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# Why can you connect LED strips at both ends simultaneously?

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(@tasan)
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Topic starter

I don't quite understand how you can connect DC power to both sides of an LED strip and get power flowing towards the middle, making the middle the least bright instead of the ends.

I guess I would be able to understand it somewhat if it were AC power, or perhaps not. Won't the two currents "collide" seeing as DC runs in one direction? Where does the power... go?

🤔

Interested in learning about electrical engineering!

(@robotbuilder)
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@tasan

Maybe something like this:

To enlarge an image, right click image and select Open link in new window.

Remember what ever flows out of a pump must also flow back in (assuming a closed system).

Pressure (volts)
Current (like gallons per minute) (amps)
Resistance (ohms)
Power (watts) (Energy released like heat or motion or light)

(@zander)
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Posted by: @tasan

I don't quite understand how you can connect DC power to both sides of an LED strip and get power flowing towards the middle, making the middle the least bright instead of the ends.

I guess I would be able to understand it somewhat if it were AC power, or perhaps not. Won't the two currents "collide" seeing as DC runs in one direction? Where does the power... go?

🤔

In most sketches you specify the number of LEDS to control. If a strip has 42 and one end is connected to a sketch that says 20 and the other end to another 20 then the 2 in the middle will be unlit. I also don't get it, but accept it and use it. I will be working with a large NeoPixel strip today and might try lighting from both ends.

Arduino says and I agree, in general, the const keyword is preferred for defining constants and should be used instead of #define
"Never wrestle with a pig....the pig loves it and you end up covered in mud..." anon
My experience hours are >75,000 and I stopped counting in 2004.
Major Languages - 360 Macro Assembler, Intel Assembler, PLI/1, Pascal, C plus numerous job control and scripting

(@will)
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Posted by: @tasan

I don't quite understand how you can connect DC power to both sides of an LED strip and get power flowing towards the middle, making the middle the least bright instead of the ends.🤔

Remember that you're using the SAME DC power source on both ends, not two separate ones. So the electrons leaving from GND return to the anode regardless of their path,

Experience is what you get when you don't get what you want.

(@robotbuilder)
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@will

Remember that you're using the SAME DC power source on both ends, not two separate ones.

That makes no difference. My example the power supplies were in parallel making it "one" power supply using two batteries.

The question was how you can have electricity flowing in opposing directions. The answer is the same way you can have water flowing in opposing directions. They divert down different pathways.

Won't the two currents "collide" seeing as DC runs in one direction? Where does the power... go?

I think @tasan is confusing electrical concepts that is why I wrote them out. I also think the water analogy is a good one providing you remember it is just an analogy.

In this example one flow is colored orange and the other flow is colored purple using ONE power supply.

To enlarge image, right click image and select Open link in new window.

I don't know why you would power it from both sides I assume it has something to do with resistance along the bus lines? I have not used these lights myself.

This post was modified 2 months ago by robotBuilder

(@will)
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Posted by: @robotbuilder

@will

Remember that you're using the SAME DC power source on both ends, not two separate ones.

That makes no difference. My example the power supplies were in parallel making it "one" power supply using two batteries.

We'll have to disagree then. I feel that powering an LED strip from both ends with 2 different power sources is a Bad Idea and should not be done.

Experience is what you get when you don't get what you want.

(@zander)
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@robotbuilder At least in the case of Neopixels yes each one 'consumes' a certain amount of power that is why the sketches specify the number of leds. if you specify 20 and the strip is longer than 20 the leds do not light after 20. I have them setup on my workbench right now and have seen this with my own 2 eyes. If you specify 40 and there is only 20 leds, the power goes ???? the video I saw about this simply said it just magically goes away. As a person somewhat educated in this subject matter I don't get it but if it works and is safe I don't lose sleep over it either.,

Arduino says and I agree, in general, the const keyword is preferred for defining constants and should be used instead of #define
"Never wrestle with a pig....the pig loves it and you end up covered in mud..." anon
My experience hours are >75,000 and I stopped counting in 2004.
Major Languages - 360 Macro Assembler, Intel Assembler, PLI/1, Pascal, C plus numerous job control and scripting

(@will)
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The sketch needs the count to know how many data to send. If you specify more than the actual LED count, it keeps sending until your count is reached, but the phantom 20 LEDs aren't there so nothing appears to happen.

Similarly, if you send colours for 20 LEDS when you have 40LEDs on the chain, then the last 20 LEDs are not initialized. If they started off they'll remain off. Not sure what'd happen if they started some other colour, I'd guess thay'd stay that colour.

Experience is what you get when you don't get what you want.

(@robotbuilder)
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Posts: 1647

Bad idea or not the impression from what was asked that it was recommended way of solving some light brightness issue. I was addressing the question about "where does the power go" and "colliding currents". I haven't really read about these strips but a quick look shows they are not all the same. There is one for example where each led module has a chip to create a constant current led strip.

(@robotbuilder)
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Posts: 1647

The sketch needs the count to know how many data to send. If you specify more than the actual LED count, it keeps sending until your count is reached, but the phantom 20 LEDs aren't there so nothing appears to happen.

Ok so these strips are complicated connections of modules? Are they addressed as individual modules? Any links to explain all this?

(@will)
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Posted by: @robotbuilder

The sketch needs the count to know how many data to send. If you specify more than the actual LED count, it keeps sending until your count is reached, but the phantom 20 LEDs aren't there so nothing appears to happen.

Ok so these strips are complicated connections of modules? Are they addressed as individual modules? Any links to explain all this?

Experience is what you get when you don't get what you want.

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(@zander)
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@robotbuilder YES, neopixels or individually addressable RGB leds have a chip on each light.

Arduino says and I agree, in general, the const keyword is preferred for defining constants and should be used instead of #define
"Never wrestle with a pig....the pig loves it and you end up covered in mud..." anon
My experience hours are >75,000 and I stopped counting in 2004.
Major Languages - 360 Macro Assembler, Intel Assembler, PLI/1, Pascal, C plus numerous job control and scripting

(@robotbuilder)
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Joined: 4 years ago
Posts: 1647

The last time I remember these led strips was this thread.

All the secrets are hidden in the fastLED library.

(@will)
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Posted by: @robotbuilder

All the secrets are hidden in the fastLED library.

That's what libraries are for 🙂

Experience is what you get when you don't get what you want.

(@robotbuilder)
Noble Member
Joined: 4 years ago
Posts: 1647

Noooo!!  I am old school. I like to know how things work at the bottom level even if I end up using a library. In this case I do know how addressable modules work and how to build one out of discrete digital chips.

This post was modified 2 months ago by robotBuilder