Not All USB Power Is Created Equal 240
jfruh writes "We've reached a point in our electronic lives where most of our gadgets draw power from a USB cable, and we have lots of USB ports to choose from — some of which live on other gadgets, some of which live on adapters that plug into your wall or car. But those ports supply wildly varying amounts of power, which can result in hours of difference in how long it takes your phone to charge. The Practical Meter, the product of a successful Kickstarter campaign, can help you figure out which power sources are going to juice up your gadgets the fastest."
Or use what already exists (Score:5, Insightful)
Re:Or use what already exists (Score:5, Informative)
Re:Or use what already exists (Score:5, Informative)
Or one of these (it also passes through USB 3.0, which is nice):
http://www.amazon.com/Centech-USB-Power-Meter/dp/B00DAR4ITE [amazon.com]
This isn't new.
Re: Or use what already exists (Score:4, Insightful)
But that company isn't paying for a slashvertisement.
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Re:Or use what already exists (Score:5, Insightful)
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If someone has some money they want to waste on a device that only has one purpose, I won't argue with them stimulating the economy. Bu
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Turn in your Geek card. That will measure Current draw by the device, NOT how much current the device can deliver.
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Very nice I may need one. Have you heard anything about how well USB Wall outlets work like these?
http://www.homedepot.com/p/U-Socket-15-Amp-AC-Standard-Duplex-Wall-Outlet-White-with-Built-in-USB-Charger-Ports-ACE-7169/203423602#.UozT2EAx7vQ [homedepot.com]
I've been considering getting a few.
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Those are utter crap. I have bought and returned two.
http://www.smarthome.com/21680WH/Cooper-Wiring-TR7745-W-Dual-USB-Charger-with-15A-Duplex-Tamper-Resistant-Receptacle-White/p.aspx [smarthome.com]
This one however works flawlessly with an iphone, ipad, nexus7 and a Nexus 4.
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I looked at that model at home depot but it was going for about $14 more than at smarthome. I think I'll be purchasing from there instead, thanks for the link.
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One issue with this (and many other power meters) is - What is the burden voltage of the ammeter? e.g. how much voltage does it drop.
Meters can often have a burden voltage of 0.1-0.2 volts when measuring currents on the order of an ampere. This might not seem like much, but considering that the original (2012) Nexus 7 drops charge current by approximately 200 mA for every 0.1 volt drop below 5.0 volts - http://forum.xda-developers.com/showthread.php?t=2065404 [xda-developers.com] - 0.2 volts can drop your power by 400 mA (aro
Re:Or use what already exists (Score:5, Funny)
business plan generator for 2013:
1. browse dx, find a neat product, that seems practical while being utterly useless for your usual hipster, preferably with nice lights.
2. ask around your hipster friends if they've heard of such a product. if they have go back to 1.
3. if not, then track down the manufacturer industry guy from china.
4. put the fucking thing on kickstarter with reduced features to put down cost(come on, going from a segment display to fucking FOur FUckING LEDS!!).
5. have them dropship the product from said china guys warehouse
6. you're in the moneeeyy you're in the moneeeeyyyy.
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Cheaper alternative ($6) (Score:2)
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Since I already have a nice multimeter and don't see the need to waste money on yet another device that does exactly the same thing, I'd rather have a simple test lead adapter that lets me plug the multimeter into a USB ports instead of having to gingerly stick probes in there. If no one was entrepreneurial enough to address that need instead of trying to force people to buy another duplicate device, then I guess I'll have to make one myself.
kickstarter link (Score:2, Informative)
http://www.kickstarter.com/projects/david-toledo/the-practical-meter-know-your-power
External DVD drives (Score:2)
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I've seen that before - USB to 5V direct. Someone vandalized a mouse, tearing the cable apart. It was smoldering and melting a hole in the keyboard when I found it.
Re:External DVD drives (Score:5, Insightful)
In what way is it "good and practical" to ignore a standard, possibly damaging electronics which assume the standard by providing a variable non-guaranteed maximum current? At worst this is a fire hazard, as you'd end up delivering an unreasonably high current. If the device isn't intelligent enough to ask for the right current, it should be delivered a safe trickle - as the USB standard asks.
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In what way is it "good and practical" to ignore a standard
For starters, it allows me to burn and read optical discs. Most external DVD drives want to suck all the power through USB and, while they might have a separate DC power connector too, finding just the right power supply is a pain in the ass. If everything was done properly, every external DVD drive would ship a discrete AC/DC power supply, because the USB spec does not actually allow delivering these crazy, over 1A currents which is needed. But what can you do...
possibly damaging electronics which assume the standard by providing a variable non-guaranteed maximum current?
