" Since the designers of the Apple charger went to a great deal of effort to build a high quality charger, I conclude they must not consider voltage sag worth worrying about. Or, more interestingly, maybe they built this sag as a feature for some reason. In any case, the chargers lose points on this."
I'm not versed in the arcane arts of EE— could anyone give me a basic definition of voltage sag? And would there be any reason to build it as a feature in a charger?
Hi, the author here. I'm using voltage sag to describe how the voltage drops as the load increases. (This has nothing to do with the presence of a battery or not.) Chargers normally are designed to output a constant voltage until the charger hits maximum power. But with the iPhone charger, the voltage is 5.2 volts if there's no load, and steadily drops to 4.6 volts as the load increases. If a device is expecting 5 volts, this could cause problems. Also, charging times could increase since the voltage is lower.
Generally, if the voltage drops under load like this, it's a sign of a poorly designed charger that doesn't regulate well. But since Apple's charger appears otherwise well-designed, it's a bit of a puzzle. One possibility is that maybe the exact voltage input to the iPhone doesn't matter, and the designers knowing this didn't care about the voltage sag. Another possibility is for some unknown reason they deliberately designed the charger this way.
It's almost always a balance between cost of manufacture and performance. I would bet money it was not designed that way as a feature.
The interesting points to remember here are:
- USB specifies 5V, not the iPhone
- Voltage drop across the iPhone's internal regulators and charging circuits are probably less than 0.5V
- Most cell phone LiIon batteries are 3.7V
So, presumably they made a charger that allows droop to 4.6V because this is a voltage still capable of charging the battery, yet also allows a less expensive charger design.
That is pretty interesting. I guess that's still within spec for USB. The iPhone probably doesn't care as long as it's a few hundred mV above the battery voltage (which would max out at about 4.2V for a lithium ion cell). Also, it looks like the big drop occurs when you go over the 1A rating of the charger.
While it's an interesting measurement, I'm not sure how relevant the current sag is to overall quality. Devices really shouldn't be using the charger in the constant current range. I guess it does reflect the overall workmanship and thought put into the design.
Also, nice job doing those ripple measurements - that was interesting stuff! It's not super surprising that the cheap knockoffs fail miserably there. Probably cheap capacitors that are way too small.
The correct term is internal resistance, and you can calculate it easily using ohms laws. The internal resistance is easiest to think of as a series resistor in line with one of the output terminals.
Too bad the Amazon Kindle charger isn't in the mix, it is my favorite just because it is so tiny and relatively high output. Would have liked to see same analysis for it.
The Kindle charger is sold separately, and you could probably get a much cheaper one than the $20 they are selling for as it's a standard micro USB port. My paperwhite USB is the same as my Logitec mouse and Android phone. Makes charging much easier as I keep them all on the same desk as I work.
The chargers look like they'll work together. For example, the Kindle charger will make my pocket camera light up its charging light, but after a minute or so, it goes out, as if the charge has completed. In fact, though, the battery is still dead. The other combinations of cell phone / camera / Kindle chargers are similar.
I dunno, I mix iPhone, iPad, Kindle, Samsung (Nexus) and Griffin chargers all the time, bring only one of them (typically iPad) on the trips, and it works out completely fine. I think that Kindle and Griffin are the slowest though.
If you check the article you will see that every charger has an Amp rating (like: 2 Amperes). This has a direct impact on the charging rate (provided that the device can handle it). Charging a device at 5V@2A is twice as fast as charging it at 5V@1A. An USB 2.0 plug can usually provide 500mA (0.5A), so the USB charging is slower than using the plug.
In real life charging is also limited by the battery temperature for safety reasons. If the battery becomes too hot during charging (because of the environment temperature, poor ventilation or just because the device has a huge consumption while charging) the charging will stop and will start again when the battery cools down. This is why we do not see chargers with bigger Amp rating: the batteries are limited to a current that doesn't get them too hot.
I bought up a half dozen Touchpad chargers when they were being cleared out for $5 a piece. I did it mainly because they were the cheapest 2A adapters that work with anything I'd come across, but it's good to know they're efficient and well-built as well.
I did the same with Palm Pre/Pixi chargers. I'm disappointed they weren't included in these tests, but industrial design-wise, they are THE best I've seen. The same size as Apple iPhone chargers, but cylindrical with soft touch surface and folding prongs.
