Understanding Voltage Drop using Pixels and what is Acceptable

fasteddy

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This question has been raised many times, so i thought it worth its own sticky post and so its not hidden deep within another thread in which this was. Further info on calculating voltage drop can be found on page 43/44 of the AusChristmasLighting 101 manual
The reality is that your calculated current will in fact be higher than the real current, The below example is talking about dumb RGB lights. Intelligent pixels use constant current to drive the LEDs, what this means is that the optimal current for that LED is maintained even if the voltage drops a volt or 2, but there are limits and if the voltage drops too low then the constant current driver cannot maintain that current as volts and current are linked and then you will see the effects of voltage drop.
The remainder of info has been copied from this thread https://auschristmaslighting.com/threads/27-channel-dc-dmx-rgb-controller.741/page-16#post-25206


........What will in fact most probably be the issue is the voltage drop created when using white which may not actually produce white because of the voltage drop because the more you load a cable up to its maximum rating the higher voltage loss

This can be easily show using Ohms law with the example below

So if your cable is rated at 18 amps then it would be around a 14 guage or 2mm2 cable with an approx resistance of 0.095 Ohms per metre

So that cable with 50% load will have a voltage loss over a 2 metre run of

2 metres x 2 (2 metres for +V + 2 metres for GND)

4 metres x 0.0095 ohms/m = 0.038 Ohms of resistance for the 2 metre length of cable

Volts = Current x Resistance

so at 50% load of 9 amps

0.038 x 9 = 0.342 volts loss

so now the same cable at 100% load

0.038 x 18 = 0.684 volts loss

So as you can see the higher you load up your cable to it maximum load the higher the voltage drop. The lower the voltage the higher % of volatge drop for the same distance.

LEDs are current devices, whch means that current determines the output of an LED and not voltage. But voltage and current are linked together through Ohms law. A strip that uses 3 x 5050 LEDs per metre will have a calculated current of 0.6 amps per metre for a total current draw of 3 amps for a 5 metre strip. but this is calculated and does not take voltage drop into consideration. Dumb strips are voltage controlled through a current limiting resistor, so the supplied voltage plays a very important part in this as shown with the Ohms Law calculation below

So lets look at the 3 LED/m dumb strip you are using.

Each section is made up of 3 x 5050 RGB LEDs connected in series, the 5050 LED is rated at 20mA per colour

With LED circuits

Series = Current is the same, voltage is divided between the LEDs in that circuit
Parralell = Current is added with each additional circuit, voltage is the same

So a dumb strip is made up of individual sections with 3 LEDs in series which is made of 50 parrallel section.

So a 5050 LED voltage rating will vary between colours as shown below

Red - 2.3volts
Green - 3.2volts
Blue - 3.2volts

When these are in series the voltage is added together

Red - 12 volts - (2.3 x 3) = 5.1 volts required to be dropped
Green - 12 volts - (3.2 x 3) = 2.4 volts required to be dropped
Blue - 12 volts - (3.2 x 3) = 2.4 volts required to be dropped

So to maintain 20 mA for the LED circuit voltage required a limiting resistor is used

Resistance = voltage/current

Red - 5.1 volts / 0.02amps = 255 ohms (normally a 270ohm resistor would be used)
Green - 2.4 volts / 0.02amps = 120 ohms
Blue - 2.4 volts / 0.02amps = 120 ohms


So the dumb LEDs current of 20mA from a 12vdc supply is maintained by these reasistor values.

So what happens when the voltage is 1.5 volts lower due to voltage drop (12 volts -1.5 volts = 10.5 volts)

Red - 10.5 volts - (2.3 x 3) = 3.6 volts required to be dropped
Green - 10.5 volts - (3.2 x 3) = 0.9 volts required to be dropped
Blue - 10.5 volts - (3.2 x 3) = 0.9 volts required to be dropped

So now that same strip that is designed for a 12 volt input is now only getting 10.5 volts input

current = volts / resistance

Red - 3.6 volts / 255 ohms = 14mA
Green - 0.9 volts / 120 ohms = 7.5mA
Blue - 0.9 volts / 120 ohms = 7.5mA