You are of course correct. As a comment abov [slashdot.org]
Re: External DVD drives (Score:5, Insightful)
A USB port should react gracefully to a defective USB device - either limiting current or cutting power if draw is too high. It should not give the malfunctioning device the opportunity to catch fire by delivering it as much current as possible.
Liberal in what you accept; conservative in what you send.
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It's not the job of the power supply to protect malfunctioning devices from themselves. It's the job of the devices to have fuses, regulators, etc. This has been true, forever.
Your wall outlets will provided whatever is requested. Your car's alternator and battery will do the same. 12V devices plugged into a cig plug will get whatever they want, if they were designed so poorly as to not have a dammed fuse.
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Yeah, but the power supply should protect itself. If the damn thing is connected directly to the laptops 5V rail it's possible to make the laptops 5V drop by plugging in a defective decvice. That in turn means all kinds of weird shit might happen, including data corruption and hardware deaths.
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No your wall outlets provide up to what the wire running to them will support past that and the breaker trips or the fuse pops (granted it's often much after but still generally safe for the wire). Newer homes have arc faults in a lot of places that protect from even more.
Same thing for your car every branch circuit has a fuse of a given rating try getting more than 15 amps through your 12v accessory outlet.
Pretty much the rule is you can not step down the permanent wire size without an over current protec
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"Same thing for your car every branch circuit has a fuse of a given rating try getting more than 15 amps through your 12v accessory outlet."
Wonder what my 500w Inverter pushing dual 250w Altec Lansings would have to say to that, considering in my car the 12V accessory has a 40 amp buss fuse (the original one that came with the car when it was made in 1998, at that.)
Oh, look, my 90's S-10 has a 40-Amp fuse. again, the original one.
Well, hey! My 87 Tercel has a 35A, but that's still better than your 15.
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Are those modern accessory outlets or old style cigarette lighter outlets?
Point still stands they are fused to protect the wiring as designed.
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Needless to say, I don't recommend alienware to anyone anymore.
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Post flagged for being so hilarious I wet myself. In future, only use your powers for good, AC.
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Straight copper wire often reacts badly (in ours and the fire department's humble opinions) to too much current. In what way is it poorly designed or defective?
Or is it the design issue? Using copper wire where a resistive element would have been a better choice? Or perhaps connecting that copper wire to some more appropriate load? Maybe forgetting to include a much smaller wire (fuse) in the circuit?
Sorry, still getting enough caffeine in me to build immunity to the obvious. Devices don't react badly t
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Any device that reacts badly to too much current is poorly designed or defective.
Yes, and if I own such a defective device, I certainly don't want it starting a fire. The thing about electronics is that most people could own a defective component and not know until it's caused some other problem.
Electronic devices can have latent defects: poor insulation that's barely adequate to protect the device through testing, shipping, and installation, but as you use the device and move a wire repeatedly, the gap in the insulation causes a short. Should the purchaser be satisfied simply by know
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If you tie the 5V from the power supply straight into the USB ports, there is zero chance of preventing a fire. My 850W power supply is designed specifically to pump 30A into the cabinet on the 5V line. The power supply has no way of knowing if it's pushing that current into a hungry CPU or into a shorted out mouse wire.
It doesn't matter if a USB device is badly designed. There are several places that all need to implement safety measures. USB specifications require that a USB host limit the power to pr
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Any device that reacts badly to too much current is poorly designed or defective. That's therese electricity works, you're half amp light bulb doesn't explode when there's nothing else in the house because there's too many amps available. A device that issues is most likely already shorted out.
Safety demands that you *never* depend on the unknown thing to be properly designed or to function properly if you can help it. If you are providing power to something, you need to limit their power draw *somehow* to keep a malfunction from causing damage. It is why fuses and circuit breakers where invented and why modern power supplies will shut down if they are asked to supply more than their rated current/power. Any device that risks permanent damage because some other product draws too much current, e
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Then there's the another category which try to limit the power and have a polyfuse or something more smarter in place.
ALL USB sockets are required to have a polyfuse, it's part of the specification.
Whether they're 500mA or 2A or whatever is another matter, but they have to have them.
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Don't really see the market (Score:3, Insightful)
At work I could plug my phone into the computer or... Buy a second plug, that seams a bit pointless even it if it does knock and hour of the phones charging time.
