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Why does Power Consumption increase so much?


Sep 16, 2022
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I'm doing an experiment with 10 lights which 10W each.

First light's distance from the SMPS(48V, 350W) is 100m and each lights are 10m away. They are connected with cable 'AWG16 2C'.

I measured lights' Power Consumption and Voltage. And made a line graph.

Until 7th light, it's almost straight. But from 8th light, Power Consumption increased 23.2W, 46.3W, 91W..

Each light is 10W but I don't know why the Voltage Drop and Power Consumption chages this much..

Can you please help me?

"watt" is not useful for the necessary calculations.

You need to know the current:

So you say the power supply is 48V .... then if the power of 10W is drawn from 48V DC the current should be
10W/48V = 0.21A

AWG16 has a resistance of 13.8mOhm/m plus 13.8mOhm/m for the return path.(internet search) = total 27.6mOhm/m

Now calculating 10 lights of 10W each in a distance of 10m each is a bit complicated.
Thus it´s easier to replace this with all 10 lights connected at half of the distance: 100W at 50m distance.

So the wire resistance is 50m x 27.6mOhm/m = 1.38 Ohm total.
10 x 0.21A = 2.1A.
The voltage drop will be 2.1A x 1.38 Ohms = 2.9V
and the power dissipation about 6.1W.

The above calculation is for constant current of 0.21A per light.

For more detailed informations we need to know the exact voltage to current behaviour of the lights.

Also we don´t know how you measured the power consumption
Show us your chart.
Also a sketch about your wiring could be helpful including measures.

Thank you for your detailed reply.
I attached file about the project. Hope This can help.
And there was a typo in my post.. It's 200m not 100m. Sorry.


  • Test overwiew.pdf
    37 KB · Views: 81

in post#2 I showed you how to do some calculations.
I won´t repeat them, you should be able to do it now on your own.
Excel could be a big help.

Still missing: the information how your light controller behaves (current vs supply voltage).
In worst case they influence each other and ... maybe they start to oscillate.
Big bulk capacitors at each light controller may improve stability.

It may be that the power source is playing up for the higher number of lamps - oscillating - this would account for the likely erroneous higher power measurements - easy to check with a scope or a DVM set to AC volts only.
Unclear points
- where is "lighting voltage" measured?
- what's the load I/V charateristic. Is it constant I or constant P?

If it's constant P (expectable for a LED light), the observed behaviour could be easily explained by wire voltage drop.
What you experience is clearly the result of a constant power load: the lower the input voltage, the more current the lights draw. You definitely need a higher voltage at the end of the 100? 200m? run. Just a few volts more may make a big difference.

Sounds like a silly question but did you check the manual of your smps for ways to increase the output voltage?
An output voltage adjustment potmeter or remote sense inputs could be available in your type.
I've even seen smps that can increase the output voltage based on the sensed output current (tdk/lambda has them i believe).

When sticking with 48V, make sure to test electrical safety: a short circuit in a light should not cause a fire. The long wires may prevent the SMPS output overcurrent protection to kick in when one of the last lights fails with a short circuit between the 48V rails.

I would not be surprised if, in the end, you need to decide to move the smps closer to the lights with a long mains cable. Of course, simply distributing mains and choosing lights that run from mains instead of 48V may be a cheaper and more power efficient solution in the end.

I hope this helps.
You say your 10th light is drawing 100W....but a 6W light would surely blow up if it was running that much power...these things are usually skimping it with heatsinking. As above i ask how you measure current drawn by the light...........
@cupoftea: the pdf is not perfectly clear but the 10th light certainly does not draw 100 watt.
Ts states turning on an additional light with each step and notes in the table the theoretical, expected total power to the lights, ignoring cable losses.

I assumed that the noted smps voltage is measured at smps output, and that the 'power meter' is a mains device in between the smps and mains or maybe on the SMPS output, and it starts to make sense.
Any dmx bus is run at low voltage and means that each light contains some smart driver. Again I make an assumption, but it makes a lot of sense that each led driver contains a switching converter that feeds a constant current, thus a roughly constant power, to strings of leds.

@Eggdbdr1213: are these guesses right?

P.S.: ltspice can simulate this.. use a bv source and put p=10 in the value field. When the cable resistance is stepwise increased, a dramatic and sudden increase of load current shows above a certain cable resistance, maybe around 0.8 ohms (to be double checked).
What kind of lights do you use? Bulbs with glow wire? These have a positive temperature coefficient and draw more current on a lower supply voltage.

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