Adding LED's to your locomotive! WARNING!

[cidchase]

The original problem was unexplained burning
of the LED across a loco motor.

We learned from Fred that the LED's PIV is only 5V.

I think krokodil was only trying to say that if a high
reverse voltage was causing the problem, that a
series diode would not protect the LED as well as a
parallel one.

Still unsure about the inductive kick (flyback) causing
the problem, but I think it's possible.

goin' back to :sleeping: :sleeping:

 

[Fred_M]

Also as a note on LEDs, the blinking package LED I used in my caboose has a PIV rating of .5 volts. So if you even hooked it to a 1.5 volt battery backwards it could possibly fail. (prob. due to the IC flasher packaged with it). FRED

 

[krokodil]

Hi everybody.

FRED was right with his circuit, because he use bicolor LEDs which are wired internally already antiparallel. In such cases the diode in series will solve all the problems, and the antiparallel LEDs in the package provide also enough protection against high freq. high power pulses.

(However such application in locomotives can create some "special light effects", as the protection diode - always one of the LEDs when active will create light, to avoid them add a capacitor, or you have to build a more complex circuitry to block out those reverse pulses - I would suggest to have one transistor for each direction)

As I mentioned earlier, in a simple LED circuitry we have to wire the external diode antiparallel to get the similar conditions as on the drawing provided by FRED (thanks).

In such cases a simple diode in series would not give ANY protection against high voltag reverse impulses.

The concern of FREDs about the short circuitry in case of antiparallel diodes does not apply to much, as instead of 1,2 V drop (in case of red LED, or cca 3 V drop in case of blue or white LEDs) are not that much in comparison to the diode drop what will be around 0,6 V.

In 12 V source, 20 mA circuitry with red LED (1,2 V drop) the series resistor is 540 Ohm cca 216 mW , when the diode is active in reverse, the power will increase to 228 mW.

In case of whithe LED (3V drop/20mA) the conditions are as follows:

Resistor: 450 Ohm/180mW in forward (LED active) and in reverse 228mW (the diode is active).

i.e. 1/4W resistors will be quite OK for this application and will provide complete protection.

 

[krokodil]

Originally posted by cidchase
The original problem was unexplained burning
of the LED across a loco motor.

Exactly, and that is the difference between the circuitry inserted by FRED and your problem. He uses a separate (clean) power pack to feed the LEDs, while you are getting the power from the locomotive board with not very good contacts to the rail and the motor EMF. (every brake of the contact - spikes - can cause up to 100 V pulses inside of the loco and also on the rails - power disconnection is most tragical problem, because of many inductive elements in the layout - transformers, motors and filters).

(Try it, make your hands wet and put them on the track, try to run an engine and you will feel the "power".
People with hearth problems, please avoid this test ....:ops: ).






:thumb:

 

[Fred_M]

Ok, so I'll post the schemes I used before I went to DCC on my locos, I just can't show a picture of them working. I kind of feel like I'm going out on a limb here, I'm hearing that my solutions don't/can't work from experts (I assume) so use them at your own risk. I'm just a computer repairman and not an electronics engineer. And where's the sketch of using transistors to run LEDs? Anyway the resistor size used is dependant on max voltage your DC powerpack makes v. specs on the LED v. LED life v. how fast you run your locos normally v. how brite you want them. Also think about using 1k linear trimmer pots as I have used them too. They are cheap and allow you to tune your LEDs (and burn them out when you give it that one final adjustment for just a bit briter LOL) Anyway I'll repeat these worked for me, your results may vary, and I advise you not to build and/or use these circuits. FRED

 

[krokodil]

Originally posted by dash10
Ok, so I'll post the schemes I used before I went to DCC on my locos, I just can't show a picture of them working. FRED

Hi FRED do not take it personally. Your schematics was correct and it will work without any problems.
You have just forgotten the different electronic environment in a locomotive.
I just do not have today a time to make a drawing (constant current source- and cannot find any similar circuitry on the WEB or i my electronic archive who needs it can search for it) to use with LEDs which improves their performance .

I am also always trying to use the simplest possible solution (the transistor control was just an idea for those who still do not trust to much).

 

[Fred_M]

Well, what I drew up ran in my locos from a few months to over a year on some. Then I switched to DCC and went back to bulbs. And I include another a friend uses for constant lighting on a dc loco that seems to work well. He uses the aprox .7 volt drop per diode in series with the motor to supply power to his lights (LED). The only drawback is it requires more voltage to run his locos at a given speed. This and my circuits can be found in old issues of model railroading mags. We/I didn't design these. FRED

 

[krokodil]

Hi FRED

this circuitry is very interesting, for my taste to many diodes eek: ).
However it will work somehow, especially when the LED is yellow or amber (probably also white an blue). In case of red or green LEDs I would be careful, because they require less then 2 V in forward, so bigger part of motor current will flow over the LED. (This is the main problem with this circuitry, that noone can control the real current over the LEDs - the motor current is split between the row of diodes and the LED - in forward, without any control or adjustment). The only adjustment is to change the number of diodes in series. The reverse diodes in this case are also correct, as the will provide the same speed in both direction.


(Just for your information I also use many LEDs in my engines and I never burned out any yet. Of course sometimes I also see the mysterious lights from those high frequency spikes, on those locomotives I try to block them, and it usually works )