Track voltage on a DCC layout (for brake van lighting)

[Lidders46]

Hi All.

I am fitting a red LED and white LED to my brake van and wanted to know what the track voltage was on my DCC layout.

On checking the internet I found it was meant to be 15.4V ac, but mine measures 22.4V ac when measured on my Clarke CMD 20 multimeter.

Is the track voltage sinusoidal or square wave.

Is that why my reading is high.

My layout is powered by a Hornby 4.0A PSU type P9300.

Help with answers to my predicament would be greatly appreciated.

Mod note - title amended to aid future search and white space removed.

[ColinB]

The waveform is a digital pulse train so not that easy to measure. I must admit when I measured it with a Fluke multimeter I think I got about 16 AC. So I assume you are trying to work out the bleed resistor for the LED. Generally it is a good idea to put a bridge rectifier in the feed from the track DCC to your LED. So wire up the bridge rectifier and see what voltage you get on the DC output. I suspect it will be 12 to 15 volts.

[Lidders46]

Hi Colin thanks for your reply.

Yes that's exactly what I'm trying to do.

I'm on the same "track" as you.

I've ordered on of these.

They're small and should fit inside the brake van.

That's interesting about the waveform being a pulse answer my question.

Thanks again

[Bulleidboy]

Mine usually reads about 14v.

[96RAF]

As your meter is designed to read sinusoidal the reading you get from a DCC signal will depend upon the sampling rate of your meter.

I have three multimeters and they read from 9 to 18 v on the AC range, but when I use my RRAMP meter I get 15.4 true DCC potential from the 4-amp PSU.

DCC as stated is a square pulsed AC, actually a serial bit stream bi-polar on zero volts.

The track voltage remains constant and when you apply throttle or press a function your controller modifies the bit stream mark-space ratio.

[Lidders46]

Thanks for your replies.

That's interesting, 96RAF, that the waveform modifies the bit stream mark-space ratio, similar to frequency modulation.

 I thought, for some strange reason it was Amplitude modulated because of the sound, but remembered that was for audio only.

One can certainly learn a lot from building and modifying model railways.

Cheers all

[96RAF]

The bitstream is a translation of the command string into 1s and 0s to a pattern defined by NMRA.

Preamble - Address - Instructions - Additional - Error Checking - Repeat.

See here - https://www.nmra.org/sites/default/files/standards/sandrp/pdf/s-9.1_electrical_standards_for_digital_command_control_2021.pdf

[37lover]

I didn't pick up this thread earlier as I thought it was simply about track voltage but now see why the question was asked. I have fitted a flashing red led to several of my Guards Vans; one was a proprietary kit bought off Ebay and the others used some basic electronic circuits cobbled together by myself. They all used a bridge rectifier and then a voltage rectifier; I experimented with a 555IC to get the flashing but it was much simpler to use a Flasher IC bought of Ebay, I think I used a 1Hz rather than the 2Hz. No on/off switch so they are on once placed on the live track.

[Lidders46]

Hi 37 Lover

I'm just going basic and putting a steady red light and the back and a white one at the front, rather than on the sides, bit unconventional, my Hornby Brake van only lends itself to back and front. If fitted to a train in the other direct I'll physically turn it around , as my layout is only 6' x 4' (late starter), although I could do a lot of shunting in a siding. wink thumbsup

[96RAF]

I have a guards van with a DIY FRED which is simply a flashing red led and a 1K ohm resister in series. Power is DCC track via an ESU coach lighting pickup. It has been sat in a live siding flashing for months.

If on DC then you definitely need a bridge rectifier to cater for the polarity reversal and possibly a voltage regulator else resister.

You can get 12v flashing leds with a built in resister but I still use my default 1K ohm.

[rpjallan]

I am using a very small surface mount bridge rectifier & a small buck board so I can regulate the voltage for different applications (or different voltage LEDs). And of course a capacitor as well...

[Nigel1707822225]

The DCC controller puts out high on one rail and low on other rail then keeps swapping them around.

Put 1K resistor in series with 1n4148 diode and LED.

[Lidders46]

Hi all, I thought I would post the result of my endeavour, of which I'm pleased. A bit of poetic license with just one white light at the front.

[Brew Man]

Just out of interest, why do you have a white light on a brake van? They are always at the rear of a train and only ever, in my experience have a red light showing. White lights are only shown at the front of trains.

[Lidders46]

Hi Brew man there is a position on each side on the brake van for a white light either side, as can be seen as a bulge with a white window, on the real size brake van photo. This I believe was to the show the engine driver the brake van and all of the wagons were still attached! My brake van doesn't have those moulded in, so that's why I have the one white light, for fun.

( poetic licence)

[Rog RJ]

The bulges on the sides are called "duckets". They are there to allow the guard to look up and down the line safely without getting his head knocked off. Nothing to do with lights.

[Brew Man]

My thoughts entirely. I worked a great many brake vans when I was a guard in the mid seventies and don't recall anything about white lights. White lights to my knowledge relate only to the fronts of trains.

[96RAF]

Lidders is essentially correct, see here . ..

http://britishmodelrailwayclubofmontreal.blogspot.com/2011/08/lamps-on-goods-brake-vans.html

[Lidders46]

Section taken from

https://www.bluebell-railway.co.uk/bluebell/wagon/55993.html

Every train had to carry a red tail lamp to indicate to signalmen and station staff that it was complete. If the train contained "unfitted" wagons, the van would also need to display a pair of side-lamps showing a white light forward and a red to the rear. The purpose of these was to indicate to a member of the loco crew looking back that the train was complete. The guard was responsible for checking that these were illuminated. Two side lamps and one tail lamp were part of a van's equipment. Between his duties, the guard was kept warm (draughts permitting) by the small coal-fired stove, which would also keep a tea can warm. Inside the van was a coal box.