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Thread: How I do my LED lighting

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    Default How I do my LED lighting

    Another user commented in my layout thread that they were not sure how to go about wiring up LEDs for a layout. I'm just going to explain how I do it, others may have their own methods. I've been using CL2 constant current LED driver for my LED lighting.

    The CL2 takes about 5V to operate, so this simply means that your available voltage for an LED string is Vsupply - 5V. If you use a 24V supply then your available voltage is 24 - 5 = 19V.

    I've measured the Vdrop of the LED colors I use -

    Green - 3.2V
    Red - 1.9V
    Amber - 2.1V
    White - 3.1V

    Please refer to the schematic from the CL2 datasheet attached. Let's say you have a building like I do that has 6 LEDs total (3 amber and 3 white LEDs). That's 3 * 3.1V(white) + 3 * 2.1V (amber) = 15.6V.

    If you use a 24V supply like I do, then your available voltage for LEDs is 24 - 5 = 19V. (remember the CL2 takes 5V for itself). So we know we need 15.6V for this string which is below 19V, so just wire all of them in series and then to the CL2 - nothing more has to be done, 20 mA will be supplied to the string. In this case you even have 19V - 15.6V = 3.4V for an additional LED. But even if you don't use all your available voltage, it will be dropped across the CL2.

    Once you use up the voltage of one CL2 simply start using another one off the same supply. Keep going until your layout is completely wired for all your desired LEDs. You can see in the attachment how I used a simple perfboard to hold 10 Cl2s, along with some screw terminals for the outputs and ground returns.

    For the wiring going to the LEDs off the CL2 - use very small wire, large wire is not needed, I use either 30 gauge magnet wire or 28 gauge stranded wire.

    Note: The nominal current of these small LEDs is 20mA, although you can run them at any current less than that to make them less bright. The CL2 device supplies constant current at 20 mA. If you want to make an LED less bright, and it's inside a building (so that painting it sloppily won't be noticed), the easiest way is to simply dab some black paint on the LED until you get the brightness you want. Also, if the plastic lens is focusing the light to tightly, and you want it diffused, scrub the lens with fine sandpaper or steel wool and the light will be nicely diffused all around.

    Another note - yes the CL2 can be supplied with up to 90 VDC. However, if you did that and used only one or two LEDs, the CL2 would get very very hot. This is because the CL2 would have to drop most of the voltage across it and waste a lot of power. I chose 24VDC power supply and I try to use at least 2 or 3 LEDs at once so that the CL2 does not have to drop too much left-over voltage across it.

    As far as outside LEDs such as street lights, I've found the full brightness is OK for my tastes. I usually paint over much of the plastic anyway to make it look like a real lamp (which doesn't shine out the top or other places of the enclosure). But if you have a specific application that you must use less current then you can always wire it up on it's own circuit (not using the CL2) using a series resistor causing it to use much less than 20 mA. If you don't know how to use series resistors with LED there are a gazillion +1 simple tutorials on the internet, or I or perhaps others can help you with that - it's very, very simple.

    This is the way I do it, but others are free to comment with their own ideas. The main point is that worrying about series resistors for a large complex LED layout is not needed, with the availability of better solutions.

    CL2 Data Sheet -

    https://www.microchip.com/wwwproducts/en/CL2
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    Microchip CL2, huh?

    Dude,I could ... give you a big man hug or something. I've been looking for a handy no-frills current source like that for a while. Don't know why I haven't stumbled on this little gem.

    Nice work! And nice write-up too!
    Never mistake a guy who talks a lot for a guy who has something to say...

    CH&FR Site and Blog: http://www.chfrrailroad.net and http://blog.chfrrailroad.net
    Appalachian Railroad Technology: http://www.apprailtech.com


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    Quote Originally Posted by TwinDad View Post
    Microchip CL2, huh?

    Dude,I could ... give you a big man hug or something ...
    Oh no!

    There might be better solutions out there today but these are easy to work with so I haven't really done any further research.

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    Hey, if it ain’t broke don’t fix it. How long have people been using the 78xx regulators or the 555 timer or the lowly 2N2222 transistor? Yet they are still sometimes (often?) the ideal solution to the problem at hand.
    Never mistake a guy who talks a lot for a guy who has something to say...

