I just tried to install an LED in a Bachmann loco on our little DC powered demo track here at the Hobby Shop. I used a 3mm bright white LED, and a 1k resistor. Now the LED just flickers, instead of constantly lighting. I know the LED and the resistor are ok, because if I take the assembly off the loco, and put it on my test bench power supply, it light at an indicated 3-4 volts.
What did I do wrong? Did I use too much resistor for the DC power here? I’ve used the same setup at home on my layout, on both DC and DCC with no problems. Should I try a 380 ohm resistor here, instead of a 1k?
While I’m asking LED questions, is there any way to dim an LED down some? These things are awfully bright!
I’m not sure what your problem may be, but I know it is not a resistor problem. If the resistor were too lagre, the LED would either not light at all, or be very dim. If it were too small, the LED would get too much current, and self-destruct.
You can dim the LED somewhat by using a resistor of a slightly larger value that what it calls for. You may not be able to make a huge difference in how bright it is, but with a little trial and error you should be able to get the brightness you are looking for. Hope this helps.
I would have thought that too, Mark, but I just hooked the bulb back into the circuit, before the resistor, and there’s no flickering of the bulb. Also, the LED may not light for two or three laps around the track, then light up for a second.
I’m thinking maybe the Bachmann wall wart transformer has too little current with the 1k ohm resistor to fire the LED? Or maybe I have a bad resistor? I only brought one LED and resistor from home, so I’ll bring a couple more tomorrow and try again. I’m also going to try a lower resistance, ( 380 or 750 ohm ) to see what happens.
I really am having fun playing with LED’s. I’m learning, and relearning, some of my basic electronics that I haven’t dealt with for years. [:D]
Something else I found is I’m going to need to paint the inside of the yellow plastic UP shell. The LED, when it lights, is so bright the whole front end of the loco glows like it’s radioactive! [:0][;)][:D][(-D]
I guess I’ll tote the whole thing home tonight, remove the windows and lenses, and spray the inside of the shell with silver, then black, to keep the light from shining through the plastic!
Exactly Mark! That’s why I think it’s some problem with either the resistor, or perhaps the LED itself. But then why does the assembly work ok out of the train on the power supply? [:(]
So what you’re saying is that the motor runs fine but the LED does not. Sounds to me like you have a good connection to the motor but not to the LED. Check all of your solder joints first and try again. But I think you may have a broken wire. This would explain the flickering. The wire is broken, but every once in awhile it gets jostled around just enough to make contact. I think your LED and resistor are fine or it shouldn’t light at all. So:
Check your solder joints and retest. If it still flickers replace the wires to the LED and try again.
I don’t think it’s a bad solder joint JPM, since I soldered the original bulb to the LED and the near side of the resistor leads, and it lights up, although it won’t light on the far side of the resistor.
I’m about to go home now, so I’ll test it on my layout at the house, then start changing pieces. I’ll let y’all know what happens.
Couple of things. It might be a bit high of a resitor value, unless you run wide open all the time - with DCC, the function output is a steady 12v or higher, depending ont he track voltage setting, and a 1K resistor is right for a golden-white LED. But if you are running DC and running at half power, say 6 volts onthe track, 1K is too much. A typical GW LED drops 3.5 volts, with a 6v track supply, to get 10ma to the LED you need only a 250 ohm resistor. With a 1k resistor, you are only getting 2.5ma. That may be below the LEDs threshold. A 560 ohm or 480 ohm is probably a better choice for DC running. Even a 480 is only 18ma at a full 12 volts to the track
Next, it could be the circuit board attached to the motor - if this Bachmann has one - full of capacitors and choke coils to limit RF interference. Make sure the LED and resistor connect to the rail pickups, not the motor side of any board that may be there.
Third, although not likely with a Bachmann, back-emf from the motor could be interfering here. My Stewart with a Buhler motor could keep the headlight illuminated while coasting with track power off.
Randy, you confirmed what I found a little bit ago!
This is a train set Bachmann, so no circuit board. I changed out the 1k for a 380 ohm resistor, and the thing works great here at the home layout now!
Funny thing, here at home, the 1K resistor would light constantly, although only at near wide open throttle. [?] I wonder why it would only occasionally fire on the shop layout? Both are DC, although the shop layout is using the Bachmann power pack, and here at home the DC layout is using my antique ( 30+ year old ) MRC Twinpower. I didn’t try to run it on DCC here at home, but it will probably never see DCC at the store, so it doesn’t matter.
Thanks for all the suggestions guys! Glad I finally got this thing going!
BTW, I did remove all the windows, headlight lens and masked the number boards, ( I couldn’t get them loose for fear of breaking them ) and then airbrushed the inside of the shell flat black to stop the “radioactive” look of the glowing bodywork. I’ll reassemble it tomorrow at the store and see how it does there on the demo track. I may repaint the inside of the cab with a light grey color to lighten it up a bit.
Also, I did add a diode tonight to prevent reverse polarity burnout of the LED while backing up. With the diode,installed, can I use a lower ohm rating resistor to fire the LED at a lower voltage?
Does the diode affect the firing voltage of the LED?
The real case is probably the difference between a lamp and a LED. A lamp has a filament thet glows when it is heated by an electric current, an Led is a solid state device whose light is derived from the passing of current thru the join of materials from which it is made. Minor fluctuations of current would probably not be noticable from a globe due to the persistance of the human eye.[:-)]
When I got to the store today, I put the loco on the test track, and it wouldn’t consistantly light the LED with the diode and the 480 ohm resistor, even though it worked ok at home on my layout. I didn’t bring any resistors with me, so I experimented one time, and tried the LED with no resistor. 2 laps later, the loco slowed way down, and the LED let all the factory installed smoke out of it! [:D] WAAAY COOL!!!
I got the wife unit to bring my “Big Bag o’ Resistors” from the house, and some more LED’s, and I went to experimenting. I kept going down on resistance, until around 270 ohm the light was pretty good, with just a slight flickering at crawl speed. I then dropped to 220 ohm, and now the LED works great! No flicker from the time the LED fires, through full throttle.
Still not sure why the Bachmann set had issues, while the light was great on my home layout? [%-)]
I don’t think you should need a diode in the circuit to keep the LED from self distructing when the power is reversed since an LED is in itself a diode. I have never used a diode with an LED and never had any problems. (Is that a double negative?)
I dunno… it was suggested I use one by one of the electronics gurus here, so I did. Also, the specs showed a max reverse voltage of like 5 volts, and the way the kids run the train here, they are likely to run it wide open in either direction! So I figured better safe than sorry! [;)]
“I don’t think you should need a diode in the circuit to keep the LED from self distructing when the power is reversed since an LED is in itself a diode. I have never used a diode with an LED and never had any problems. (Is that a double negative?)”
Tom, you are correct. LED means Light Emitting Diode, in a headlight application with a normal LED you would not need to add another diode.
A resister is need for voltages above the LED’s rated voltage.
A capacitor is useful to cut out flickering, but not required for the LED’s “safety”.