I currently have in operation a 12 volt 2 amp transformer operating my LED signals and misc. LED structure lighting. Everything works / looks great at this point, but the future calls for much more.
If I were to replace this transformer with say a 5 amp, would there be any concern regarding all the LED / resistor installations that are currently working fine ??? I’ve had nightmares of actually doing this, turning on the supply and blowing out all my signal lights !!! [xx(]
Should I be concerned or would it be kinda iffy ???
Just as others divide up their layouts into power districts, why not do the same for your lighting. You could have one for you interior and one for your exterior lighting. I use slide switches to control the ones that I want to turn on or shut off.
If it were me, I would just buy a good 16-18VA power pack. I like and use the MRC 1370 Rail Power. I got mine off eBay for ~$20. That would be more than ample for what you need.
Mark, you could also buy 1.5V regulated wall transformers that will save you the worry altogether. I use one for my 1.2mm 30mA 1.5V exterior light bulbs that I have wired in parallel. I bought mine from Miniatronics. Although they aren’t cheap, I believe they also come with built-in protection in them, as well.
If you were to change from a 12 Volt to 20 Volt transformer, you would need to change all of the resistors, but changing from a 12 Volt, 2 Amp to 12 Volt, 5 Amp transformer would make no difference except that you could power more LEDs with it.
The typical LED draws 20 mA of current, so 5 LEDs would draw 1 Amp. A 2 Amp transformer can safely handle 10 LEDs before it becomes overloaded. With 5 Amps, you could have 25 LEDs.
Slight math error there … 2 amps divided by 20 mA would be 100 LEDs. A 5 amp supply should be able to handle 250.
Another thought I’ve had as well - resistors are also considered a “load” in some cases. Do they factor into the power consumption as well … say for example using 680 ohm resistors as opposed to 1000 ohms - the LEDs would still be well protected and there isn’t much difference in light output ??? With a total of 100 LEDs, that would be 68,000 total ohms resistance vs. 100,000 ohms … or is that not relevant ?
From my limited electrical knowledge, resistors do constitute a load and would have to be figured in if you were trying to light, say, 250 LED’s. Anything below that it probably inconsequential in calculating load. I agree with Tom, though. Buy one of the MRC 1370 transformers that are going for almost nothing now with the switch to DCC. I got mine for $15 in new condition on e-bay. I currently have 58 incandescent bulbs and 10 LED’s running on it and it doesn’t even get warm. I haven’t calculated what the total capacity is but I think I could light a small town. [:)] The big advantage for me is wiring everything to the variable DC terminal though a power buss. I can then control the intensity of the lights to suit the room lighting or my mood. The only change I had to make was to run my LED’s on lower value resistors than normal so they will still be bright at 9 or 10 volts. That plus Posi-Tap connectors have made lighting the layout a pleasure instead of the torture that it used to be.
The resistor does use power, but it is wasted power. If you use less resistance (with the same LED and the same voltage power supply) you will get higher current in both the LED and the resistor, and the resistor will dissipate more power. The formulas are:
Current = (power supply voltage - LED voltage) / resistance
Power in the resistor =
[(power supply voltage - LED voltage) * (power supply voltage - LED voltage)] / resistance
If I had a 12-volt power supply and a bunch of LEDs of the same current and voltage, I would connect them in series, which allows using a lower resistance, plus a lot fewer resistors. Unless, of course, that would make the wiring inconvenient, or unless I wanted to switch them on and off individually.
You can’t do this for an unlimited number of LEDs. The total of all the LED voltages in series must be less than your power supply voltage. If you have more than the number that gives that total voltage, then you can make multiple strings of LEDs.
For auxiliary panel, structure and (future) signal power, I have assembled a motley collection of old toy train power packs (collected at yard sales,) wall warts from dead devices (rated variously from 5 to 19.8 VDC) and cheap filament transformers (6.3/12.6VAC center tapped.) As I need to activate a circuit I dig up an appropriate one and plug it into one of the several circuit breaker equipped outlet strips powered from my layout master switch, which connects to a dedicated circuit formerly meant to power the garage door opener.
By using a number of separate transformers I never have to worry about overloading one, and if one should fail in operation it won’t kill the entire layout. Since they are all pretty much standard designs, I will have a replacement ready to install in the unlikely event that one fails.