Powering LED headlight from the motor circuit (Grey+Orange wires)

[nickelplate759]

I'm building a multi-unit engine for track cleaning.  Two powered units (based on old Atlas/Kato RS3s I have lying around) and a center dummy unit to hold a decoder and keep-alive.  I plan to use a single decoder, but I don't want to run too many wires between units, but I do want directional headlights at each end.

At a minimum I need to run 4 wires between units - black, red, orange and gray.  I don't want to add headlight wires.   Is there any reason I can't add an LED with appropriate resistor in parallel with the motor in my lead and trailing units (powered from the orange and grey wires)?  It seems to me that's equivalent to how the lights are powered in a straight-DC unit.

[daniel_leavitt2000]

Only it wouldn't work when the motor is off and the  kick start used for fine motor control will cause problems.

You could use a diode resistor and LED and run straight from the tracks, but I'm not sure if it would flicker.

I think you can go single wire by using the function wire, then using a diode on one of the rail pickups to get a common +.... I believe this is how MP wired the headlights in their DCC/sound stream.

[peteski]

George,  If you want to power a light from the motor outputs (in parallel with the motors), the simplest would be to use a 12V incandescent bulb.  It would simply glow with the brightness equivalent to the motor speed.  If you want directional feature. add a diode in series with each bulb.

You can also use an LED/resistor combo, but I would also install a protection diode with it for when the motor polarity reverses.  White LEDs have rather low reverse breakdown voltage. Since motor is powered using PWM voltage, depending on the decoder brand (and how its PWM circuit is designed) the LED might flicker.  You can try adding a capacitor to the LED circuit to minimize flicker, but that gets a bit more complicated.

I like Daniel's ideal of powering the LED from the DCC track voltage.  Since that voltage flips polarity many times a second, the LED will always glow (it would be flickering faster than the eye can see).  Again, you need to use a resistor and a protection diode.

If I was going to use track power I would insert a bridge rectifier between the track and LED.  The LED will not flicker that way, and only a resistor is needed in the LED circuit - the bridge rectifier becomes that "protection diode".  Of course with track power the LED will be always on.

[nickelplate759]

George,  If you want to power a light from the motor outputs (in parallel with the motors), the simplest would be to use a 12V incandescent bulb.  It would simply glow with the brightness equivalent to the motor speed.  If you want directional feature. add a diode in series with each bulb.
...

That's a darn good idea.  Heck, that pretty much describes what is ALREADY in the lightboards in Atlas/Kato RS3 units I am using.  All I really need to do is change the power source from track to the motor leads.   Cheap & Simple - I like it.

[Maletrain]

A side question, which will demonstrate that I am not an electronics guy. 

When using a protection diode to keep an LED from seeing excessive back voltage, I am familiar with the set-up where the diode is put in parallel with the LED in a configuration where the protection diode is conducting when the LED sees back voltage, so the back voltage on the LED is limited to a safe value. 

But, I am wondering if the protection diode could protect the LED if it was wired in series with the LED, configured so that both conduct or both try to block the back voltage.  Of course, the protection diode would need to be designed to handle the max back voltage plus a margin.  I am interested in doing this between the A and B phases of a DCC circuit, so I am wondering if the square wave nature of that AC would have any detrimental effect.  Wiring the components in series would greatly simplify the way I make the indication system that I have in mind.

[peteski]

Either  way of hooking up the protection diode will work.
The parallel circuit works as described - the protection diode shunts the voltage down to about 0.7V when the circuit is powered in reverse polarity.

Another variation of the parallel protection circuit is to make both diodes LEDs.  That way either diode will glow (and protect the other) depending on the polarity.  That is the way directional lights are implemented by the manufacturers of some models.

Protection diode is a run-of-the-mill rectifier or small-signal silicon diode.  Those have very low reverse leakage current (in microAmps or smaller).  LEDs, on the other hand, have much larger reverse leakage current.  In the series connected circuit, the protection diode (with its low leakage current) will have the brunt of the voltage across it.  The LED will only see a small voltage across it.

The fast-changing DC voltage will not negatively affect LEDs.