I loved the "Touchstone" inductive chargers. I bought a whole box of them and the backs that were supposed to go on Pre's (these have the receiver coil in them) just to use in projects.
After fiddling with those awful 30 pin apple connectors, they feel like pure sci-fi.
No, the current rating is the maximum output of the charger. It's up to connected device to draw as much current as it needs, rather than the charger pushing the current into the device.
Afaik it's not that clear cut. If some kind of fault occurs in the device causing it to draw more current than designed then the lower power charger could be safer assuming it has some kind of current limiter or fuse. Ie high power charger would happily give faulty device more current possibly causing more damage (heating up, exploding batteries etc), while a low power charger could prevent that by tripping a fuse or limiting current.
Of course I'm not sure if this is applicable to USB chargers which all are relatively low power.
No, but it is inefficient. A larger charger will have a larger 'dead load', the internal consumption of the charger. So if you plug in a very small consumer into a such a charger then percentage wise you're losing a lot of energy to heat.
Author here - it's a bit more complicated than that. For a linear power supply (old-fashioned wall wart), all the unused power is converted to heat, so you'd waste a lot of energy. But for switching power supplies (such as USB chargers), theoretically only the power that is needed is used, so the efficiency shouldn't depend much on the load. In practice, larger chargers might have better overall efficiency since they can implement better circuits (for space and price reasons). But larger chargers might not optimize as much for low loads. So it's hard to say offhand whether a large charger or small charger would be more efficient for a smaller load.
I did a quick test to see what really happens, plugging a Samsung phone into a iPhone charger and an iPad charger. In both cases, the charger used 3.0 watts of wall power. (The phone was turned off and charging since if it is turned on, the load fluctuates a whole lot as the phone does random things.) So my conclusion is that the size of the charger doesn't affect efficiency.
For the general case, all other parameters being equal (supply mode, quality and so on): bigger charger -> larger dead load.
An iPad charger is not a 'large' charger, it's a fairly small step up from an iPhone charger, since you are reporting 3.0 watts of 'wall power' but your multiplication of scope measured values does not correct for power factor you are likely off by quite a bit on both measurements.
GP mentioned a HP touchpad charger to charge a phone, I don't have a HP touch pad charger here but the specs are quite terrible [1], you'd have to measure with that specific charger to answer the specific question or you'd have to do a comparison of a large range of chargers with accurate measurement methodology in order to really answer the general question.
As it is your conclusion contradicts practical engineering and I'm afraid it will not hold up in a better test, which would be to try a number of switched mode supplies of various sizes designs with various loads. Plugging in one device and doing a hasty (wrong, ignoring phase shift) measurement does not warrant your conclusion.
To measure efficiency you're going to have to take the power factor[2] into account, this can be quite hard to do, and theoretical efficiency doesn't matter for a practical test (you're measuring, not theorizing).
The wave forms that switched mode chargers [3] output and consequently the kind of load they represent to the grid is so irregular that most non-caloric and power factor corrected measurements will give values that are not accurate. That noise that is present on the output wires will be to some extent visible on the input side.
A normal Watt meter will work best with transformer based supplies or resistive loads, accuracy for small switched mode loads will be anywhere from 'so so' to 'terrible' depending on the make and model power meter. Good brands (for instance Fluke) do most calculations right and will be able to deal with CFLs and other phase shifted loads, bad brands (I won't name them but they're killing it in the domestic watt meter department) will give wildly in-accurate results.
But even a quality meter like a Fluke will still have trouble with this kind of spiky load, especially if it is small.
It would probably be a good idea to (properly) describe your test rig along with the results it says:
"I measured the AC input voltage and current with an oscilloscope. The oscilloscope's math functions multiplied the voltage and current at each instant to compute the instantaneous power, and then computed the average power over time. For safety and to avoid vaporizing the oscilloscope I used an isolation transformer. My measurements are fairly close to Apple's[15], which is reassuring. "
But you can't really do it that way and get accurate results, instantaneous power draw using a switching supply changes several hundred thousand times per second and is likely phase-shifted so a simple multiplication is not going to work.
Accurately measuring (low) power draw from switched mode consumers is a really tricky problem, it's easy enough to read some numbers from a display but I can assure you that this is not a simple problem to work on if you want to get meaningful results.
