TTS decoder light function brightness adjustment

[Rcik]

Hi

Is anyone aware of what CV changes need to be made to adjust the light brightness on a TTS chip ( if it is possible to do)? I would like to reduce the brightness of the forward lights if possible. 

[Fishmanoz]

Lights (directional) on all chips are supplied from the yellow and white function wire outputs combined with the blue common positive wire,  Internal cab lights are similar but will be blue and either green or purple, more likely green.  These outputs in a TTS decoder will be full voltage, up to 100mA, and no adjustment.  Easy to check - all the supported CVs are covered in the loco leaflet.  If none are shown to  adjust function output voltage, then it can't be adjusted.

 

However, all is not lost.  There must in the circuits between the decoder function outputs and the LED lights series resistors to reduce the voltage as the full 12-15 volts is too much for the LED to handle (assuming not a 12 V type, unlikely to be).  Find these resistors and replace with ones of higher value (try doubling their value), so reducing the volts across the LEDs and the brightness.  If you aren't up to this job yourself, maybe your local model shop can do it for you.

[Chrissaf]

Fishy is correct in principal with what he says, but just a small technical correction required to save you being sent up the garden path.

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A LED is a constant voltage device. Changing the resistor value will not change the voltage across the LED itself (as Fishy indicated), but it will change the current flowing through it. The constant voltage nature of the LED means that it is difficult, but not impossible, to use a resistor to dim a LED.

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To dim a LED using a resistor, the resistor needs to be in series with one leg of the LED, and be a fairly significant value. Say anything from 4K7 to 15K. Just add it at a convenient wiring location near to the LED position, don't bother trying to find and replace the factory fitted one. The factory fitted one is likely to be in the region of 470R to 1K and significantly less in value than any resistor you will add.

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However, one can not guarantee that making this mod will have the desired effect in a model loco. A loco may include a 'constant current' driver circuit on the circuit board providing the final interface to the LED after the decoder. This may be unlikely on a TTS model as the model has the decoder fitted at the factory. The 'constant current' driver circuit is more likely to be found on a DC (or possibly a DCC Ready) model. Why? - because a 'constant current' driver circuit compensates for the track voltage going up and down depending upon loco speed in a DC analogue control environment. Adding a resistor to a LED being fed via a 'constant current' source will have little effect as the circuit will constantly compensate for the resistor until such time as the increased resistor value causes the LED to go out all together. It is the current, not voltage that controls LED brightness.

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My honest opinion is don't bother, leave well enough alone electronically and go for a physical light reducing solution. A thinned coat of appropriate colour (white or red) paint over the lens of the LED will reduce the light transmitted. As long as the paint coat is thin, a lot of light will still bleed through it.

[Rcik]

Well thank you both for each comprehensive reply.  I now know not to spend more time searching for cv references for the lights. 

Both suggestions make sense to me and I do have some small resistors that I tried using in an old hornby (ex dapol) class 56, but without success that time. I may live with lights as they are for the time being ( I can always not switch them on at function key ) and, one day, try one of the suggestions. 

Thank you again.