At home I do have choice, but why would I really worry as each night it gets charged and has all night. So again an hour does not really matter.
What am I missing from this?
Re:Don't really see the market (Score:5, Informative)
What am I missing from this?
One example: my Nexus 7 draws so much power, even when sleeping, that it is possible to connect it to a weakly charging USB port, come back a few hours later, and it has a lower charge level. I'm sure the same is true for other tablets, and possibly even some phones.
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One example: my Nexus 7 draws so much power, even when sleeping, that it is possible to connect it to a weakly charging USB port, come back a few hours later, and it has a lower charge level. I'm sure the same is true for other tablets, and possibly even some phones.
Interesting. My wife has a Nexus 7 (2012 edition). It charges just fine (albeit relatively slowly) from 500mA USB chargers. It charges faster with the 2A charger that comes with it, but I've never had issues with it losing charge while plugged in to a standard charger.
How weak is your "weakly charging" USB port? Is it one on a keyboard or some other low-power accessory, or is it a port on the computer itself?
Re:Don't really see the market (Score:5, Informative)
One example: my Nexus 7 draws so much power, even when sleeping, that it is possible to connect it to a weakly charging USB port, come back a few hours later, and it has a lower charge level. I'm sure the same is true for other tablets, and possibly even some phones.
Interesting. My wife has a Nexus 7 (2012 edition). It charges just fine (albeit relatively slowly) from 500mA USB chargers. It charges faster with the 2A charger that comes with it, but I've never had issues with it losing charge while plugged in to a standard charger.
How weak is your "weakly charging" USB port? Is it one on a keyboard or some other low-power accessory, or is it a port on the computer itself?
The Color and Tablet Nook devices have two different charge rates. If you use the official "USB" cable with the LED indicator in it, it charges at a 1A (2A?) rate. If you use a stock micro USB cable, it charges at the official 500ma rate. The decision is made by the Nook itself, based on info from extra pins that are in the custom cable.
Which (blankety-blank-censored-blank) is no longer available. And since the cables are no longer made or sold and since they were notoriously prone to fail means that I've been trickle-charging my unit for about a year now.
Moral of story: always check new toys for screwball cables before buying.
Custom cables are almost always unnecessary (Score:4, Informative)
Which (blankety-blank-censored-blank) is no longer available. And since the cables are no longer made or sold and since they were notoriously prone to fail means that I've been trickle-charging my unit for about a year now.
Exactly why I avoid devices with weird custom cables whenever possible in consumer electronics. It's been my experience that unless a custom cable is so popular as to become a standard itself (like Apple's Lightning) that eventually you are going to run into a problem. Furthermore it adds to the cost of the device (custom cables = $) and it usually means that the company making the device had lazy and/or incompetent engineers. Now admittedly the USB spec is pretty flawed, particularly when it comes to power, but even so I've still seen lots of devices that could have used standard USB (or Firewire etc) had they taken the time to do so.
Now sometimes the standard needs to be updated. I think USB should be beefed up to handle up to 100 watts [computerworld.com] with all due haste.
Bear in mind that my day job is to run a company that makes custom cables. Think about that. I make a living off of custom cables, have the ability and equipment to make a copy of pretty much any cable, and I still think they are a bad idea for most consumer electronics.
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Interesting. Back in July they were talking like a special production run would be made (no cables in inventory for months), but come July, nothing. Apparently they did it and didn't say anything - I had a pre-order in for all the good it did.
If you plug in a stock micro USB connector, even if it's jacked into a 2A power converter, the Nook will say "Not Charging". But come back in an hour or 2 and it will be back up to full power.
I don't think the trickle rate's enough to charge it while in use - especiall
Re:Don't really see the market (Score:5, Informative)
The USB spec - ya know, that thing that every device carrying the USB logo is supposed to follow - permits a connected device to draw a maximum of 100 mA until it is properly recognized (enumerated) by the host. This is probably what the GP is referring to: 0.5 W of available power (less after conversion efficiency) isn't a whole lot for a device like a Nexus 7.
After being enumerated, the connected device can request higher current levels, up to 500 mA max. It isn't supposed to draw more unless the host permits it. For many modern portable electronics (e.g., smartphones) that have a 3-10 Whr battery, a 2.5-W maximum charge rate isn't much.
There are amendments to the spec that allow for greater power: in 2009, the spec created a Charging Downstream Port, which allows for up to 1.5 A from the host after enumeration; and the Dedicated Charging Port (DCP), which shorts the two data lines together and allows for 1.5 A charge power without enumeration.
