I bought some used PRR HO scale Bachmann Cars that had lights. Most of the lighting equiment had been removed and I am fine with that. I still have the light bar and think I know why the lights where turned off, they have light bulbs and not LED's from Bachmann.
I am running DCC and know LED's need resistors. With the proper resistor can a LED work with out burning out with no decoder from straight DCC power? What OHM do I need? I all so think I will need 2 to 3 LED's depending on the car. Run in parrel or serices? I want the lowest power draw I can get.
Thank You folks for the coming answer.s.
Cuda Ken
I hate Rust
With the proper resistor, you should be able to use LEDs on DCC. They are after all, diodes. Resistor value will depend on the desired brightness.
A LED consumes typically 20 mA, and creates a voltage drop of 3 V. You should check the specs of the desired LED for the appropriate values. So, as a first step, you have to check if your decoder can provide the required amperage, No. of LEDs x 20 mA (or whatever rate). If not, you will need an external driver, be it a transistor or an ULN2003/ULN2803 (you will need a power source, I will come later to this point).
Now you know your decoder can handle the current needed to light the LEDs, you have still to "choke" the voltage to limit the current to the premissible value (20 mA for parallel connection, or No. of LEDs x 20 mA for series, which no one should do BTW). I assume your decoder supplies regulated 16V. Your LEDs create a voltage drop of 3V if connected in parallel (again, chec the value). So the remaining driving "force" will be 16-3=13V. The current "pushed" by these 13 V should never exceed the 20 mA. Therefore you need to choke them by 13/20 = 0.65 Ohm, right? No, wrong, 0.65 x 1000 = 650 Ohm connected in series with EACH LED. Add 10% safety factor and go to the next higher resistance and you should be somewhere close to 800 Ohm.
Of course you can add a single resistance of No. of LEDs x the 800 mentioned above connected in series with the LED tree (three LEDs connected in parallel), but you should check for the power dissipation and whether your resistance can handle it - Better not do it unless the space is limited.What if your decoder cannot provide the required 60 mA and you need an externla "relay"? The decoder can easily be connected to the relay (ULN2003 for example) with now porblem, but who supplies the power? You do not have a battery in your little train, do you? The ideal solution is a rectifier bridge connected to the wheels. This will rectify the square wave of the DCC and produce a constant 16 V, or slightly less. BTW, this is the same way the decoder gets the power and provide it to the motor. Take the output of the rectifier bridge, hook it to your ULN2003, et voila, you have enough power to light the house.
Regards
Walid
It may be 100 times easier if the following guess is true: A decoder does not really supply current to the light pin, but connect it to the ground. If this is the case, you can add the resistor directly between the LED and the current pick up, and the other leg to the decoder. You will have effectively half of the brightness sind the LED will use a single half of the DCC wave. Probalby you should check with a voltmeter first what the decoder really does.
Interesting idea, but he said he wasn't using a decoder in his passenger cars.
Henry
COB Potomac & Northern
Shenandoah Valley
BigDaddy Interesting idea, but he said he wasn't using a decoder in his passenger cars.
Oooops.... in that case forget about the text book I wrote above. Hook a resistor to the current pick up, connect the other side to on side of the LED, the other side of the LED to the other current pick up. The resistor value can be estimated as mentioned in the previous post.
If the light is too dim for your taste connect a bridge rectifier IC to the current pick up. Refer to the data sheet of the rectifier IC as you may need a capacitor. Use the two pins of the rectifier marked (+) and (-) as a power source and connect the LEDs to it, but do not forget the resistor in between.
I replied to a similar thread a few months ago:
http://cs.trains.com/mrr/f/744/t/267418.aspx
I used a diode and two resistors with my LED strips. Seems to work ok.
Gary
garyaI replied to a similar thread a few months ago:
I'll try to make it clickable:
Cheers, Ed
gmpullman garya I replied to a similar thread a few months ago: I'll try to make it clickable: http://cs.trains.com/mrr/f/744/t/267418.aspx Cheers, Ed
garya I replied to a similar thread a few months ago:
I tried 3 times before I gave up...
Somedays they aren't clickable
Hey thanks for all the answers folks!
Walid Thank you for your answers. Honstley had no idea what you where typing about but thanks for the effort!
I am going to check the other link.
Thank You all Again, Ken
If it's rectified from 14.4Volts Part 1: Voltage that reaches circuit14.4V - 1.4V (rectifier drop)= 13.0 Volts
Part 2: Calculating required voltage drop with a single LED13.0 V - Vforward led = V drop 13.0V - 3.2V = 9.8V WARNING: Some white LED's only have a Vf of 2.7V. So read the specs!Part 3: Calculating the required resistorV = IR 9.8 = .020 * R 9.8 / 0.020 = R
R = 490 Ohms MINIMUM The closest standard resistor is 510 Ohms. I would advocate for at least 20% safety zone. That .016 ma current (which is super bright still) That's a 612 Ohm resistor. The closest standard resistor is 620 Ohms.Here is my personal parts list
https://www.amazon.com/gp/product/B00B889A7W/ref=oh_aui_detailpage_o05_s01?ie=UTF8&psc=1https://www.amazon.com/220uF-8X12-Aluminum-Electrolytic-Capacitors/dp/B074RJ215G/ref=sr_1_13?ie=UTF8&qid=1523290331&sr=8-13&keywords=220uF+capacitor+25Vhttps://www.amazon.com/Projects-100EP5141K20-1-2k-Resistors-Pack/dp/B0778NVY3C/ref=sr_1_3?s=industrial&ie=UTF8&qid=1523290451&sr=1-3&keywords=1.2k+ohm+resistor
And here's the picture of it put together
https://drive.google.com/file/d/1fGxBuw34rfiOtjH_YHt_dFarRyoGSbVN/view?usp=sharing
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!
(and I know we all do it) After running the calcs, the most likely go-to answer is - 1K resistor. STILL plenty bright, less than half the LED's current rating.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
rrinker (and I know we all do it) After running the calcs, the most likely go-to answer is - 1K resistor. STILL plenty bright, less than half the LED's current rating. --Randy