducts with cab trailing (loco leading) > dropping resistor > capacitor? > red LED > right side 

You'd need two separate circuits, with a capacitor for each one, if you wanted flicker-free light in both directions.  My only concern is how the resistor and capacitor might interact, possibly requiring some experimenting to keep from burning out the LED.  I don't know enough electronics to answer that one!

[peteski]

I think this is for a push-pull commuter train.  Going one way, the loco has it's headlight on, and the cab car would have markers on the cab (trailing) end.  Going the other way, the cab car is leading the train, so its headlight would be on.

But Ron stated "car", not "cars", so that is why I'm confused. If it was a single commuter push-pull car then it would utilize read and white lights on both ends of the car.  Maybe if Ron can explain it better, I can provide the circuit he is looking for.

[u18b]

Sorry for any confusion. 
It is the same car.  One car.   

And both the lights are on one end.  The same end.
The model has two side by side lightbulbs that shine up into two separate compartments.

One shines up to the headlight.
The other shines to the red marker lights.

So Nate’s description is acurrate.

So I’ll be able to use two white LEDs.

So obviously, both LEDs draw from the track. 
But when the polarity switches, the lit one goes off and the other one comes on.
In that sense, it is like any other analog model lights we are used to.

My ultimate concern is the flicker induced by the inherently poor pick up scheme.

I know little SMD caps are widely used now to prevent flicker - but mostly from some kind of back EMF for the light that is off ( without the cap, the off light still fickers).

But here I’m dealing with power that is completely interrupted for a few milliseconds.

So that is the simple circuit I’m trying to determine.

I appreciate the questions, clarifications and suggestions.

[Point353]

But Ron stated "car", not "cars", so that is why I'm confused. If it was a single commuter push-pull car then it would utilize read and white lights on both ends of the car.  Maybe if Ron can explain it better, I can provide the circuit he is looking for.

Likely, "car" and not "cars" because he's only referring to a single cab car, which has the head/tail lights (on the same end) - not the rest of cars, which lack the lights.

[nkalanaga]

Point353:  That's what I assumed as well.  He wanted both lights in the same car, on the same end, and the rest of the train was irrelevant.

I've seen single-unit "cab cars", such as RDCs and most interurbans, but they aren't usually referred to as "push-pull".  This same circuit would work for one of them, just wire the headlight on one end and the markers on the other together.

[jdcolombo]

To get back to Ron's original question:

Yes, for what you want you can use a diode and a cap for each LED, with the circuit supply rails reversed (obviously) for one of the LEDs.  The diode should come before the cap, which would be wired in parallel with the LED, but BEFORE the LED dropping resistor (you don't want to wire the cap after the dropping resistor, because it will supply full current to the LED in that case and burn out the LED).   What you described in your original post works.  See following diagram:

https://model-railroad-hobbyist.com/node/23755

The diagram shows a 100-ohm protection resistor to avoid in-rush current problems, but I doubt you need it for something that is going to run on analog DC only.  Make sure the cap is rated for at least 20v; probably a 35v one would be even better since the DC voltage to the track might be as high as 16v or more - it really depends on what kind of power pack he's using.  Assuming the worst is the best approach.

John C.

[u18b]

Thank you John.

That's what I needed.  I wasn't thinking about the resistor being in front of or behind the capacitor.
(and glad to know I'm not the only one to ask that question).

[peteski]

The circuit in John's post is basically correct. Here is my take on this.



R1 limits the in-rush current to the capacitor.
I chose the value of R2 (LED current limiter/brightens control) so even at 12V (max. throttle setting) the current does not exceed LED's maximum current, yet it will still have plenty of brightness at lover voltages (since most modelers do not run their DC trains at full throttle).  As shown, at 12V the LED current will be 11mA.  At 6V it will be 3mA (still plenty bright).

I also designed a modified version of the circuit.



The only difference is the added protection of a Zener Diode.  Since many DC throttles have peak voltage, or voltage spikes exceeding 12V by quite a bit, the Zener diode together with R1 will clip any higher incoming voltages down to 15V.  Main reason to do this is to protect the voltage-sensitive Tantalum caps from catastrophic failures (flames).  Last think we want to do is to burn up the precious model.

Where in the first circuit I recommended using caps rated for 35V (to have a margin of safety in case the DC throttle exceeds 12V), with the Zener diode I feel it is safe to use caps rated for 20V.  Especially the Polymer-Tantalum caps which are not as prone to burning up as the standard Tantalum caps.

To make the circuit as compact as possible I recommend using SMD (lead-less) components. Those can be stacked and soldered together, and if the solder pads do not contact directly, use some thin bare wire. I use stripped 30AWG wire wrap wire for this task. Then use thin insulated decoder wires to connect the clump of components to the LEDs, and to the track pickups.

A 100uF capacitor might be large enough to stop the flicker.  If a larger capacitance is needed, I recommend stacking multiple 100uF caps in parallel (that is most economical due to the prices of higher capacitance Tantalum-Polymer caps)

Here is a list of components from the Digikey site.  If you prefer to use components with leads, I can look those up for you too.

C1 options
CAP TANT POLY 100UF 20V 478-11484-1-ND $1.72
CAP TANT POLY 100UF 35V 478-9993-2-ND  $4.94

CAP TANT POLY 220UF 20V 478-13377-2-ND $4.56
CAP TANT POLY 220UF 25V 478-12543-2-ND $5.00

There is no 220uF 35V Tantalum-Polymer cap available at Digikey, so I selected one rated for 25V.  But I recommend using either the 220uF 20V or 2x 100uF 20V cap with the Zener diode.  That is of course if more that 100uF is needed to prevent flicker.  I would also not risk getting caps from eBay - using quality components from reliable industrial suppliers of electronic components is the safest way to go.

R1: RES 47 OHM 5% 1/4W 1206 RMCF1206JT47R0TR-ND $0.10
R2: RES 680 OHM 5% 1/4W 1206 RMCF1206JT680RTR-ND $0.10

Z1: DIODE ZENER 15V 1W DO213AB ZM4744A-GS08TR-ND $0.49

D1: DIODE GEN PURP 75V 200MA DO213AA 1N4148UR-1-ND $0.84

The other thing worth mentioning is that increasing the value of R2 will increase the amount of time the cap will keep the LED illuminated (but it will reduce the LEDs brightness). And as you suspected, you need one complete circuit for each direction of travel.