Basic LED Wiring Diagram

Model Railroader General Discussion (Model Railroader)
1

I’ve looked and found diagrams…of complicated setups I don’t understand, that may or may not apply to LEDs and resistors and power supplies I can find locally.

I’ve looked and found text descriptions by the quadrillions.

What I really need is a simple wiring schematic, a drawing that includes the type (AC, DC, variable voltage variable current) and rating (how many amps, volts, mhos, whatever is necessary) of the power supply, a simple on-off switch, so that ALL the structure lighting is either on or off, whatever resistors are needed, where they go in the circuit, what their ratings are, how many of what type and rating LEDs I can string together with given power supplies and resistors, and where the wires go, with dots where wires connect, and little semi-circles where wires cross without connecting.

I can read a simple diagram, but not a complex one.

Text descriptions, electrical circuits and my brain just don’t mesh very well.

I THINK the reason I haven’t seen such a simple diagram is that the issue is more complex than I wish it was, that maybe the LEDs types depend on the voltage of the power supply, and maybe the resistors depend on some other variable, but if such a drawing existed, one solution for one set of components, say, a 12 volt toy train transformer and X LEDs maximum (and Y LEDs minimum if need be), and Z value resistors, then I wouldn’t need to understand all the theory and could light the layout using the monkey see, monkey do principle.

Anybody know of a simple drawing like that?

2

If I can’t find one, I’ll make one. Or several. There isn’t only one answer, and there will be a little text!

3

OK. As you suspected, there isn’t one answer to the question. And, I’m going to start with a little text, to try to explain why, hopefully in an understandable way. The simple cases are pretty easy, and I did find some drawings for those, so that will be a place to start. We can go from there to more interesting, though someone else may have to pick it up since I’m going on vacation for a week and may not be posting.[:)]

The first point is that LEDs are diodes. In general that means that they pass current in one direction, with a more or less fixed voltage drop across them. There is a region between ‘off’ and ‘on’ where things are not quite that simple, but as a rule we don’t try to operate them there, so it isn’t an issue. Given that they will pass current in one direction and not in the other, they like DC. AC can be used, but they will only be on half the time (usually that is plenty to see), and they are not rated to like even moderately high reverse voltages so they may need some protection from that. That can be gotten into later.

Every LED has two ratings that we care about in our kinds of applications, the forward voltage, and the forward current. We don’t plan to operate with reverse voltage on them (for now) so none of that is relevant.

Here is an example datasheet:

http://www.rohm.com/products/databook/led/pdf/slr-342.pdf

We’ll use the red one for this example, the SLR-342VR. We don’t want to push the maximum forward current, nicely this datasheet shows a typical value of 10 mA, with a maximum of 25 mA. (Ignore the peak value. that is referring to a pulse and we don’t want to worry about that.) Often, only the max rating is given. Usually half of that is a reasonable operating state. The typical forward voltage is 2.0 V.

4

Jeffers,

I’ve found this website highly useful in figuring out resistor requirements:

http://metku.net/index.html?sect=view&n=1&path=mods/ledcalc/index_eng

5

That one is pretty cool!

6

One other common topic is whether to use a single resistor for a string of parallel LEDs or a resistor per LED. Unless we are talking about a couple of LEDs then always go with a resistor per LED and even with just a couple a resistor per LED is preferred. There are numerous reasons for this.

7

[quote user=“Vail and Southwestern RR”]

OK. As you suspected, there isn’t one answer to the question. And, I’m going to start with a little text, to try to explain why, hopefully in an understandable way. The simple cases are pretty easy, and I did find some drawings for those, so that will be a place to start. We can go from there to more interesting, though someone else may have to pick it up since I’m going on vacation for a week and may not be posting.[:)]

The first point is that LEDs are diodes. In general that means that they pass current in one direction, with a more or less fixed voltage drop across them. There is a region between ‘off’ and ‘on’ where things are not quite that simple, but as a rule we don’t try to operate them there, so it isn’t an issue. Given that they will pass current in one direction and not in the other, they like DC. AC can be used, but they will only be on half the time (usually that is plenty to see), and they are not rated to like even moderately high reverse voltages so they may need some protection from that. That can be gotten into later.

