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Noel

Kingsbridge - DCC / Electronics

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I've been trying to figure out the simplest and most cost effective solution for coach lighting and tail lamps for brake vans, coaches, etc.  I've looked at some of Train-Tech's battery offerings and others, but decided to experiment by trying to 'grow' my own.

Experiment no 1 - Tail Lamp - Runs off track DCC, no decoder required, flicker free.  Same circuit would do for coach lighting. On all the time.

IMG_3816.jpg

 

It's about 1978 since I last used a bread board, but I have to say its fun going back. 

IMG_3817.jpg

 

Components used in test

  • 3v Red LED (or LED lighting strip for coach interior)
  • Bridge rectifier (converts 16v pulsed DCC/AC to 12v DC) - 50v rated
  • 1k Resistor to drop voltage to approx 3v suitable for LEDs
  • 1k Trimmer resistor to adjust light level
  • 1000uF capacitor - avoids flicker if wheel pickup interrupted while running (e.g. over points, etc) - 25v rated

The little Red LED that I have sourced is almost the same shape as a tail lamp with a flat bottom, curved top and 1.8m lens so I will paint it white except for the 'lens' and it might pass as a tail lamp.  

. . . more to follow when I fit it to a brake van and test run on track, then a coach. Have to decide wither I will use axle spring pickups or phosphor bronze strips to wheel backs for electrical pickup.

PS: I ordered these parts only last night from Farnell and Radionics and to my astonishment both were delivered this morning!!!

Edited by Noel
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I wonder if it might be worth painting it black and then over-painting it white, to avoid 'bleed through'?

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23 minutes ago, Broithe said:

I wonder if it might be worth painting it black and then over-painting it white, to avoid 'bleed through'?

Thats a good idea. :tumbsup:

This is the LED I'm using for tail lamps http://ie.farnell.com/kingbright/l-2060hd/led-1-8mm-red/dp/1142456

Its the same one sold by Train-tech as a tail lamp.

42250824.jpg

Edited by Noel
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8 minutes ago, Noel said:

Thats a good idea. :tumbsup:

Actually, thinking about it a bit more, I might suggest using silver paint - it's generally just as opaque as black but is a better 'undercoat' for white. I generally use that for added opacity on the resin buildings, if they're going to be lit - in those circumstances you have the bonus of less light attrition from the paint surface.

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SSM have whitemetal taillamps, old and new style that can be installed with 0.8mm LEDs and 1.8k ohm resistors (included in the pack). 

A 2.2 µF capacitor in parallel with the Led will reduce flickering.

Edited by Weshty
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Confession

 parapet.gif

Beware peak track voltage can only be measured accurately on DCC with an oscilloscope, and can reach over 20v at the top of the pulse wave, so use at least 25v rated capacitors.  I mistakenly used a 16v capacitor during tests and spectacularly blew one up on the LED lighting test rig, which could have damaged a piece of rolling stock.

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Generally, people will quote RMS voltage, as it is the most 'useful' way of describing an AC set-up in terms of power supply available - divide that figure by 0.707 to get an idea of what the peak-to-peak voltage will be, from an insulation point of view. Or just add 50% roughly...

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just be aware, typically DCC and most pulse based waveforms are usually described in peak-to-peak values and not RMS, which is largely useless for complex waveforms , Sine wave or near sine wave is typically quoted in RMS

Also an unregulated transformer output is typically quoted at full power,  and its peak to peak voltage in general , so a 16VAC 20VA traffo, will be specified to output peak 16 V at a load of 1.25 amps,  The load regulation figure for the transformer will also be quoted and is often in the range 10%-20%, who means that an unloaded 20VA 16VAC traffo, could have a fifth higher voltage on little or no load, ( i.e. 19 VAC or even a bit more in some cases) 

 

Edited by Junctionmad

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35 minutes ago, Junctionmad said:

just be aware, typically DCC and most pulse based waveforms are usually described in peak-to-peak values and not RMS, which is largely useless for complex waveforms , Sine wave or near sine wave is typically quoted in RMS

Also an unregulated transformer output is typically quoted at full power,  and its peak to peak voltage in general , so a 16VAC 20VA traffo, will be specified to output peak 16 V at a load of 1.25 amps,  The load regulation figure for the transformer will also be quoted and is often in the range 10%-20%, who means that an unloaded 20VA 16VAC traffo, could have a fifth higher voltage on little or no load, ( i.e. 19 VAC or even a bit more in some cases) 

 

Wot he said 😄 he's the resident forum electronics engineer (who built bits for the space shuttle and 'other stuff' ).

Edited by Noel

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16v capacitor I blew up on the test rig.  Have switched to 25v capacitors since. :) You learn by mistakes and the advise of those who know better. Luckily this didn't happen inside a wagon or a coach.  It made quite a noise.

IMG_3912.jpg

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