Just out of curiosity, what type of circuit you are designing?    BY "phases" do you mean you want an LED across the DCC rail power leads, or between boosters, or auto reverser sections?

[Maletrain]

Peteski,

Thanks for the reassurance. 

To answer your question, I am thinking about making a sort-of-modular set-up for control panel turnout position indication.  I want to use 2-color LEDs (common anode or cathode) to compare the frog phase to the rail phases.  So, the frog connects to the common and each rail connects to one of the other leads.  Since I am using DCC, the LED connected between the out-of-phase wires will both light and see back voltage.  Connections are by the same type of 3-wire cables used for servos - except I will make sure that the connectors are reversed so that I can't put track power into my servo driver.  Those cables can plug directly to the LED leads.  But, I will need to make a special cable section that has a proper value resistor on the common lead and a protection diode on the each of the other 2 LED leads.

Can I draw on your experience for a recommendation for a good protection diode at Mouser or Digikey?  The smaller physically the better, so long as it is rated at high enough reverse voltage for long life at about 14 volts (DCC).

[peteski]

That sounds like a good design.
So basically, at any given time one LED will see zero volts (since the frog will be connected to one rail), while the other LED will have DCC voltage applied to that circuit.  Since DCc voltage flips multiple times a second, either common anode or cathode (most common) 2-color LED will work.

As for as the reverse voltage goes, it is mainly the LEDs based on the blue chips (like white, blue and true-green). Those are the ones which have forward voltage around 3V.  Most 3-color red/green or red/yellow LEDs use another type of LED which has much higher max. reverse voltage - a protection diode might not even be needed.  Check the specs (maximum ratings values)of your LEDs to see what the reverse voltages is for each LED.  If the rating is high enough, you won't need protection diodes.

As for the protection diodes, I assume that you are interested in the leaded packages (not tiny SMD components).  If you won't have more than 20mA passing through the LEDsm a 1N914 or a 1N4148 are small signal diodes which should be safe to use.  Slightly larger, but more robust, are general purpose diodes in the 1N4000 series. Like 1N4001, 4002, 4003, and I think up to 4007.  Those are rated for 1A forward current.

If you would like to control the brightness of each LED, then you can use a separate resistor on each of the LED leads.  If you not too worried about individual brightness, you can just place single resistor on the common lead of the LED to cut down on the parts count.

[mmagliaro]

Okay, now you've piqued my curiosity, Peteski.

If I hook a protection diode in series with the LED, and a spike comes along, but the protection diode doesn't turn on right away, doesn't it effectively look like a short (or in other words, just a piece of wire) for that first instant, allowing the LED to drop the full voltage of the spike?

I guess what I'm asking is, something like a 1n4148 (which is a "fast switching" diode) is probably faster to turn on than any LED, but are general purpose diodes like a 1N4001 really that fast?

[peteski]

You are correct Max. the 1N400X series diodes are relatively slow.  The 1N914 or 1N4148 diode is much better suited for this application.  If one wants to really get down to the details, there also is the junction capacitance of the protection diode and the LED itself.  I usually don't get that deep into it.

Good article on this (for a similar application) is https://web.archive.org/web/20170102085826/http://www.cliftonlaboratories.com/diode_turn-on_time.htm

The generally preferred method for LED protection is the parallel-connected method (with either another LED or a standard silicon diode), but the series-connection method does work too.  Adding a capacitor across the LED in the series-connection scheme also adds a margin of safety (as it will short out fast voltage spikes).

Google search for "protecting LEDs from reverse voltage" brings up a number of good discussions.
Like https://forum.allaboutcircuits.com/threads/led-reverse-voltage-protection.127294/

Either way, in the case of using standard red and green LED, the point is moot since their allowed reverse voltage will likely be higher than what they will be exposed to in that circuit.

Also if I understand correctly what Maletrain is doing, parallel connected protection diodes should work there too.



Optionally, to control each LEDs current (brightness) that single resistor could replaced by 2 resistors, each inserted between the stock rails leads and where the LED/diode leads are tied together.

EDIT: corrected some typos.

[PrajwalKale]

Thank you for the information this was very useful.