What type of power does DCC send to the rails?

[Bart1701]

I want to install a small red light on the fascia of my layout - over by the door to the room that I close when I leave the room. I have sometimes forgotten to turn off my Master Power Switch and the layout has stayed "powered up" until the next time I come into the room and notice my mistake. I figured I would have a good chance of noticing a bright red light as I am closing the door to remind me that the power was still on.

I wanted to install a light that would be tied into the main DCC power bus so that it would be illuminated whenever the master power is on.

But, exactly what type of power does DCC send out (specifically from a Digitrax Zephyr unti, if that matters).

- Is it AC, DC or some unique kind of power associated with DCC?
- How many volts does it put out?

I found a LED at Radio Shack that would be what I want. It is rated at 12 volts DC and 100 mA. I connected it to the track last night with some jumper wires, and it lit up quite nicely. But, before I connect it permanently, I want to make sure I won't be burning the house down by doing this!

Thanks,
Bart

[jagged ben]

You're unlikely to burn down the house at 100ma.   You'd really have to search around for a wire that is too small to carry that current.   If you use 20 gauge wire or larger you'll be just fine.    Your DCC system has some overload protection as well.

DCC power is AC.  (It's not your typical sine-wave AC, it's basically a square wave, but that's not too significant in this discussion.)  Your LED only works in one direction which means it's only using the half of the waveform that is the right polarity to light up the LED.  What you should probably do is place a diode in series with the LED to prevent a reverse voltage, which could damage the LED.    Your LED may last longer that way.

You also either got lucky or guessed right by choosing an LED whose voltage is pretty close to a typical DCC voltage.   Most DCC systems can have the voltage adjusted to various settings, usually between 12V and 18V.   If your system is set to 12V then that's why your 12V rated LED didn't burn up.  If you'd chosen a 3V LED it would have burned out immediately.  If your LED is burning really bright then it may just be that the voltage is within the tolerance of the LED's rating, but the LED won't last so long.   If that's the case, or if you decide to increase the DCC voltage, you should also put a resistor in series with the LED.

[peteski]

Bart,
I suspect that the light you found is a standard "pilot"or "panel" light which uses an old-fashion 12V rated incandescent light bulb, not an LED.  Or is it rated incorrectly.

If an LED was conducting 100mA it would glow so brightly you couldn't look at it directly!  LED panel lamps are usually rated in the 20mA range.  But light bulb-based one could be 100mA.  Does the light housing get a bit warm after few minutes of being on?  After all the light source inside is dissipating 1.2 Watts of power.   LED-based light woudl heat up much less (since it woudl dissipate only around 0.25 Watts.

If it is a light bulb based lamp then there is nothing else to add. Your DCC system is probably putting out around 12V and a light bulb works just as well on DC and on AC or DCC power.  But if it is really LED-based light then adding a diode and possibly extra resistor would make it last longer.

[Missaberoad]

would something like this controling the circuit powering the layout work? it could even be at the door...


http://www.homedepot.com/p/Leviton-15-Amp-Combination-Switch-with-25-Watt-Neon-Pilot-Light-White-R52-05226-0WS/202035018

[peteski]

I got another chuckle from the URL for that switch.  It seems to indicate a 25W neon light!   I don't think such a beast exists. That would make that little switch get might hot!  Neon lights are probably rated around 0.25W or less. 

[Bart1701]

Here is a picture of the light. Looking through the red plastic casing is hard to determine if it's a bulb or an LED.



It's a Radio Shack item that they describe as follows:
Model: 272-336 (Catalog #: 2720336) 12-Volt, Jumbo Red Lamp Assembly. These 12V jumbo red lamp assemblies are perfect for home and school projects, electronic repairs and hobbies like model railroads.

I'll connect it for a while to see if the case gets a bit warm.

I thought the drawer at Radio Shack was labeled "LEDs", but I am thinking it may have been labeled "LEDs and LAMPS".  Based on the comments so far, I am thinking it is a bulb and not an LED.

But, if anyone has any knowledge about this item, feel free to share it!

Also, here is a photo of my Master Power switch over by my Control Panel. Look familiar?



Thanks,
Bart

[peteski]

Looks like it will be an old-fashion light bulb.  Especially if it gets warm after few minutes and if the package or the lamp case doesn't mention polarity on it.  100mA load is going to be equivalent of about one extra locomotive on your DCC track.

[Bart1701]

I just had it connected to the layout for about 20 minutes. Yes, the case did get warm (very warm) - so that should definitely make it a bulb and not an LED. Since I have a small layout ( U-shaped with 2 HCDs and a 4 foot connecting section between them), about 2 trains running at one time is my maximum. Adding the equivalent of another locomotive should not be a concern for me.

Thanks to everyone for their DCC and electrical knowledge and for your advice..especially Peteski who always seems to posts timely and valuable replies to my questions!

Bart

[peteski]

Thanks to everyone for their DCC and electrical knowledge and for your advice..especially Peteski who always seems to posts timely and valuable replies to my questions!

Bart