5v lamp from track supply

[Ian_M]

I've bought some 1:100 street lamps from an ebay supplier in China. They don't look too bad and they were very cheap. The listing says they need a 5v supply and that they are LEDs, however they are quite clearly filament lamps. I've tested them with a couple of 1.5v AA batteries, and get a dim light whichever polarity I connect it up with.

 

For reasons of space and practicality I'd prefer to power them from my DCC track supply if possible.

 

I did a quick test and as expected a direct connection to the track lights up the lamps far too brightly - I wouldn't expect them to last very long at all with 16V passing! 

 

The obvious solution to me seems to be to connect a resistor in series with the lamps to reduce current. I duly got some 1k resistors, but now find that the lamps don't light at all with them in a test circuit on the dcc supply. I know the lamps without resistors are still OK as I can test with my dim AAs. 

 

What have I done wrong? Are the resistors too high value? I choose 1k as that seems to be what people use with 5v LEDs on DCC. Have I misunderstood something about the dcc supply being AC like?

 

Thanks in advance. 

 

Ian

[Junctionmad]

Grain of wheat type filaments will need far higher current then leds , 50-70mA typically , so your resistor will be far too high a value 

 

for DCC voltages 15v and the lamps operating voltage , 3-6v , a typical resistor would be the nearest one to approx 150 ohms. Note that you will need a resistor capable of handling upto 0.75 of a watt , so don’t  use the wrong range of resistor. You might find you need slightly higher  or lower resistors to select the appropriate brightness for your application 

Edited February 10, 2019 by Junctionmad

[Suzie]

Just wire them in groups of three in series.

[norman]

Reduce the 16 volts to 8 volts with a diode.

Norman

[Suzie]

You will create asymmetry on DCC if you half wave rectify the track supply for large loads. 

[DCB]

Just get a 5 volt DC power supply, any old computer USB port will do.  Its a very bad idea to connect low current items to a DCC power supply as the wiring is generally so small in cross section that a partial short well below the tripping amperage of the power unit could start a fire.   Not a terribly good idea to run anything 5 volt from a 16 volt supply when 5 volt supplies are widely available.    

[Junctionmad]

7 hours ago, DavidCBroad said:

Just get a 5 volt DC power supply, any old computer USB port will do.  Its a very bad idea to connect low current items to a DCC power supply as the wiring is generally so small in cross section that a partial short well below the tripping amperage of the power unit could start a fire.   Not a terribly good idea to run anything 5 volt from a 16 volt supply when 5 volt supplies are widely available.    

This comment makes no sense , firstly the dropping resistor can be close to the track feed and even 7/0.2 wire can easy carry far more then the typical DCC trip current as fault current , of you followed your logic , you’d never wire anything including DCC decoders to dcc track power 

Edited February 11, 2019 by Junctionmad

[Pete the Elaner]

I may be completely unjustified with this but I would be concerned that an inductor (coil or bulb) or capacitor on a DCC circuit could distort the wave. An inductor would cause back emf spikes & a capacitor would smooth it out a bit.

Why not power the lighting from a decoder & treat it just like any other DCC device? You could then turn it on/off & even dim the lights with the throttle. You could even adjust the speed curve to prevent the lights being set too brightly.

[Suzie]

The biggest disruption connecting loads such as this to the DCC track bus will do is stop Railcom from working. If you use Railcom you need to feed the lamp circuit via a pair of inverse parallel diodes.

[Ian_M]

Thanks for the advice everyone.  You've  raised a few things I hadn't thought of before. 

 

 

[Junctionmad]

9 hours ago, Pete the Elaner said:

I may be completely unjustified with this but I would be concerned that an inductor (coil or bulb) or capacitor on a DCC circuit could distort the wave. An inductor would cause back emf spikes & a capacitor would smooth it out a bit.

Why not power the lighting from a decoder & treat it just like any other DCC device? You could then turn it on/off & even dim the lights with the throttle. You could even adjust the speed curve to prevent the lights being set too brightly.

You need have no such concerns for typical accessory loads.   Remember the booster drives the dcc signal with its full load ability so it’s unlikely to be affected by loads that are a fraction of its drive capability 

Edited February 11, 2019 by Junctionmad

[jdp298]

If you want to run a lot of street lights, then grain of wheat bulbs aggregate pretty quickly. By the time you've got 20 or 30, that's actually adding up to a fair amount of current. There are other chinese suppliers - www.hezhiqing.com is my preferred one, that will sell you cheap warm white LED lamps and lights.

 

There are a lot (and more to follow) on mine, so in the end I went with a 12v 5A separate supply from cpc.farnell.com - PW0391003 . You should expect to cut off the plug and wire it into something you can use. Also means all the things meant to run trains only do that.