I am trying to calculate max current draw for SMD 5050 LED strip. Datasheet says that 1 channel draws 20mA. So I do the calculations: 20mA * 300leds * 3channels = 18A

I already have the same strip with power supply (12V 5A) and it is working fine when all channels are high (white). If it would be 18A, wouldn't it burn? I don't understand what am I doing wrong in the calculations. :( Could anyone help please?

LED Datasheet: https://www.google.com/url?sa=t&source=web&rct=j&url=https://e-radionica.com/productdata/RGB5050LED.pdf&ved=2ahUKEwj8yvCTr87oAhXhoosKHRDqCvAQFjALegQIAxAB&usg=AOvVaw3B5Ja9lwTl9we5GdXwR4Gp

Strip Datasheet: https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.iled.com/class/INNOVAEditor/assets/gallery2/5100-5105.pdf&ved=2ahUKEwjC163XwM7oAhWys4sKHS7FDw8QFjALegQIBBAC&usg=AOvVaw3dRnzlhi1q8HrHmk2auw3g

  • \$\begingroup\$ You forgot that "max" is "max". \$\endgroup\$ – Marcus Müller Apr 4 at 9:11
  • \$\begingroup\$ You haven't provided links for the datasheets you are working from. "1 channel draws 20mA" is more likely "one LED draws 20 mA". "I already have a strip with power supply (12V 5A) and it is working fine." We don't know how many LEDs are on that strip so that sentence doesn't help. Hit the edit link below your question and add in the missing information. \$\endgroup\$ – Transistor Apr 4 at 9:13
  • \$\begingroup\$ Right, we've got the LED datasheet. What about the "strip" datasheet? \$\endgroup\$ – Transistor Apr 4 at 9:49

enter image description here

Figure 1. From the datasheet it is clear that the LEDs are wired with three in series.

What you have missed is that each 20 mA powers three LEDs in series. This is the most efficient arrangement for a 12 V supply and little power is lost in the series resistors.

Re-running your calculation we get:

$$ I = \frac {20 \ \text {mA} \times 300 \ LEDs \times 3 \ \text {channels}} {3 \ \text {LEDs / string}} = 6 \ \text A $$

From the comments:

Calculations say that max current is 6A, but my power supply is 5A. When I turn the color to white, everything is fine. But why?

  1. Measure the actual current being drawn. Remember to put your ammeter in series with the load - not directly across the supply.
  2. The PSU won't just stop at 5.001 A. Typically the voltage will start to collapse to keep the load to a safe value. It depends on the type of PSU.
  3. The PSU rating may be conservative.
  4. The PSU might be running hot and it will die prematurely.
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  • \$\begingroup\$ Thank you for your response! There is only 1 thing I don't get right now. Calculations say that max current is 6A, but my power supply is 5A. When I turn the color to white, everything is fine. But why? \$\endgroup\$ – Nauris Grinbergs Apr 4 at 12:40
  • \$\begingroup\$ See the update. \$\endgroup\$ – Transistor Apr 4 at 13:02
  • \$\begingroup\$ Okay, this is a very good answer, now it's clear for me, thank youuu! :) \$\endgroup\$ – Nauris Grinbergs Apr 4 at 13:36

It is drawing less than the paper 6 amps for several reasons.

  • voltage drop down the long strip. Those internal pathways are PCB traces, not superconductors.
  • cheap items of foreign manufacture are not running 100% to spec.
  • actually, LED strips are sized to work safely on 14 volts, i.e. Automobile battery float voltage. So you are not running them at full spec.
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