Individual companies, such as Apple and Samsung, supply their own USB chargers that allow for even greater charge current, but do so in a way that technically violates the USB spec.
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This device seems to be able to bypass the "enumeration" thing and allow them to charge at max rate It also acts as a firewall to keep the USB port from downloading data from your device.
http://www.kickstarter.com/projects/1137339450/lockedusb-adapter-usb-charger-firewall-and-power-o [kickstarter.com]
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This doesn't add up. The Nexus 7 has a 16Wh battery. The worst USB port possible (100mA) provides half a watt. Either a) your N7, rated for 10h active / 300h standby, is guzzling so fast that it would deplete a full battery in 32h of all-standby, or b) the USB port is defective.
Re:Don't really see the market (Score:5, Informative)
Your Nexus 7 has runaway background processes. Otherwise there's no way it draws more than 500mA in standby - it would be empty after just a few hours. Check your battery stats to find the culprit...
My girlfriend's Nexus 7 charges just fine off of good old 500mA USB2.0 ports when it's in standby...
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There is a known issue with some Android devices where they don't go into sleep mode and will discharge rapidly even when connected to a charger.
I had this with my Nexus 4 but it's been reported in many different devices.
Lots of random advice in forums on how to fix it but the problem still seems to exist.
http://androidforums.com/samsung-galaxy-s4-active/733289-s4-active-wont-go-in-sleep-mode-when-charging.html [androidforums.com]
http://forums.androidcentral.com/google-nexus-7-tablet-2013/314761-battery-drain-sleep-mode.html [androidcentral.com]
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What am I missing from this?
One example: my Nexus 7 draws so much power, even when sleeping, that it is possible to connect it to a weakly charging USB port, come back a few hours later, and it has a lower charge level. I'm sure the same is true for other tablets, and possibly even some phones.
I used to have the same problem with an old PC too. If I plugged most devices into my main computer they charged just fine, I had a really shit old small form factor packard bell thing I used to leave always switched on as a router though and if I plugged anything into that to charge via USB it ended up actually drawing power out of the device instead. This was not due to the device though, anything I plugged in to charge did the same thing.
Weirdly though I could plug anything that needed power to run like
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A lot of the replies here are incredulous about Nexus 7 power.
My Nexus 7 2012 edition would charge up, even if the screen and wifi was on, if left on a 500mA laptop USB port (usb debugging / storage enabled).
My Nexus 7 2013 edition would not charge up, even if the screen and wifi was off, if left on a 500mA laptop USB port (even with usb debugging / storage disabled). It would drain slowly. It required a 1A from a wall-wart to tread water with the screen on. It took a 2A wall-wart to actually charge up
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Exactly! For me the cable was the problem as well. The Nexus 7 2012 I mentioned comes with a wallsocket-to-usb charger, and a special USB cable. I managed to mislay that cable, and with most ordinary cables I had very long or even negative charging rates, even with the original charger. As far as I can tell the trick is to find a cable with low wire resistance, either because it is short, or because it has thick wires.
In any case, I think the whole discussion illustrates that some kind of measurement instru
Then turn it off while charging (Score:2)
You probably arent using it then anyway.
Bad phone design (Score:3)
One example: my Nexus 7 draws so much power, even when sleeping, that it is possible to connect it to a weakly charging USB port, come back a few hours later, and it has a lower charge level.
If you need a portable fusion reactor to power your tablet and/or phone *while it is sleeping* then there's something really wrong with your device.
(Bad hardware design, bad OS power management, or you installed too much background shit).
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What am I missing from this?
One example: my Nexus 7 draws so much power, even when sleeping...
OK, we can stop right here and determine what the hell the problem is...and it's occurring before you even find a power source.
Go fix or replace your tablet. One should not be trying to power a black hole.
I may have accidentally modded the above "redundant" by starting to mod it "informative" and then deciding to read the rest of the thread to see if I was going to want to comment instead of modding.
If so, my apologies to the PP
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Having a quick top-up before getting on the train home is rather useful...
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Your car charger would be a big one... Fire up Google Navigation for your weekend drive out-of-town, and connect it to your $2 cigarette lighter charger, and before you get to your destination, your phone shuts off because it has run out of juice. Other apps like the free MapQuest use less power and offer better routing, but even those commonly use more than the 500mAH basic old USB chargers can supply, so your battery will be drained rather than charge.
