DCC Conversion of Directional Lighting

[Chrissaf]

Probably a silly question, but have you turned the lights on? Func 0 I think!

[Chrissaf]

I am convert an old Hornby Class 47 to DCC using the R8249 chip. The loco was running fine but with front and rear direction lights both on. I then used the yellow (rear), white(front) and blue wires to connect the lights to the chip. I left the existing resistors in place.  Now I have no lights but a running loco.

Can anyone help solve my problem?

[Chrissaf]

With DCC, not only do you need to use a Function (F0 as Eric suggested) to turn on the Directional Lighting circuit. The 128 Speed Steps Bit in CV29 also needs to be enabled. Your R8249 might possibly have CV29 currently set as 0 instead of 6. If it is indeed 0 then 128 Speed Steps are disabled and set as 14 Speed Steps instead.

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If your controller is a Hornby Select controller, then you will not be able to read or write CV29 to correct this issue.

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For more information on CV29 see this web site:

http://www.2mm.org.uk/articles/cv29%20calculator.htm

[dave52]

Yes F0 is on. Tried F1 & F2 for fun as well. Does it matter which way blue and yellow/white are connected.

[dave52]

Yes. CV 29 is set to 6. I am using an elink box.

[Chrissaf]

LEDs are polarity sensitive and will only work (light up) one way round. The anode of the LED goes to the Blue wire. The cathode of the LED goes to the white or yellow wire as appropriate. The current limiting protection resistor can go either side of the LED, it doesn't matter as long as it is in series with the LED.

[dave52]

How do tell which is the anode or cathode if the lights are LED. 

[Howbi]

On new LEDs the longer lead is positive..........HB

 

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[96RAF]

If you have already trimmed the legs then try one of these

 

As you can just make out in HB's diagram there is a flat on the cathoe side of the LED lens.

 

Rob

[RDS]

... The 128 Speed Steps Bit in CV29 also needs to be enabled...

 

I don't use 128 steps but all my Directional Lights work perfectly.

[dave52]

Thanks for the replies to date. I have test the bulb by contacting it to the live track. It lit whichever way the wires were contacted. So not an LED. Any more thoughts. 

[Chrissaf]

Its decoder dependant. On some you do, on some you don't. None of my directional lighting will work without 128 speed steps being enabled. It is also documented on the web as being a solution to lights not working.

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[96RAF]

That is possible RDS - this snip is from the Elite manual Page 29. It refers to the type of speed table in use.

It is the TTS decoder that absolutely demands 128 and that is more for the sound than the lights.

 

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[Chrissaf]

Connecting an LED to a live DCC track (I hope that you had a resistor in series as well) will still light a LED. DCC track voltage is a digital square wave that has an AC content. Thus the polarity of the track voltage swings back and forth between being positive and negative. You have to test for LED polarity with a DC voltage source, like a 9 volt battery for instance. Do not forget to include the resistor in the test circuit else you risk instantly blowing the LED.

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As to the question how do you identify the LED anode?

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LEDs come in different case styles. Some are round (like the one that HB posted). Some are square. Each have their own way of identifying the anode. Also, once installed onto a circuit board, the method of identifying the anode can be obscured. Thus the only way to easily tell the anode is by testing with a DC voltage source and a resistor.

[96RAF]

Dave if yours look like these they are filament bulbs not LEDs and on DCC they will get hot.

 

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This is the simple wiring circuit for bulbs. You may wish to add a resistor to drop the brightness, reduce the heat a little and protect the bulbs against overvoltage, but conversion to LEDs is a better way.

 

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[Chrissaf]

If they are indeed bulbs and not LEDs.

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Bulbs can draw a lot of current, maybe as much as 60 - 70mA each. Put two of these in parallel across a decoder function output (typically rated at a maximum of 100mA) and the bulb current can potentially blow the function output circuit on the decoder. Thus after a brief flash, the bulbs stop lighting up but the loco motor control of the decoder can potentially still function. My personal choice is to ALWAYS replace bulbs with LEDs.

[dave52]

Have solved the problem by removing the resistors. Lights are now functioning, but having seen the previous posts will replace them with LEDs. Thanks for the help.