Why am I not reading more power draw here?

Greetings all, I'm working with a strip of some WS2812B (120 LEDs) and have them connected to a benchtop power supply. I'm doing some simple animations and messing around with the FastLED library and have a question about current draw I'm hoping someone can help me understand.

I have a breadboard pot that controls brightness and when I have my strip set to all white, full brightness, at 5.0v the power supply (in constant voltage mode) is only showing 2.57 amps. Shouldn't this be closer to 7.2 amps? 120 LEDs * 60 mA = 7200 mA / 1000 = 7.2 amps.

I'm trying to figure out big a power supply I need once I disconnect it from the benchtop PSU and everything I've read seems to tell me that 60 WS2812Bs require 3.6 amps--which corresponds to my 7.2 amps for 120 LEDs yet the reading on the power supply is only 2.57 amps so I'm a bit confused.

I am not sure why you think that each LED uses 60 mA. The information that I have suggests 18 mA which gives a result much closer to what you measured.

Please provide a link to the datasheet that you are using.

vaj4088:
I am not sure why you think that each LED uses 60 mA. The information that I have suggests 18 mA which gives a result much closer to what you measured.

Please provide a link to the datasheet that you are using.

On the contrary! Please provide a link to the WS2812 datasheet that you are using. :roll_eyes:

Page 3 of your datasheet seems to have the parameter, labeled as "Low voltage output
current". It is a poor datasheet. It is not really current that is output outside the device, parameter is misspelled, symbol is misspelled, typical is not abbreviated correctly, I am still looking for condition "ROUT", etc.

Measurements are unfortunately better than this datasheet.

I will not provide a link. I am not asking any questions. I used bing.com for my search engine. I bet that Google would work also.

Well I think the general assumption (a terrible word, I know) is that each RGB channel is ~20mA (which that linked datasheet indicates on page 4) and to get white you need all three to be on and, at full brightness, you end up with 20mA*3 = 60mA per LED for full-brightness white?

vaj4088:
Page 3 of your datasheet seems to have the parameter, labelled as "Low voltage output current".

Yes, per LED; there are three LEDs. Total 55 mA.

When referring to WS2812s in discussions, I actually refer to this figure rather than 60 mA to be consistent with that specification.

Thanks for the information!

We still have no theory why the OP's strip draws less than half the expected value.

@OP, please post your code and schematic after reading the forum guide in the sticky post.

PaulRB:
We still have no theory why the OP’s strip draws less than half the expected value.

I think it is down to how this current is being measured. A lot of difficulty is due to the pulsed nature of the current. Even at full brightness they are not on all the time and that is what can cause errors.

Alternatively the current could be wrong because the strip is not wired up correctly with power going in at both ends. Even with that the resistance of the copper tracks Connecting them together could be limiting the current draw, and possibly affecting the colour and brightness.
The eyes are not good at picking these things up. Just experiment with 8 or so LEDs and you will see the numbers you are expecting, I know I did when I tried this.