The second would be a li
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In all those cases, though, I read the specs on the charger, and/or observe how fast my phone is charging, and don't NEED a stupid meter to tell me what I'll find out in a few minutes.
In fact it doesn't MATTER how much power your charger can supply, if it isn't wired in a way your cell phone recognizes it, it won't TRY to draw that much power, anyhow. And Apple and Android were oh-so-nice to choose mutually-incompatible methods of signaling this, so one charger cannot work for both. Though expensive "char
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It's not sex... (Score:4, Insightful)
Being the fastest might not be the best for your battery life.
Faster isn't always better (Score:5, Interesting)
I believe most types of battery when charged faster actually degrade faster.
Life fast die fast ;-)
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The device should only draw as much power as it wants, which is controlled by the charging circuit in the phone, regardless of how much is available. Which raises the obvious question of how they're getting iPhones which want a 120-minute charge cycle, to charge in 90 minutes by using a special cable.
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Remember, power over USB is negotiated. It doesn't just supply 2A to each port. The USB 3 spec allows for up to 900mA, but will only provide 150mA if the device doesn't ask for more. There is a separate "charging" spec which supplies up to 5A, though it wasn't supported by motherboards until fairly recently.
The phones want a 90 minute charge time; it's just that they communicate their want for more power in a way that not all USB hosts understand -- often they use some non-standard way that only their power
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Smartass fast - run-on fast.
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Your threshold for what counts as stress + your tense overuse of italics suggest you may wish to consult your physician before you have a coronary.
Basically an Ammeter (Score:5, Informative)
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And a bad one, too, with a terrible resolution. Just get an amp meter and a cable, that should be under $10 together.
Typical slashvertising. I miss the good old days.
Re:Basically an Ammeter (Score:4, Funny)
Typical slashvertising
Impossible, I disabled ads thanks to
As our way of thanking you for your positive contributions to Slashdot, you are eligible to disable advertising.
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That won't work as USB amperage is based on resister sensing.
won't help for Samsung note 2 (Score:4, Interesting)
Samsung seems to measure the "reliability" of the supply or the cable, and limits power based on those values. Then the same supply will charge at different rates depending on the cable used.
Re:won't help for Samsung note 2 (Score:5, Informative)
Indeed they do. If the charger says "I can supply 1.5A" but due to thin wires in the long, cheap cable that results in a significant voltage drop the device backs down to a lower level.
Re:won't help for Samsung note 2 (Score:5, Funny)
This is why I build my own USB leads using #00 welding cable.
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I don't think it has to do with voltage drop. A cable usually will not show a voltage drop ~0 resistance for 2m of cable. And, same cable will charge other devices without any problem. A cable might burn out with too much current. Maybe that's the worry? I haven't really heard of that happening....
Even the originally supplied cable is rejected sometimes. And, if you get a bad charging rate, just unplug and replug the cable for another roll of the dice.
No, I think it is a simple bug in the charging control i
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Voltage loss over such a cable is very real. At 0.14 mm^2 (AWG 26) you get 0.14 mOhm/m. For a 2m cable, 2 wires you end up with 0.56 Ohm. At 1.5 A that's a voltage drop of 0.84V.
Also note that such a cable is only rated for 0.36 A
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Voltage loss over such a cable is very real. At 0.14 mm^2 (AWG 26) you get 0.14 mOhm/m. For a 2m cable, 2 wires you end up with 0.56 Ohm. At 1.5 A that's a voltage drop of 0.84V.
A factor of 1000 snuck out on you here. You went from milliohms to ohms in a second.
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No no, resistance has nothing to do with it. The USB charger's output is current limited, which means that if the device tries to pull more current than the charger is programmed for, the charger's output voltage will drop. That is, the output will be limited to (for example) 500mA or 1A or 1.5A or whatever and if the device tries to pull more the output will STILL be limited to that value and the voltage will drop from 5V to compensate.
Smart devices can usually ignore the USB resistor-based programming p
Isn't there a spec on how much power ... (Score:2)
Also, don't USB devices usually have to request the high power mode explicitly? Some USB power supplies are "dumb" and only supply power, but don't speak USB. Some devices are curteous enough not to draw 500mA if they haven't received permission from the USB host to do so. In this case, they'll slowly load with 100mA ...
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... USB ports have to supply (and USB devices may draw)? Drawing more power is outside the spec.