Thank you for your detailed comment. I went to a great deal of effort in my article to measure the power consumption accurately, accounting for the power factor, but I left out most of the details since most people don't care. I'm not multiplying the average current and average voltage to compute watts, because that obviously would not work due to the power factor. Instead, I'm multiplying the instantaneous voltage and current 50,000 times a second and summing this up, which gives the actual power, corrected for the power factor. (While the internal current changes tens of thousands of times a second, the line current changes slowly due to the input filtering, so this is plenty of resolution. I'm using a Tektronix TDS5104B 1 GHz oscilloscope, so I have a pretty accurate view of the input voltage and current.)
The main sources of error in my measurements are the cheap isolation transformer (which causes a bit of line voltage distortion under load), the current sense resistor, the tolerances of the voltage divider resistors, and noise in the measurements. So I wouldn't claim these measurements to be better than 10%.
You can take a look at one of the oscilloscope power graphs at https://picasaweb.google.com/lh/photo/pbrO8BQz38kDo9xU5ejffd...
Yellow is the input voltage, and turquoise is the input current. The non-sinusoidal current shows the non-unity power factor. Note that there's no phase shift, but instead the current flow happens only at the voltage peaks (which is a consequence of the input diode bridge, not of the switching power supply per se.) At the bottom of the image is the instantaneous power, computed from the instantaneous voltage and current.
For the iPad vs iPod measurement above, I didn't have the oscilloscope handy so I used a Kill-A-Watt, which does in fact take the power factor into account.
Going back to your statement that "bigger charger -> larger dead load". By "dead load", do you mean the power consumption under no load, which I call "vampire power" in the article? This varies widely between chargers, having more to do with the design than the size of the charger. But in any case, this wasted power is pretty much irrelevant under load. For instance, 100 mW is a typical vampire power usage. So if a hypothetical larger charger has twice that wasted power, at a 3 watt load, this is only a 3% difference.
The peaks in your scope image clearly show a phase shift, which is kind of logical if you take into account the fact that the main component in a switched mode supply is a coil.
If you look a little more carefully at your scope trace you'll see the coils reactance at work in the lower trace, the peak is where the FET in the supply is closed and is drawing real power, the purple trace past the peak and beyond the 0 crossing is inverted and drops slowly back to 0 before the next peak hits. If you use the controls on your scope to zoom in on the bottom trace by increasing the vertical sensitivity you'll get a much better idea of what I'm getting at here. You'll see '0' voltage and yet current is still flowing.
You can't correct for power factor by simply increasing the resolution and averaging. The base frequency of your oscilloscope does not enter into the discussion here, it could be 500 Hz for all I care and that would be enough.
Furthermore, the power factor of a switched mode supply changes as a function of the load applied and gets (much) worse if that load is also reactive or capacitive. Under some circumstances it is possible to draw negative power from the wall socket if you do a naive measurement, or you'll see wall socket power decrease as output current increases.
All this is possible because voltage and current are more or less out of phase with each other.
The kill-a-watt will work well with some reactive loads (such as CFLs) as long as they're of the ballast type.
A switched mode supply presents challenges that can't be met at the cost constraints of a consumer device like that.
Vampire power is a new term, I'm not familiar with it. Dead load (or simply the losses) is anything that does not end up in your consumer (the live load), I'm not sure if that is an accurate translation of the terms. It normally goes up as a function of the amount of power consumed, the base line (consumption without any load at all) is probably your 'vampire power'.
Total efficiency is 100 * ((output power)/(input power)) and will in practice be anywhere from 60% to 98% depending on how well load and supply are matched, and can vary wildly from one powersupply to another due to component variations.
Finally, classical power factor correction applies to sinusoidal wave forms, as you've already discovered switched mode supplies waveforms on both the input and the output side are anything but sinusoidal further complicating an already hairy problem.
The lesson here is that if you don't want to pay for an official apple charger, do not buy a counterfeit one just buy any name brand one with the right wattage rating.
But there will be no damage which is more important. My iPad simply slow charges (1A) on my old iPhone & my newer/current Samsung & LG 1A phone charger. Meanwhile my iPad charger works on all my devices - retired iPod Mini, my old iPhone 3G (now iAlarm Clock), Samsung Admire, LG Motion 4G, iPod Touch 7G. In the latter case, charging isn't any faster of course since the devices will only pull 500mA (Admire) or 1A.
