Hi All,
Here goes nothing, I have a Kato 4 wheel Tram chassis that i want to chip with directional lighting the decoder i have is a Bachmann/ESU 8 pin decoder (ive made a 8pin socket as i do not wish to directly solder the decoder to the chassis motor and pickup inputs). I need advice on the directional lighting im ok with the other things related to the DCC chipping. Please see below for information on my decoder and what i want to for loco lighting:
Decoder is Bachmann 36-553 3 function 8 Pin with configurable Back EMF, 700mA motor output, 180mA output for each function line 350mA max out across all 3 functions
Wiring of decoder is
Pin Function
-----------------------------------------------------------------
1 Motor Right
2 Rear Light
3 Function F1
4 Left Rail Pickup
5 Motor Left
6 Front Light
7 Function Common Positive
8 Right Rail Pickup
i want to have one red and one yellow led at each end of the loco, so when the loco is in motion, the end travelling forwards lights the yellow led and the rear end of the loco lights red led. i have done some calculations and according to ohms law i need 27ohm resistors to keep a led safe voltage of 3 - 5 volts can anyone confirm this please?
Sorry for the long post and if it is unclear please feel free to ask questions
Thanks in advance
Lee
Your resistor value looks very low. How did you calculate it?
Red and yellow LEDs have a forward voltage drop of about 2 volts.
If your DCC function voltage is (say) 12 volts then you are dropping 10 V across the resistor.
If the LEDs are low current types you want something like 10 mA to flow through the LED (and resistor).
The resistance is then in the order of V = I.R --> R = V/I = 10 V / 10 mA = 1 k
Hi Graeme thank you for your reply
i calculated using ohms law using target voltage i need for the leds rather than using the voltage of the dcc command station output voltage of 16 volts i think i will recalculate tonight thank you again
Lee
Hi Lee,
A couple of other things occured to me. I am not familiar with that particular decoder, but the voltage available for the functions outputs will be a bit less than the track voltage.
I guess there will be a full wave rectifier on the decoder to turn the DCC waveform into a DC power supply so that will drop about 1.4 V. The function output will be controlled by a transistor. If it is a FET with a low ON resistance it can probably be ignored, but if it is a Darlington pair that will drop another 1.4 V. You will possibly have a couple of volts less than the track voltage to play with.
It also occured to me that you are possibly wiring one red and one yellow LED in series so that the red LED at one end is powered by the same function output at the yellow one at the other. Then of course you would double the voltage drop across the LEDs to about 4 V in your calculation.
Alternatively you could still power the LEDs from the same function output but wire them in parallel. They would each need their own resistor or one LED will 'hog' all the current and the other one may not light. The total current draw will be double but as you have plenty of current available from the function output this might be the way to go.
If the LEDs are not matched for brightness wiring in parallel may be better and you may need to experiment with different resistor sizes to get equal brightness, but it is unlikely. However it is just possible that you have a 'superbright' or 'hyper bright' red LED and an ordinary yellow one. Similarly if you decided to use a white LED, its voltage drop and brightness will be different.
Since I have a mottlely collection of components I usually test LEDs (especially red ones) first for a match or to adjust the resistor size. Newer LEDs can be significantly brighter than older ones due to design improvements.
Having said all that, most of the recommendations on-line are to use a 1 K resistor! ;D
hi GraemeD,
sorry for the delay in getting back to you, once again you have come up with more helpful advice i will when i get chance to try it out i might consider bread boarding the outputs then experiment until safe and desired current and brightness.
Many thanks, i will post my findings here for others to learn from.
Lee
The resister value I use for LEDs from my DCC decoders is 1K. This gives a not too over bright light.