Also, don't USB devices usually have to request the high power mode explicitly? Some USB power supplies are "dumb" and only supply power, but don't speak USB. Some devices are curteous enough not to draw 500mA if they haven't received permission from the USB host to do so. In this case, they'll slowly load with 100mA ...
There are several generations of specs. The original was hard-wired for 500ma max. Later versions can negotiate.
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Which manufacturers blissfully ignored. Apple with there secret handshakes. Nearly all of the USB power specs required to much intelligence at the charger end thus cost. The latest is rather fun supping up to 100w. The previous supplying 1.5a assumed simply by shorting the data lines together.
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Unfortunately, that method is not reliable, because every manufacturer implements it differently - it is not a part of the USB spec.
Following the spec, the only way to know the current rating is to either negotiate with the host (not usually possible for a standalone, "dumb" charger) or to have the dat
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Unfortunately, that method is not reliable, because every manufacturer implements it differently - it is not a part of the USB spec.
And you have hit the crux of this issue. There is no standard for this so manufacturers have implemented *dumb* ways around this which are incompatible.
Personally, I think there should be to additions to the standard that include the "dumb" option for devices needing up to 1 Amp. If the hub/charger can do 1 amp, hold the pins the right way and supply 1 amp. If the device requires more than 1 amp, then make it mandatory for the device to request what it wants using the USB data bus and the source device c
Charging spec compatibility? (Score:2)
Different devices have subtly different ways of asking how much currently they can draw. Your iDevices and Androids and whatever are fairly interoperable with each other's chargers now, but there's still the occasional stupid outlier like the PS Vita that insists on having a specific shorting of the USB pins before it'll draw more than 500mA. I'd like to see a gizmo that could not only measure the current available, but act as a universal adaptor for those sorts of devices.
Change the spec (Score:2)
I'd like to see a gizmo that could not only measure the current available, but act as a universal adaptor for those sorts of devices.
The answer is not a different gizmo to work around existing limitations in the spec but updating the USB spec to reflect real world conditions and handle more power and handle power more gracefully than it does now. There is some evidence [computerworld.com] that this might occur in the near future.
Apple vs. Other Devices (Score:4, Insightful)
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the properiaty stuff is in the cable, it should charge. maybe it expects the datalines to be in other state though than they are on your charger.
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I have a multitude of Apple and Android devices and a multitude of Apple and non-Apple usb chargers and power sources, and also have a little non-Apple dongle charger in the car in the front.
It all works just fine. Apple might have weird charging cables but they still have usb on one end and they've worked with the dozen or so different usb charging sources I've got in the house and in the car.
Most USB chargers only throw out 2-5W. It's gonna take a while to charge-up Apple's big batteries with one of tho
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I also have a 1.5A cigarette light.. errrr "Power Port" charger that will charge my gt-i9300 S3 just fine but will NOT charge my wife's international Note II. It will for a short while if she's using it, then the phone will start making the charger connected sound over and over. The only way for it to charge her phone is to turn the phone off. The Note II just pulls too much power with the screen, battery etc and seems to overheat the charger. (Not that I'd expect this to happen with an iPhone but thought I
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What's happening there is that the phone is crow-baring the charger down to under 2V and then mis-interpreting its own voltage sensor (on the phone) thinking that the charger is connecting and disconnecting. Might also be tripping the short-circuit detector on the charger and causing it to cycle.
That's a problem with the phone. No phone should crow-bar the 5V the USB outlet provides, particularly not one that outputs >1A. (This is using your description of the charger being hot). All USB power sources
Use a simple voltmeter (Score:2)
This is /. after all. http://www.accesscomms.com.au/reference/usb.htm [accesscomms.com.au]
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That isn't going to tell you how much power that port is capable of outputting or will output.
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The idea is good but there are better solutions, as indicated by other posters.
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An volt meter is just an amp meter that has a resister in series with it..
Also, it might be better to use a volt meter and a very small shut resister to measure a current in a circuit you don't want to disrupt. Such a scheme could be less disruptive to the circuit. Having all the current flow though your meter and the leads to/from the meter is more likely to be disruptive.
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Perhaps AC has a spare voltmeter and was planning to add a shunt resistance in parallel?
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Two-prong chargers just synthesize a ground. Well, there are two grounds actually... one for the AC side and one for the output side. On the AC side its the average of the AC sine wave between the two prongs taken off the middle of the rectifier. The charger's output is traditionally isolated with a transformer (a small transformer on the output of the switching power supply instead of a big one on the input), which isolates the output and also handles the flyback function for the switching power supply.