    CH&FR Site and Blog: http://www.chfrrailroad.net and http://blog.chfrrailroad.net
    Appalachian Railroad Technology: http://www.apprailtech.com


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    How about we make this an article?

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    Here's another option I just found on Ebay. I hadn't really looked around at what is available recently. These provicde a constant 20 mA and handle up to 45 V and only drop 2 V across themselves. I ordered 5 for $10 so I can experiment with them. I will post the results in this thread.



    https://www.ebay.com/itm/5-x-20mA-LED-Driver-No-Resistors-Constant-Current-Power-Supply-DC-V-Voltage/162925859408?ssPageName=STRK%3AMEBIDX%3AIT&_trksid =p2057872.m2749.l2649


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    Got the little LED drivers in yesterday. They do work as advertised. They only drop 2V as opposed to the 5V of the CL2 device, so as far as that is concerned I like them better. They come on a little board that allows you to snap off one at a time, or you could mount it on another small perfboard and run wires from that. The package comes with a length of clear shrink-wrap so you can just install the driver as an inline device in your wiring and cover it with the shrink wrap.
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    Hi Nscale, just saw your guide here linked and I wanted to ask some very 101 questions, actually probably 100 level questions.

    On my last layout I started using some LED's. I got a wall-wort that supplied plenty of (I believe) 8v power. I then checked the voltage of my LED's and did the math to make sure each serial-wired LED added up to a touch over 12v because if you applied too many volts they'd burn out, but if you're a little over the only harm is making them a touch dimmer (which is good in our scale anyways).

    This is how I attempted to avoid dealing with resistors and the like, but it did make wiring a little tricky and sometimes I just needed 1 more LED somewhere but couldn't wire it until I found 4 more friends to share the 8v.

    From what I understand about these drivers is that they do the voltage math for you and act as a self-regulating resistor? So if you string 4 2v lights together it some how detects the string wants only 8v combined and only sends that voltage along? but if you then add or remove a light, it then adjusts? So as long as all the lights in your string want the same voltage, they'll always be protected from over-voltage? The only thing to watch out for is only having a couple LED's on a single driver and supplying it with a lot of volts as it will have to convert those extra volts into heat?

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    You're spot on with your last paragraph. Just need to make sure the maximum output voltage is higher than the LEDs in series need (or they won't light) and - as you say - make sure the LED driver doesn't overheat.

    About the rest, I need to take a little detour: A diode will be destroyed if there is more current flowing through it than it can take. And: The current through a diode is very much non-linear with the voltage on it, check the WIkipedia image: https://en.wikipedia.org/wiki/File:D...rrent_wiki.png - at 0.2V, there is nearly no current at all. At 0.5V, there is about 5mA (nominal current) flowing. At 1V, we're off the chart. (That voltage level at which the nominal current flows depends on the diode, regular silicon diodes are around 0.5V-0.6V, red LEDs 1.6V, yellow and green 2V-2.5V, blue and white in the 3V-4V range.) And what's worse: If the diode heats up, that voltage level drops, which increases the current, which heats the diode up more...

    So what do you do to keep the LEDs happy and healthy? Either the way you do, stay far away from the combined threshold voltages of the series of the LEDs and hope for the best, or with a series resistor that "removes" the remaining voltage at the nominal current - so when the LED heats up, takes up less voltage, the current will increase, increasing the voltage drop over the resistor - and that will stop the current increasing further before the LED overheats. Or with one of those LED drivers that regulate the current by taking away the excess voltage.

    HTH,
    Heiko

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    Quote Originally Posted by baronjutter View Post
    From what I understand about these drivers is that they do the voltage math for you and act as a self-regulating resistor? So if you string 4 2v lights together it some how detects the string wants only 8v combined and only sends that voltage along? but if you then add or remove a light, it then adjusts? So as long as all the lights in your string want the same voltage, they'll always be protected from over-voltage? The only thing to watch out for is only having a couple LED's on a single driver and supplying it with a lot of volts as it will have to convert those extra volts into heat?
    What Heiko said - the driver takes care of the regulation. It's all explained in my first post.

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