Can't get full power out of my 5V USB power supply - Page 1
Forum section: electronics / projects

author: H-Milch
author_rank: Newbie
timestamp: March 09, 2017, 04:19:58 pm
content: So I'm trying to power a 12V LCD driver (needs about 12W or 1A) from a USB power supply using a step up converter. The problem is I'm only getting about 180mA on the 12V side (which means the LCD driver is stuck in a boot-loop as long as it hasn't enough power), so the USB PSU isn't delivering all its power.Is there any magical way to trigger the PSU to deliver 2A? And yes, I already shorted the data pins.

author: Buriedcode
author_rank: Super Contributor
timestamp: March 09, 2017, 04:30:02 pm
content: Depends on this power supply you speak of. There are a vast number of incredibly cheap USB supplies out there, some are surprisingly good for the price, but some won't provide anywhere near the rated current without severe voltage sag or laughable ripple.Some maths:Say your boost is 80% efficiency.Your "LCD driver" draws.. 12*1 = 12W.The input would therefore be: 12W/0.8 = 15W.For a 5V input: 15W/5V = 3A.I don't think I've seen a 3A USB power supply. Try using a lab power supply first - or something that can supply >3A. What kind of boost converter are you using?If you're "only getting 180mA on the 12V side" then this is all your LCD driver is drawing. If you try to draw too much current from a boost converter, often its voltage will drop, so I suspect it is drawing 180mA, but at a lower output voltage.

author: GoneTomorrow
author_rank: Regular Contributor
timestamp: March 10, 2017, 07:42:56 am
content: Quote from: Buriedcode on March 09, 2017, 04:30:02 pmMy Nexus 5X came with a 3A USB charger (15W). It is type C though.

author: wraper
author_rank: Unknown
timestamp: March 10, 2017, 09:04:22 am
content: Quote from: H-Milch on March 09, 2017, 04:19:58 pmWhat is that PSU? If some sort of Chinese crap/counterfeit then likely it's ratings are completely fake.

author: NottheDan
author_rank: Frequent Contributor
timestamp: March 10, 2017, 11:31:54 am
content: Is the cable good. I've got a couple of cheap ones that you won't get more than 500mA through from a known good 2.5A max PSU. Also, does the PSU possibly expect something on the data lines to provide more than 500mA?

author: jeroen79
author_rank: Frequent Contributor
timestamp: March 10, 2017, 12:56:48 pm
content: You have too much variables at once:-Your 5V PSU-The converter-The displaydriverIf you want to test the PSU you should add a known load (a set of resistors will do) and see how well it handles it.Same for the other parts.Add a known supply and/or load and see how they behave.Once you have verified that they work well individually you can start testing them combined.

author: DBecker
author_rank: Frequent Contributor
timestamp: March 11, 2017, 11:54:25 pm
content: Testing the power supply is a good first start.USB power supplies indicate what current they can provide. The device doesn't inform the supply -- it just draws the current. (The USB host is supposed to read the options then configure the device, avoiding configurations that draw more than available. But that pretty much went by the wayside when USB started being used for charging rather than just for peripheral devices.)I have several varieties of USB voltage and current monitors. Some even report watts and watt-hours.I pair those meters with USB loads. The simple ones are two power resistors with a switch for about $1. They get dangerously, painfully hot. Plus you'll really want more than just two different loads. It's worth getting the $5 adjustable ones with a fan.

author: DaJMasta
author_rank: Super Contributor
timestamp: March 12, 2017, 02:35:16 am
content: Take a look at the USB power specs. I believe 1.1 and similar are only up to 500mA, then USB2.0 can be as high as .9A in normal situations and 1.5A in special downstream power situations. There are power adapters that provide more, but the only USB spec that supports a 3A current draw is 3.0 and above, and only when configured as such - your average port not configured for high power delivery will be much lower.

author: wraper
author_rank: Unknown
timestamp: March 13, 2017, 09:57:14 am
content: Quote from: DaJMasta on March 12, 2017, 02:35:16 amUSB version is completely irrelevant for chargers and phones/tablets they are made for.