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I have a very simple circuit for a small RGB lamp I'm building. My plan was to power it off one of my many spare old USB phone chargers via micro-usb. It's just a string of 60 WS2812B LEDs with a NodeMCU as controller. I'm using a capacitor to smooth out the input power and resistor on the data line. In theory, the max draw from the LEDs is about 3.6A and the controller is somewhere around 200mA. With a 5V 3A phone charger, that should give me an acceptable amount of illumination from the LEDs while still suppling the controller enough power.

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The issue I'm having is that my circuit is only getting around 350mA total. That's enough to run the controller and dimly light all the LEDs or have a few lit brightly. The brighter I set the LEDs, the more the voltage drops until the controller can't run. I've tried different chargers and different USB cables, but I seem to never get more than about 350-400mA. The circuit's really simple, so I'm guessing that's not the issue (showing it here in case I'm wrong about that). Is this an issue with the chargers limiting the current they send out? I know there are different ways the chargers 'advertise' their supply options, but I thought that was all on the supply side, not the draw side. Do I need to do something with the D+/D- pins to get the chargers to supply more current?

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  • \$\begingroup\$ Wouldn't the WS series LEDs be PWM controlled and the pulse current would be still 3.6 A (even at 10% pulse width) and may cause the PSU to go into current limit? \$\endgroup\$
    – Transistor
    Sep 22, 2021 at 20:58
  • \$\begingroup\$ @Transistor I'm not really sure what you mean, but I tried dropping the number of LEDs to 30, which should have a max draw of 1.8A, which is well within the abilities of the charger and the circuit is still only getting about 400mA. \$\endgroup\$ Sep 22, 2021 at 22:23
  • \$\begingroup\$ Sorry, then I don't know. \$\endgroup\$
    – Transistor
    Sep 22, 2021 at 22:28
  • \$\begingroup\$ Why don't you test ability of your power source with a simple massive load resistor as a start? \$\endgroup\$ Sep 23, 2021 at 16:59
  • \$\begingroup\$ I checked my circuit on a bench power supply and it pulled 2A, so I've now confirmed it's the USB chargers that are the issue. \$\endgroup\$ Sep 24, 2021 at 3:00

2 Answers 2

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Under the assumption that the +5V line is coming straight out of the USB jack and nothing else is connected to this particular USB cable which connects to the charger: Power-negotiation may be a problem here. This means: The charger has somewhat like 500 mA as default limit and waits for a controller to signal a higher current draw. Use a 'as dumb as possible' charger to verify - but your chances are low, because these features are quested by the usb standard. At least afaik.

Furthermore, I would not mess with the D+/D- lines and USB at all - just use a fixed 5V wall adapater - all problems solved!

But, if you insist on using a Phone-Charger please have a look at this topic: What is the ideal way to handle data pins D+ and D- on a USB power adapter to be compatible with fast charging on devices? As you read you will see why 'messing with this stuff' is not worth the time.

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  • \$\begingroup\$ Your assumption about the +5V line coming straight from the USB is correct. As to the link, I read that before asking my question. Unfortunately, the answers seem to all be talking about building a charger capable of notifying a USB device that it can draw more power, not about building a USB device that draws more power. Did I miss something there? \$\endgroup\$ Sep 22, 2021 at 22:12
  • \$\begingroup\$ Do the reverse thinking - I have only flown over it, but there might be information related to the device side too. But again: Save yourself the trouble and invest 10 bucks. digikey.de/product-detail/de/ideal-power-ltd/15DYS818-050300W-1/… \$\endgroup\$ Sep 23, 2021 at 2:40
  • \$\begingroup\$ Yeah, I ordered a 6A power supply a couple hours ago. It's a shame though. There are 4 adults in my house who have all gone through multiple phones. We have so many of those chargers all over the place. I was really hoping to use them for projects, but I guess they're only good for very low power. \$\endgroup\$ Sep 23, 2021 at 4:23
  • \$\begingroup\$ Glad to hear. Yeah, USB-Power Delivery got messy with time. Tough it is extremly powerfull. Think about the new standards having 10Gbit/s and 100W Power. You can do amazing stuff with this! Will, if you ask me, in some time replace PCIe and other internal systems busses to allow for distributed systems in smal setups! But way to go. \$\endgroup\$ Sep 23, 2021 at 6:58
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After much debugging and ordering a 6A power supply I solved it. There are actually two different things going on here. The first is that my USB chargers don't want to deliver much power. My new 6A power supply solved that issue, but my circuit was still only pulling around 400mA. When connected to my bench power supply it pulled 2A, so that wasn't what I expected. I realized I had wired up the power a little different on the bench. Not connecting things differently, just in different places on the breadboard.

That was it. The breadboard was providing enough resistance to prevent the circuit from getting enough power. After shortening my connections as much as possible, the circuit was pulling 2A from my 6A supply. I've never dealt with more than a few mA on my own circuits before, so I didn't even think about resistance of the breadboard.

Out of curiosity, I wired up a 2A USB with short connections. It delivered 800mA (twice as much as it was), which isn't enough for this circuit, but it gives me hope that I'll find uses for all these chargers I have lying around.

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  • \$\begingroup\$ Glad to hear you solved the problem! Please keep enjoying electronics! \$\endgroup\$ Sep 26, 2021 at 0:42
  • \$\begingroup\$ USB-PD devices have built in voltage and current control. Since you aren't requesting a higher voltage/current your power is limited to USB defaults. You can buy USB-PD "trigger" boards and adapters for a few dollars online (or make your own). These will let you set the voltage and current to whatever you want (at least assuming the power supply can provide them). \$\endgroup\$ Sep 26, 2021 at 0:52
  • \$\begingroup\$ I've looked around quite a bit for specs for micro usb power delivery or a pre-built power delivery board but all I could find was usb-pd for usb-c. Do you have a link to something for micro-usb? \$\endgroup\$ Sep 26, 2021 at 21:02

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