I have a 15V DCC power bus, no auto reverse. Can I wire a LED onto that bus, say a 2.1V 20mA with a 680Ohm 1/2W Resistor?
Dont see why not Digitax uses them on their UP5 loconet inter connect fasia panels. In fact you could manually put some around the layout for power monitoring. Not sure on the resistor size but uaually a 1k resistor 1/4 watt would do the trick. Some one may chime in and tell us to use maybe a bi polar LED. Steve............what was your plan??
Not my idea but I have used this circuit I found on line.
Using an LED to indicate DCC track power on.
You will need a voltage-dropping resistor in series with the LED. You will also need a reverse polarity protection diode.
VPS = 14.4V VL = 2.6V VD = 0.7V IL = 20mA = 0.02A R = 555 ohms. You can use 560 ohms from Radio Shack. Using a 560 from Radio Shack, W computes to be 0.224. Use a 1/4W resistor or larger. Radio Shack has 560 available in ½W size. Use that
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
awesome, that helps a lot, I'll do so.
The proper way to do the protection diode is to wire it anti-parallel tot he LED, not in series like the pictures circuit. That won't protect the LED fromt he full reverse voltage the way it's drawn.
Flat side of regular diode to triangle side of LED, triangle side of regular diode to flat side of LED. One junction to one rail, the other junction to the resistor.
'flat side' and 'triangle side' are highly technical terms
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
I had a Radio Shack 5mm LED with a 1/2 watt 3300ohm resistor on hand, so hooked them up to my CVP Easy DCC 15.6V power buss for remote power monitoring. Worked fine, except a bit dim, guess the resistor value is too large. The LED stayed lit regardless of locomotive direction or which side of the buss the leads were attached too. LED goes out when the buss is shorted.
Regards, Peter
Yeah you probably want more like a 1K resistor.
Since it's across the track, you are simply seeing the DCC power, which doesn't really change much when locos are run and doesn;t change when the loco direction changes. The only exception would be on those systems that can run a non-decoder loco on address 00. In one direction, an ordinary LED would get dimmer, in the other direction it would get brighter.
The problem with leaving it at a simple LED and resistor is that LEDs can;t stand much reverse voltage. A standard silicon diode can stand 50, 100, 200, 400 or more volts 'the wrong way' - this is the Peak Inverse Volatage parameter (PIV). LEDs are typically no more than about 5V PIV, so approximately half the time they are getting around 15V when connected to DCC track. Commonly, a bi-color LED with 2 wires is used - this is really two LEDs back to back, commonly a green and red, so it glows kind of orangish unless the address 00 is in use (not supported on Easy DCC), or unless something is broken. The one LED protects the other. To use just a single LED< you want an ordinary type diode (1N914 or similar) connected anode to cathode and cathode to anode witht he LED. Thus the LED will never see more than one diode drop in reverse - .6-.7 volts.
Excessive reverse voltage doesn;t instantly destroy an LED, like excessive current would, but it does greatly shorten the life.
So could you use a bi polar LED? Steve
The kind with 2 wires, not 3, yes.
Evans Designs makes three sizes of LEDs that come with resistors etc. attached so they can be used with any DC or AC power source up to 19 volts. I have a couple of buildings that have LEDs inside lit directly from DCC track power, no problems.
http://www.walthers.com/exec/productinfo/266-U50
I have used the LED this way to indicate if a roundhouse track or stub ended siding has power. Nothing to do with direction of a loco. Sometimes a siding needs to be underpowered to store a loco.
Never had a LED burn out. Also have one on the program track.