Author Topic: 5a output per port, max pixels, etc.  (Read 419 times)

Offline pixelpuppy

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Re: 5a output per port, max pixels, etc.
« Reply #15 on: December 09, 2018, 08:12:11 AM »

Light strings are a hose and then attach both ends to a faucet. 

I am sorry to differ. Your hose analogy would apply to voltage. Current is the same for the entire length of the string, ohms law. Dont believe me, measure current at any point in a series circuit and it will be the same. It is the voltage drop across the wire and each pixel circuit board that lowers the brightness at the far end pixels. That is resistance. E=I*R.  E=voltage, I=current, R=resistance. So resistance adds up over distance, reducing voltage. Sorry, I just could not let that analogy go without a bit of correction.39 yr Telecom Technician.


Actually, Emuney18 was more correct than you may think.  The water analogy applies to both voltage and current, where water pressure is analogous to voltage and water flow is analogous to electrical current.  They are both part of the total equation.   Yes, the resistance over the length adds up and creates voltage (pressure) drop.  But, using the ohms law that you referenced, a voltage drop also means a current drop given the same resistance.  E=I*R so if you lower E (votage drop) without lowering  R (fatter wire or hose) you also lower I (current)

Your statement about current being the same at any point in a SERIES circuit is correct.  However, pixel strings are not series circuits.  Power in pixel strings is a PARALLEL circuit which means that current flow at each pixel (each hole in the hose) can and will be different.  Consider the common case where each pixel can be running a different color and at different intensity  A WHITE pixel next to a BLUE pixel on the same string do not both draw the same current.
-Mark

Offline pixelpuppy

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Re: 5a output per port, max pixels, etc.
« Reply #16 on: December 09, 2018, 08:43:42 AM »
What determines the amount of current on that string, the PS or the lights?  For a 30amp PS is it always 30 or only what the lights are asking for at a given time?


As aknflyer stated, its all Ohms Law.  The amount of current flowing through a circuit is determined by the voltage divided by the resistance.   A 30amp power supply is not constantly flowing 30 amps.  That rating is the maximum it will supply.  Actual current flow varies depending on the load (resistance) and the voltage.  The supply *should* provide a constant Voltage (typically 5v or 12v).  As Jon stated, some cheaper supplies may not be able to maintain constant voltage with high current draw.


With that said, there are power supplies that are made for "constant current" rather than "constant voltage" but that is a different discussion.   Most of the power supplies we talk about in this forum (and most power supplies in general) are constant voltage supplies.

Offline Emuney18

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Re: 5a output per port, max pixels, etc.
« Reply #17 on: December 09, 2018, 12:07:01 PM »
Thank you all for these comments. Makes alot of sense and I hope others benefit from it.  I use mostly MeanWell supplies and never come close to even 80% of the max current output based on me measuring the current of 100 of my lights.  I just got a kill a watt meter and plan to do some readings of my mega tree for my entire playlist to see how close my numbers are to what I think they are. 
How different are the resistance values for each pixel in practice?  I have used a couple voltage drop calculators and they must assume some average resistance value so I'm guessing someone better than me has measured it.


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Offline aknflyer

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Re: 5a output per port, max pixels, etc.
« Reply #18 on: December 09, 2018, 06:54:21 PM »
@aknflyer thanks for correcting me, I can admit when I was wrong and I guess I am still confused about current.  What determines the amount of current on that string, the PS or the lights?  For a 30amp PS is it always 30 or only what the lights are asking for at a given time?


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JohnB256 is correct about the power supply will provide what the string needs.
It gets trickier when you have "active components" in the circuit, the data is always telling the pixels to turn off, on, change color. So I guess I should qualify my statment to read "with a static display, current will stay the same in all parts of the circuit.  The voltage will drop towards the end of the string due to wire resistance over distance.

Offline aknflyer

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Re: 5a output per port, max pixels, etc.
« Reply #19 on: December 09, 2018, 07:02:33 PM »

Light strings are a hose and then attach both ends to a faucet. 

I am sorry to differ. Your hose analogy would apply to voltage. Current is the same for the entire length of the string, ohms law. Dont believe me, measure current at any point in a series circuit and it will be the same. It is the voltage drop across the wire and each pixel circuit board that lowers the brightness at the far end pixels. That is resistance. E=I*R.  E=voltage, I=current, R=resistance. So resistance adds up over distance, reducing voltage. Sorry, I just could not let that analogy go without a bit of correction.39 yr Telecom Technician.


Actually, Emuney18 was more correct than you may think.  The water analogy applies to both voltage and current, where water pressure is analogous to voltage and water flow is analogous to electrical current.  They are both part of the total equation.   Yes, the resistance over the length adds up and creates voltage (pressure) drop.  But, using the ohms law that you referenced, a voltage drop also means a current drop given the same resistance.  E=I*R so if you lower E (votage drop) without lowering  R (fatter wire or hose) you also lower I (current)

Your statement about current being the same at any point in a SERIES circuit is correct.  However, pixel strings are not series circuits.  Power in pixel strings is a PARALLEL circuit which means that current flow at each pixel (each hole in the hose) can and will be different.  Consider the common case where each pixel can be running a different color and at different intensity  A WHITE pixel next to a BLUE pixel on the same string do not both draw the same current.

I see your point about multiple parallel circuits. I stand corrected.

 

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