Why do I have to use power injection with RGB and Low Voltage Lighting

I have seen and been asked this question many times and thought i might show why power injection is required for low voltage applications and why a low voltage strip doesnt get the same length as a rope light

It has to do with Ohms law and the resistance of a cable, the voltage and the current.

An example is the difference between a 240v rope light and a 12vdc RGB strip, a 240 volt rope light can get much further runs then a 12vdc strip

so with some calculations we can show the difference the voltage makes to current for the same rated wattage used.

In this example we will use a 100 watt load


So the current required would be:

Current = power(watts) / Voltage

Current = 100 watts / 240 volts

= 0.42 amps @ 240 volts



Now for the same wattage used but running at 12vdc

current = 100 watts / 12 volts

= 8.3 amps @ 12 volts



The reason that strips are only 12vdc and 5vdc is because an LED has a rated forward voltage that it will run with, so the LEDs are connnected in series.

Example. An LED may have a forward voltage rating of 3 volts, if you connect 3 of theses LEDs in series you will get 9 volts and 3 volts will be required to be dropped through a resistor. This allows the strips to have a much smaller cuttable section than what is found on rope lights. If a strip was rated at 240 volts or a 240v rope light used a similar rated LED then this would require around 70+ LEDs connected in series to become an efficient LED circuit.



Now the next thing is resistance of a cable which effects the voltage and this as well can be shown using ohms law.

Voltage = Current (amps) x resistance (ohms)

So different cable sizes have a different resistance value, generally the larger the cable the lower the resistance.



20 guage (approx 1mm2) wire has a resitance rating of 1.28 ohms per 100 feet

16 guage (approx 2.5mm2) wire has a resitance rating of 0.40 ohms per 100 feet

So the voltage drop differences for a 5 amp load over 100 feet would look like this



20 gauge (1mm2): Voltage = 5 amps x 1.28 ohms = 6.4 volts dropped over 100 feet



16 gauge(2.5mm2): Voltage = 5 amps x 0.40 ohms = 2 volts dropped over 100 feet



So by using a lower voltage the % of voltage drop is much higher with lower voltages than with higher voltages over a given distance and this is why cable choice is also very important when using low voltage lighting and why power injection is used to get these extra distances.


I hope that makes it a bit more easier to understand why it is so

 

Thanks for pointing out that error, i actually used the value of 21 gauge, damn eyes are getting bad. Anyway i got the figures from this chart and you are correct with the value for the 20 guage.

http://www.cirris.com/testing/resistance/wire.html


foot thing, i tried to keep it easy for our American friends i just used feet as a general example as the figures from the chart used feet and the thread was just to give people a general idea surrounding low voltage and voltage loss as the maths is the same.

 

Very good point to raise

In most cases the power injection must come from the same power supply as most power supplies will have issues being connected together as they are not designed for parrallel connection. There are some expensive power supplies that are designed to do this
Definetly the Chinese PSU that Ray sells would not do well if connected this way.

 

I assume you’re talking about testing for voltage drop. If this is the case then it’s just a matter of connecting your test leads of your voltage meter to the positive and negative wires and take down the reading. Then do this same test at the power supply and then work out the difference between the two values. This test should be done when you have the cable connected with the actual load you’re going to use being on as current draw is also a factor in voltage drop.

I hope that answers your question