The only chargers that I have that won't charge my all of my modern iDevices are old ones meant for iPods (500mA, no signal on the data pins) and my old Belkin iPhone car charger doesn't work with my iPad (Probably only 500mA w/ signal for iPhones. I'd have to check).
Another interesting tidbit: It appears that my car's Pioneer DEH 9400 head unit (aftermarket of course) seems to inform my devices that it can supply 1A of power through its two USB ports. This is the first time I've seen that outside of dedicated chargers and Macs (I assume of course that some PCs do this as well but I haven't seen one yet).
> This is the first time I've seen that outside of dedicated chargers and Macs (I assume of course that some PCs do this as well but I haven't seen one yet).
My Asus motherboard can do that with the right software/driver installed and it seems that all products made by Asus these days support this. It's an optional install you can find in the Asus support website for your devices.
The only risk you take in that scenario is that your device may not charge as fast as you expect (or at all, if in use.) Same with the 3 different wattages of MagSafe adapters.
I'm not so sure about that. HP and USB aren't exactly in people's search list as frequently as Apple. Beyond that, I clicked wondering if Apple was selling a general USB charger and not an iPad / iPhone accessory.
A lot of people have suggested additional chargers I should examine. To narrow it down, I've created a poll on the article page (upper left). If there's sufficient interest I'll take a look at your favorite charger, so cast your vote.
Good question! I test all the products in the adafruit shop for quality and functionality :)
I tried out half a dozen 5v/1a adapters and this one was the winner - best performance and a very good price for UL listed. The most important points: It has the 'iDevice' resistors and has very good voltage regulation - extremely clean, no big voltage spikes. When loaded down 1A it stays around 5.2V, I actually pulled 1.5A and it was still above 5V. I opened a few up to look at the construction and found it well designed with good soldering, strain relief etc. In the product description I explain why its a 5.25V not 5V adapter - its perfect for high-current devices like the Raspberry Pi and we have thousands of customers who have happily used it for the Pi.
I bought two for ~$3 apiece at Fry's a few months ago. One is OK, the other was and is DOA. The one that mostly works sometimes requires jiggling in the jack to make contact, but generally they are a garbage, F-, would not buy again product.
Hi Rhizome, adafruit does not distribute the specific USB charger we carry at Frys - sorry to hear you had a bad experience with them but it was certainly not the same one from the adafruit shop the parent poster was referring to!
I assure you that just because two adapters look the same do not have any relation to the insides. (After all, counterfeit Apple adapters look the same as genuine ones, but you wouldn't think they are the same on the inside after reading this article.) The black plug casing is a common molding style used by many factories - but its the electronics inside that count :) The best way to know if they are identical is to look for the UL certificate number, those are unique to each design.
I can assure you that as a run-of-the-mill customer, I will likely never check a UL code. I don't know if you get your casings from the same place, putting your own electronics inside, but I feel that if you want to trade on your identity as a purveyor of quality products you're going to have to look at least a little different from bad products. To borrow and twist a line from Seinfeld, "Adjacent to trash, is trash."
ladyada does not manufacture that charger; the website offers it, along with other general-purpose knickknacks, to compliment the main offerings- DIY electronics kits. This last bit is significant- ladyada's run-of-the-mill customer is not going to be your standard Joe looking for a charger for his cell phone.
I was traveling in China and picked up a charger that looks exactly the same from some street vendor. When it comes to cheap, no-name accessories, no one is going to spend money designing custom casing.
Wow, really? What if they're all made in China using the same enclosures, does that bring us back to the "check the UL code" method? I know I don't memorize UL codes, so that may be a nonstarter in at least a few situations.
Hi Rhizome, I am not sure how much of an electronics hobbyist you are. But if you are into electronics hardware, you may have had a chance to stumble upon ladyada's store 'http://www.adafruit.com and maybe even took a look at her background at 'http://www.adafruit.com/about/. My best guess is she knows what she is talking about. As someone above mentioned, her run-of-the-mill customers are not your average Joe customer.
I believe that she is meticulous about what she sells in here store and tests them out before she makes it available to her customers. She is not rebranding the products as hers. She is just assuring her customers that she is doing due diligence prior to selling it on her site.
I like Fry's just like the next guy and they have great deals, but I don't expect to get high tech stuff there or necessarily high quality stuff. They are just another big box store but also catering to your average hobbyist. I don't know if the purchasing department there puts as much effort into checking out the products they sell.
