Posts: 2,959
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I have been trying to wire a headlight into an old MDC 2-8-0. I have white LEDs that I pulled from a cheap flashlight. At first, i tried a 330 ohm current limiting resistor. It flickered a few times, then quit. So, I dug out the LED and put in another one, this time with a 1000 ohm resistor. The loco did not even make it once around the layout before the LED quit. I am using standard DC. I am starting to think these are cheap LEDs and something else is killing them - back EMF from the open frame motor, or perhaps just the reverse current when putting the loco in reverse. Any thoughts?
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Kevin
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Posts: 1,061
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Joined: Jan 2009
Kevin its not the LED's . they are extremely sensitive to voltage spikes you need to put a filter cap in parallel to the led to drain off and voltage spikes i also use diode across the led to drain off reverse voltage spikes. if you need will dig out diagram for using LEDs for lighting on DC .
jim
PS I know this because when I first started using LEDs i went through several before i decovered what was killing then.
Posts: 87
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Joined: Mar 2010
one thing to look at (if you are using cheapo leds from a torch well- you get what you pay for..)
look at the PIV rating of the leds
many cheap leds (often meant for torches..) have a PIV of only 5v with a forward (driven) voltage of 1.8 or 2.2v
if the PIV is too low they will likely go all day forwards, but the smoke comes out when reversed in a dc layout
otherwise all I can suggest is looking at the current drawn by the led (most arent happy if they are above 20 mA ie 0.020A)
the resistor you are using is definately well within the range I would use for a dc (or even a DCC) layout- if anything its a bit high (making the LED dimmer than it needs to be)
(to figure out the resistor you need- its quite simple)
find the `forward voltage drop' of the led (usually 2-3v depending on type and colour)
find the supply voltage
find the Imax (maximum rated current for the led)
use ohms law to calculate
ie
(supply volts - `minus' FVD)
divided by
current in amps
ie (Vs-Vled) / I
(remember that its current measured in amps- most LEDS spec in mA so for eg 20mA equals 0.02A)
poopsie chicken tush
