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Thread: Power injection question

  1. #1
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    Default Power injection question

    I am drawing out my layout and have a question on power injection. Imagine I have a string that is *really* long. Lets say its about 250 12v bullet pixels. I understand that as the power goes from the one end of the string to the far end it will drop. Can I run a separate wire basically parallel to the string whose sole purpose is to be for power injection, and T that power into the line at places it is needed, or with the lateral length of that wire coming from the very same power supply suffer the same exact voltage drop, or is the drop exhibited within the string more an artifact of power being consume by each prior pixel?

    I ask because I am going to encase bullet pixels in a combination of 1/2" pvc and J channel, and if I can simply run a "power source" wire down the channel and T it in as needed, I'd like to do that.

    I'm just not certain if there is a point where I have to use a 2nd power supply that is physically closer to the injection point?

  2. #2
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    Default Re: Power injection question

    You can use the same supply if it has enough current capability. Keep in mind you will get voltage drop on that secondary wire as well. If you use something like 14gauge or better, you would most likely be OK.. Again, depends on distance. Google voltage drop calculator and plug in some numbers.

  3. #3
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    Default Re: Power injection question

    Your request almost sounded like you are planning to run a single wire down the length of the pipe. When injecting power, you must inject both power and ground.

    ALL wire has resistance over distance. The longer the distance between the power source and the consumer of the power, the greater the VOLTAGE drop near the consumption point. The heavier (thicker, more capable) the wire, the lower the resistance per foot = lower voltage drop per foot at a given current drain.

    When you inject, you connect the V+ and V- from the PSU (through the thick wire) to the strand to be powered. You also connect a much thinner (aka something in the line of 22AWG) pair of wires between the previous strands Data and V- to the next strands Data and V-.


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  4. #4
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    Default Re: Power injection question

    Quote Originally Posted by MartinMueller2003 View Post
    ALL wire has resistance over distance. The longer the distance between the power source and the consumer of the power, the greater the VOLTAGE drop near the consumption point. The heavier (thicker, more capable) the wire, the lower the resistance per foot = lower voltage drop per foot at a given current drain. .
    I'd like to second and add to what Martin said here. The biggest reason for the power drop through the line of pixels, or the strip, or any other method, is that the size of the wire (or trace in the strips) is comparatively small and creates more resistance. Also, most pixels are soldered going in and out of each pixel. Solder joints, in themselves, create resistance. By the time the power gets through your 250 pixels it is insufficient to power the pixels correctly. What you are doing with the power injection is providing a second source, probably with a larger sized wire. (Note, I'm talking about the same power supply here.)

    With 12 volt, I'd try to have the initial power source (front of the string), the end of the string, and possibly (if needed) somewhere in the middle. Nothing says you can't physically parallel wire with your light string, from the same source.
    Last edited by algerdes; 06-29-2018 at 03:45 PM.
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  5. #5
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    Default Re: Power injection question

    Great points and things I hadn't considered. Thank you everyone!

  6. #6
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    Default Re: Power injection question

    I got news for you: 250 pixels isn't a long string.

    Define your application first and then figure power placement. For example, on a mega tree run at 25% brightness you could probably power your string with power at the beginning of the string and at the end of the string, only, and your power cable length would be the distance from your power supply to the beginning or end of the string. In my case, that's less than 2 meters. Conversely, on a roofline, you may be 30 feet from the power supply, at which point your wire gauge and resistance is a bigger issue. So in that situation, placing the power supply in the middle of your string would be preferred.

    What I do for rooflines is have multiple power supplies placed under the eaves where strands connect, or within a certain distance of each connection point. I make "isolation tees" by cutting the V+ line on the "data in" side of the 3-2-3 tee, and inject from the new power supply at that point. Data and ground pass through the fee, and V+ from the new PS flows downstream, but not upstream.

  7. #7
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    Default Re: Power injection question

    Quote Originally Posted by Kensington View Post
    What I do for rooflines is have multiple power supplies placed under the eaves where strands connect, or within a certain distance of each connection point. I make "isolation tees" by cutting the V+ line on the "data in" side of the 3-2-3 tee, and inject from the new power supply at that point. Data and ground pass through the fee, and V+ from the new PS flows downstream, but not upstream.
    Any chance we could see a picture of this? Thanks!

  8. #8
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    Default Re: Power injection question

    Quote Originally Posted by Kensington View Post
    I got news for you: 250 pixels isn't a long string.
    Late but thought i'd share that I agree with Kensington. If using a 250 pixel string, you can probably just PI at the end of the strand and you'll be fine if using 14ga or better. My typical rule is as follows:

    [controller] ---> 100 pixels --> PI --> 240 pixels --> PI --> 240 Pixels --> PI --> 120 Pixels

    Remember that power flows in both directions and using this rule, a pixel would never be more than 120 pixels away from power. If you introduce a second power supply along your strand, remember to keep the ground continuous but to cut the positive lead at the start of injection from the second power supply.

    power-injection.jpg
    Last edited by corydorning; 09-14-2018 at 04:59 PM.

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