I expect that even if you were that dedicated to ferreting out the best chargers, there's plenty of variation in quality when dealing with chargers that are manufactured in the hundreds of thousands and move through the supply chain very quickly. I've been very happy with my HP Touchpad chargers but that's not the universal experience.
It's off-topic, but I've been trying to figure it out for a while: I have a two-port "Gear4" USB charger with swappable plugs, so it's great for travelling. However, my new work BlackBerry won't accept a charge from it, complaining about it being too low powered - however, the Gear4 charger says it's 2A, the BB charger is 750mA.
The charger has to signal that it is capable of providing more than the standard current. There are two ways this can be done - one is via the USB standard, and the other is the Apple way. Needless to say most chargers go the Apple way, so you should generally look for chargers that claim to not be compatible with Apple. Here is an example story of trying to find one: https://plus.google.com/117091380454742934025/posts/5suTrJrp...
In some cases the chargers and devices have gone a third way. This is very typical for tablets as they need a lot more current in order to charge within a reasonable time frame. At that point many manufacturers have made up their own way of signalling the ability.
On a similar note: I have a second generation iPod nano. Years ago I took a spare cable for it and cut the data wires, hoping to be able to plug it into a computer to charge while still being able to play music. (I saw this tip somewhere online.)
It worked with my brother's iPod classic (maybe 5th gen?), but not my nano; later I saw something on Adafruit about how the iPhone needed specific voltages on the data pins to charge, and I figured that was what was going on.
But recently, I got a battery pack, which can output to microUSB, iPod, or some other things. It works with my iPod, the same one I had years ago. But the cables it came with have swappable ends, I think 2.something mm jacks. At any rate they only have two points of connection. Here's what it looks like: http://i.imgur.com/8TliS.jpeg
So, how does this work? As far as I can tell, this should be exactly the same situation as a standard USB-iPod charger with its data cables cut, but it's not behaving the same.
It's possible I'm getting some details wrong, but can anyone shed some light on this?
Everyone has decided it is a good idea to use non-standard USB chargers to deliver better than USB power to devices. So as a result the Asus charger won't charge an iPad above USB specs, and the reverse is also true.
Given that the company now appears to deal mostly in iPhone cases with zero USB chargers listed on their site, their technical claims and/or QC may be suspect.
I got a nice charger from Buffalo (respected Japanese brand) that does a max of 4A (=can charge 2 iPads at once) and has 4 USB ports: 2 with the Apple power negotiation standard, and 2 with the Everyone Else/Android power negotiation standard.
Yes. I've never given to a Kickstarter campaign but would totally contribute to this. Proprietary chargers feel like such a ripoff at ~$30-$50, but clearly the $5 generics are a bad buy, so what you're suggesting is a real opportunity.
Very interesting review, I enjoyed the dissection and review of what most people don't even think about (wall chargers).
On a non-technical note, Apple chargers should lose simply because of how goddamned large they are. They basically eat two spots on a power strip and are easily knocked out of wall sockets because of their weight.
One thing that always annoys me when Apple products take over the world is how they are limited to American constraints. Sony Ericsson made a brilliant charger that plugged into the wall and you could just stand your phone onto. Apple would never think of that since the American plug standard is too weak to carry the same weight a Schuko can. And now everyone has to copy Apple instead of innovating...
I find the Apple chargers ideal for international use, because they have interchangeable plug heads for different countries. When I'm traveling, I only need to take the chargers that I need, and swap the plugs as required. That seems to work better than buying 3rd party plug adaptors (which don't always grip well, sometimes causing the plug to fall out of the socket).
The problem isn't for me personally - I don't own any Macs. The issue is other people who have Macs and plug them into wall outlets in public places and cover up other slots. And I've never seen anyone carrying around an extension cable.
Unless they are in the middle of a power slot, taking up another plug no matter which way they turn. The alternative is unplugging everyone's gear from the strip so the Mac user can get an end-plug.
And god forbid if there are two Macs plugged into the same strip...
I have a fake IPad charger, it barely charges my IPhone and can't charge the IPad at all. It was labelled as an Apple charger when I bought it for $10 and had good reviews.
I'm not versed in the arcane arts of EE— could anyone give me a basic definition of voltage sag? And would there be any reason to build it as a feature in a charger?