I have 16 turnouts on my layout. Each will have two indicator LEDs (one red, one green) on my control panel, so I will need to have 16 LEDs lit at any one time. What would be the best way of doing this? So far I’ve managed to get one going using a 1K resistor without blowing anything up! [:)]
Most LED’s operate on such a small amount of current that a 12VDC supply with just one amp could power numerous LED’s, providing you use the resistor to protect the total voltage to the LED. Most LED’s use 30 mil amps or less and the 1K resistor drops much of the voltage . 1000 mil amps divided by 30 = 33 LED’s.
This is only an example since I don’t know the output of the 12VDC power supply, but I hope you get the idea. The LED rating I listed is about average, but the SMT’s use less.
Hope this helps.
And I’d wire the LEDs in parallel, right? In other words, I’d have a 12VDC buss going through the control panel and hang the LEDs off this buss with one 1K resistor at the power supply end of the positive cable?
I read an article somewhere that advised to wire a resistor to each LED. The reason was if an LED burns out with several others wired to the same resistor the volts/amps will increase to them because the burn out one stops drawing current. I’m starting to use LEDs in my locos, replacing lights as they fail. I do have some LEDs on the layout powered from walwarts(the box looking transformer that plugs into the wall with a wire running from it). This type has the voltage (12VDC) and amperage rating (300mA on mine) printed on it. As long as your supply is putting out enough amps and your LEDs are protected by the right size resistor your all right. I think most of my LEDs on the layout have a 470 Ohm resistor or a variable resistor set close to that. I like using the variable resistor so I can easily change the brightness of the LED. Hope this helps.
Use a resistor for every LED. Wire them in parallel. A 1K resistor across a typical red or green LED with a 12v power supply will set the current to right around 10ma, so even a half amp (500ma) power supply will run 50.
Typical red or green LED drops 2.8 volts. 12-2.8 = 9.2 volts, divided by 1000 ohms - 0.0092a, just over 9ma. You might want to go with a slightly lower resistor value if the LEDs aren’t bright enough - 680 ohm works good for a 12v power supply. With a 680 ohm resistor you have the same 9.2v divided by 680 ohms, 0.0135a, 13.5ma. Still well within the safe range of the LED and possibly a bit brighter.
–Randy
You might also think about using 2-color LEDs. They come in 2 flavors - the 2-wire version will be red when wired plus/minus, and green when wired minus/plus. The 3-wire version has a common lead and you put voltage on one or the other to get the color. I like these because you’ve only got to mount 1 LED. I used them in signals and on control panels for that reason.
Has anyone used a surplus “brick” power supply left over from old computer accessories, etc., to drive MR stuff? You know, those blocks that plug into the terminal strip (and take up 3 spaces because they’re so darn fat) and put out some DC voltage for speakers or whatever?
Yup, I do. Got about a dozen or so for $1each at a garage sale. They were from all kinds of household “stuff”, with a bunch of different voltages and current ratings, both AC and DC, some regulated DC, etc. Even if I have to add a resistor network or regulator to get what I need, it’s still cheaper than buying a new supply.
Steve
Wakaiti, you could save on resistors (not that they cost much to start with!) by joining together the anodes (positive leads) of the red and green LEDs for each turnout together and using one resistor to + voltage to supply both. Since only one or the other LED will be on at one time, it’s electrically the same as one resistor/one LED.
With a 1,000 ohm resistor each LED is drawing 10 mA so divide 10 into the milliamp rating of your power source to see how many LEDs you can power from that source. I wouldn’t go past about 80% of the supply’s capacity, expecially if it’s one of those “wall wart” types.