USB power - a question

[teckneeculler]

What's the power output of a computer's USB port? I've just realized that I have no idea, despite using these things for umpteen years.
Whatever it is, I assume that it must be the same as the output from the little USB adapters that plug into household power sockets?

 

[file3456]

It's 5 volts and 500 mA (Milliamps) for a computer. That amp part (current) is typical for computers in that it's 500 mA. So that's why you may need a good, reputable USB powered hub to provide more USB ports allotting more Amps.

That's on the output side of things. On the input for a powered USB hub, that wall wart adapter could be consuming 2 Amps! In the U.S. we use 120 volts. So the wattage formula is volts * Amps = watts. 120 * 2 is 240 watts. So that powered USB hub could be consuming the equivalent of two 100 watt light bulbs and a 40 watt light bulb. Just keep that in mind because if you have lots of electronics on the same circuit and a space heater you'll come close to popping the breaker since they are mostly rated at 15 Amps per room in the U.S. Some as little as 10 Amps before the breaker trips.

Moving along. The USB wall wart adapters for phones, tablets and lots of of consumer electronics output 5 volts, but depending on the needs of the device being charged have a certain Amp output. Usually around 2 Amps on the output. Enough to charge a tablet. You can see the input and output current and voltage on the adapter its self.

Keep in mind that you don't want to charge a phone or what ever with a cheap USB cable or an under powered USB wall adapter. It'll first cause the adapter to get very warm since the device is pulling all the current possible, and it could damage your device. You just want to make sure you have more than enough Amps on the output side.

They sell small battery charges for USB devices. I have a couple. My Tripp-Lite is rated at over 10,000 mAH (10AH Amp Hours). Looks kinda like this. Not the same model. Which means I can pull an Amp per hour for 10 hours. So I should be able to charge my phone at least 10 times. But that's just approximate.

In Windows XP you could see how much current the USB bus was currently consuming in Amps by going to the device manager and into the USB devices. Doesn't appear Windows 7 has that.

You might be interested in my video here:
How to tell if a USB phone cable is genuine - YouTube

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An analogy for Amps and Voltage is this:

Think of electricity like a river. The amount of water flow is like Amps. The pressure of that water is like Voltage.

So for Amps it's a Coulomb per second or 1 Amp per second which is equal to the charge of 6.241 x 10[SUP]18[/SUP] protons. LOL For water your flow rate (amount per time) would be like gallons per hour. Or gallons per minute. Or Kilograms per hour. Or even cubic feet per hour. You get the point that Amps are a flow measured with time, a "rate."

You have a rate of possible delivery and a rate of consumption.

Even applies to economics and the freaking post office! LOL

 

[teckneeculler]

That's interesting, thanks.
One of the reasons I asked is I have an oldish Samsung Galaxy Tab S 10.5 that I use so infrequently (to watch movies in bed) that I always have to search around for a charging cable. Then I eventually remember that I just need to plug it into one of the output ports on my Asus monitor with a regular USB A to C cable.
Confusingly for me, this charging operation works whether the Galaxy is on or off (tho when it's off is a way faster charge) but if it's switched on I can also transfer movies to the tablet, or, flowing in the other direction, watch its movie content on the Asus monitor.

 

[MisterEd]

You are talking about two different things. The USB ports on a computer are all 5 volts. Their output current depends upon what standard they are. They can USB 2.0, USB 3.0, or USB-C.

USB chargers can vary both the voltage and current depending upon their capability and that of the battery they are charging. For example the charger for my Samsung S21 phone can rapid charge it at 2.6 amps at 9.3 volts for a total of 25 watts. I have seen other chargers with capabilities of 700ma, 1000ma, 1500ma, 2000ma, or 3000mA. Note that both the charger and battery being charged have to support rapid charging to take advantage of the extra charging current available. For example, one time after a power failure I used my laptop to charge my phone. The phone was probably charged at only 500mA because the laptop wasn't capable of rapid charging.



USB Charging and Power Delivery
USB Charging and Power Delivery | Tripp Lite

 

[teckneeculler]

Okay, I think I'm beginning to get it. I can charge my Samsung tablet from my computer using a USB A-C cable.
If I use a USB 2.0 port it'll charge at 2.5W, a USB 3.0\3.1 port will do it at 4.5W.
I also have a couple of 'wall warts' (hate that name) that I guess might do the charging, too.
One is a Sony - 5V x 850mA, the other an Apple with a gutsier 5.2V x 2.4A (12.48W).
Is there an upper limit to the wattage that the Samsung tablet would accept? The manual says nothing about that.

 

[file3456]

Electronics will typically draw the current needed for charging or consumption while in use. So look on your tablet. What is the input charging current rating? if it's 2 Amps (2000 mA) then you should use a charger capable of that amperage on the output or more. Again, it'll say on the charger.

In the U.S. you'll want chargers and what have you to be UL listed. This is the Underwriter Laboratories. If companies (if they care and are willing to spend the money) get their products UL tested to meet certain criteria for safety, then they can (lawfully) place the coveted UL sticker or marking on the device. Sometimes the UL label is holographic to help prevent a forgery. But I wouldn't trust that label unless you know for sure. China likes to fake those. That's where Customs and the FTC step in.

So having said that, be leary of buying consumer electronics not UL listed. Not necessary a sign of a bad or unsafe product, but when the company does go the extra mile to apply for being UL listed tells you a little about the company and whether the device meets certain safety standards. You should be able to look up a product at UL's website, but I think they charge for the Info. The other Cert is NEMA. But that's more for battery backups, surge protectors and what have you. I'd gone so far as to look up a company's ISO certifications if they have any. Even for companies in China which is where 99% of all consumer electronics come from.

If you shop for electronics on Amazon, finding UL listed products are like finding a needle in a haystack. I often times have to use the Q/A and see. Then I have to trust the validity of the response. And Amazon reviews as well as other revenue, I mean, review sites can't be trusted. I learned this from the investigations and reputation management firm KwikChex. Well, I always suspected the marketing and capital growth capability of reviews to be of shenanigans anyway. (And that Wikipedia page needs some editing. LOL)