Every LED has two ratings that we care about in our kinds of applications, the forward voltage, and the forward current. We don’t plan to operate with reverse voltage on them (for now) so none of that is relevant.

Here is an example datasheet:

http://www.rohm.com/products/databook/led/pdf/slr-342.pdf

We’ll use the red one for this example, the SLR-342VR. We don’t want to push the maximum forward current, nicely this datasheet shows a typical value of 10 mA, with a maximum of 25 mA. (Ignore the peak value. that is referring to a pulse and we don’t want to worry about that.) Often, only the max rating is given. Usually half of that is a reasonable operating state. Th

8

I pull that up, look at it, and I’m like jeeze…I’m learning to crawl and Hyun’s handing me the flight manual for an F-22, but you know what?

I looked at it again and I actually understand it.

Good stuff, thanks.

9

Wanna be crystal clear on this…

Two LEDs in series, one resistor limiting current for both, ok, yes?

Two LEDs in series, times ten legs in parallel, using a single large resistor in-line with the PS on the main trunk, to control current across the whole array, bad idea, yes?

10

Two LEDs in series, one resistor limiting current for both, ok, yes?

Yes, Ok. In series two resistors would add no value except to reduce the power dissapation by each resistor.

Two LEDs in series, times ten legs in parallel, using a single large resistor in-line with the PS on the main trunk, to control current across the whole array, bad idea, yes?

Yes, very bad idea

11

I think you’re making things far too complicated. Use a current limiting resistor in line for each LED. If you want to run more LEDs off of one resistor you’ll need to remember to increase the wattage of the resistor so that it has greater ability to dissipate the heat it will generate. I think you should also consider have more than one switch to control the lights in the town. This allows you to control the various lights independently of other light circuits You can also vary the ohms of the resistor to control the brightness and intensity of the LED. You also didn’t mention what kind of LEDs you’ll be using - white, blue white, yelloglow. The characterisitics of each may vary slightly. I’d also consider using smd LEDs. The other thing to consider is the angle of view, which can vary. When used to light a building you would want a small light source with a relatively wide angle of view to avoid a concentration of light.

Also, it is in the year 2525, not 2545.

12

an LED is EXACTLY like any diode, and they use LITTLE current. A resistor in series prevent from blowing the diode. (LED) a 1K resistor aughta do you fine. The LED should drop 1.5 volts across it so 8 in series on 12 volts theoretically could work without resistors. An LED is very simple like any light bulb, it just only lights one direction of current. You could light a whole town on one cheapo plug in the wall power supply.

13

You Think Right. That is exactly the reason. One cannot do something complicated such as was described with a simple diagram. Everyone wants the world to be simple. Remember when computers were supposed to make life easier, everything simple, and give us much more free time?

But I agree with a prior poster. As long as one considers it complicated it is. When one looks at each piece of a circuit diagram as a separate thing it is actually quite easy. Draw a single LED circuit with the on off switch. Figure out how to add a second simple one to it. Apply that and add a third. Pretty soon one has a complicated circuit by just adding simple things to the simple thing.

We have a similar issue at the club. Some members want the electrical system to do all these wonderous things, but those same members don’t want to move out of the elementary school as far as learning how it works. They gripe when it doesn’t work as fancy as they want. Change it and then they gripe when it is too complicated (for them) to fix when it breaks. Then they gripe when no one else will fix it because everyone else is tired of listening to all the griping and whining.

14

Theoretically yes

Not really. As long as you do not exceed max LED current you are OK. Run them a bit undervoltage. the resistor just limits max current

Theoretically 100% in LEDs and 0% in resistor. You’re correct about wasted heat. Ideally, you feed the series string of LED’s from a constant CURRENT supply (not a fixed voltage supply) with no resistor. These are hard but not impossible to find.

[quote]

Second q…different color, brightness, and design LEDs on one series/parallel circuit like the one at the bottom of your linked page…a whole town’s worth. Will minor series leg current variations cause major array problems? Perfectly matching one kind of resistor and it’s dependant LEDs with a different series of LEDs and it’s resistor won’t be possible, not perfectly anyway. I can do simple Kirch