So now you see that with just a 1.5 volt loss you then under drive the LEDs and because the difference between the red LED and green and blue LED output, the more voltage drop the lower the light output, the lower current drawn and the more red you will start to see in your white light

Sorry for the long winded technical answer but I thought showing the theory behind this may help people understand the importance of good design and how voltage drop effects the use of dumb RGB lighting.
 

fasteddy

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I thought i would add to this to help poeple understand the difference between current, size cable and distance of the cable run and how they all effect eachother when making thecorrec cable choice or weather it is better to have a power supply closer to the lights.
So voltage drop can be worked out and shown in the below examples on how different things effect the total voltage drop based on current and resistance of different guage sized cables

So this is shown as Voltage drop = current x resistance

So regardles of the voltage the voltage drop is the same, what is different is with higher voltages the % of voltage drop is much less than with lower voltages.

Example
You have a load that is 1 string of 50 pixels at 0.3 watts at 5vdc
0.3 x 50 = 15 watts total string power

Getting Current Load
Current = 15watts / 5 volts
= 3 amps
You have a load that is 1 string of 50 pixels at 0.3 watts at 12vdc running from a 50 foot run
0.3 x 50 = 15 watts
Current = 15watts / 12 volts
= 1.25 amps

Getting Cable Resistance Values
Now we will use the above 2 examples with a range of different cables
24 guage cable resistance = 0.86 ohms/metre (0.262 ohms/foot)
20 guage cable resistance = 0.39 ohms/metre (0.119 ohms/foot)
16 guage cable resistance = 0.014 ohms/metre (0.0043 ohms/foot)
12 guage cable resistance = 0.0057 ohms/metre (0.0017 ohms/foot)

Getting the total Cable Resistance
Now to get the total resistance we double the cable run distance as this is both the supply and return path cable reistance that must be taken into account.
24 guage total cable resistance = 0.86 ohms/metre (0.262 ohms/foot) x 30.5 metres (100 feet)
= 26.23 ohms total cable resistance
20 guage total cable resistance = 0.39 ohms/metre (0.119 ohms/foot) x 30.5 metres (100 feet)
= 11.895 ohms total cable resistance
16 guage total cable resistance = 0.014 ohms/metre (0.0043 ohms/foot) x 30.5 metres (100 feet)
= 0.427 ohms total cable resistance
12 guage total cable resistance = 0.0057 ohms/metre (0.0017 ohms/foot) x 30.5 metres (100 feet)
= 0.174 ohms total cable resistance

Getting Voltage drop on Cable length
So now we get the Vdrop of each example
24 Guage vdrop = 26.23 ohms x 3 amps
= 78.69 volts
20 Guage vdrop = 11.895 ohms x 3 amps
= 35.68 volts
16 Guage vdrop = 0.427 ohms x 3 amps
= 1.281 volts
12 Guage vdrop = 0.174 ohms ohms x 3 amps
= 0.522 volts

Current Effects on Voltage Drop
And now we will show how current also effect the voltage drop by adding a second string as load
3 amps x 2 = 6amps
24 Guage vdrop = 26.23 ohms x 6 amps
= 157.38 volts
20 Guage vdrop = 11.895 ohms x 6 amps
= 71.37 volts
16 Guage vdrop = 0.427 ohms x 6 amps
= 2.56 volts
12 Guage vdrop = 0.174 ohms ohms x 6 amps
= 1.04 volts
 

Bill Ellick

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Excellent info Eddy!
Thanks for posting this for all to see.
Hopefully this will make it a little easier to show newer folks who have never dabbled in electrical workings how things must be "figured in" when designing.
There is much more than simply plugging things together and watching the lights flash with this stuff and the onslaught of pixels with lower working voltages really brings out some of the reasons that folks need to study this a little and be aware of important considerations when putting together a large amount of pixels or even leds in a display.


I nominate Eddy as "Hobbyist of the Year" for all his work with this stuff and for all his help to others!
 

Mr Griz

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Aug 17, 2013
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This is a great post, couldn't understand why my 4-15M strips on a single 12V power supply which is also running my controller would work on some effects but not the other. When I have the strips strobe there is not enough power, so lesson learned, next year I will add additional power supplies and quite a bit more organization. Is okay for this year though.
 
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