Basic Questions about AC to DC Charging - Page 1
Forum section: electronics / beginners

author: Electro Fan
author_rank: Super Contributor
timestamp: May 15, 2016, 09:24:22 pm
content: Some real basic questions about AC to DC charging....I have a Bluetooth earphone/headset; it uses a microUSB charger spec'd at 5V, 1000mA but I couldn't find the charger cable that has a microUSB connector on end and a regular USB connector on the other end.At Radio Shack I saw a plug adapter (just the adapter, not a cable) with a microUSB on one end and two small prongs on the other end - it was for a kit that accepts the two prongs on the universal end of a charger cable (so the charger can work with microUSB, miniUSB, etc. by using various two prong to whatever adapters). My initial thought was to purchase the two prong to microUSB adapter and connect the two prongs (with small alligator clips) to my (30V, 5A max) linear AC to DC Power Supply with the power supply set to 5V and 1000mA - with the thinking being that the 5V would make the headset happy and the maximum current the headset could draw would be 1000mA. Then I wondered whether the current could possibly "overrun" the headset circuit. I wondered whether perhaps a standard (Apple) 5V, 1A AC to DC power adapter (for a USB to whatever-connector cable) has some circuitry that recognizes when a device is fully charged?This got me thinking about how the headset would actually store current - perhaps it is as simple as a capacitor inside the headset that will accept up to 1V?I decided not purchase the two prong to microUSB adapter and found a regular USB to microUSB cable for use with an Apple 5V 1A power adapter. Rather than plugging it all together directly I inserted a Portapow Monitor between the Apple power adapter and the headset. The Portapow showed 5.07V and .08A (80mA). Presumably the headset only wanted to draw 80mA initially and over time the headset will get recharged based on whatever rate at which the current actually flows.So, the questions are:1) If I had plugged the headset into the linear Power Supply with the PS set for 5V and 1000mA would the headset have still only drawn 80mA and would the whole setup have acted just like what I eventually used? Or would access to the full 1000mA have caused/allowed the headset to consume the current too quickly?2) Is the load presented by the headset the determining factor in how much current it draws (and at what rate it draws the current), or is there some "regulator" built into the Apple power charger that spools out the current at a preferred rate? (Or is their some sort of current consumption regulator built into the headset? see next question...)3) What is actually holding the charge inside the headset? A capacitor, or something else?4) In the case of capacitors, do they consume current up to the point of storing their rated Voltage and then quit consuming current and hold steady at their max Voltage or will a capacitor eventually fail if current is continually made available at the rated Voltage? (Is there any "stress" on a Power Supply if it keeps feeding Voltage to a capacitor that is fully charged (assuming the PS is set to the max Voltage for which the capacitor is rated)? Or do the PS and the capacitor reach some happy equilibrium?Thanks for the help with all this basic stuff.

author: Alex Trofimov
author_rank: Contributor
timestamp: May 16, 2016, 11:04:04 am
content: Hi!1. Power supply only limits curent when it exceeds the limit that have been set. So you can set any limit above 1A, it doesn't matter. I would set 2A, in case...)2. I presume there's a charge controller in the headset.3. It's most likely a Li-ion battery.The charger, that you are talking about, what is it? It plugs to a mains socket and have USB-A connector for a cable between it and the headset? If I get i right, it must be just an AC/DC adapter, not charger, because to specify a charger at 5V doesn't make sence to me.

author: Zbig
author_rank: Frequent Contributor
timestamp: May 16, 2016, 12:16:03 pm
content: On what planet, which parallel universe there's no at least fifty Micro USB cables in each and every household? Pretty much every morning, when I reach to the drawer for a fresh pair of socks, there's also two or three of the damn things tangled - they just won't stay there. The moment I turn my head during the lunch, they crawl onto my plate and pretend they're spaghetti. There's at least one bundled with each roll of TP I buy. *)*) Ok, ok, I exaggerated a bit. But seriously, are you trying to say youactually had problems finding a micro USB cable? I don't believe that for a second

author: Electro Fan
author_rank: Super Contributor
timestamp: May 16, 2016, 05:10:15 pm
content: Quote from: Zbig on May 16, 2016, 12:16:03 pmI know, sort of strange but after sorting through cables and looking in all the likely places for a microUSB cable, no microUSB cables - just USB A, B, mini, etc. A day later and I have finally found one - but for some reason I guess I don't have many devices that use a microUSB.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 16, 2016, 05:25:29 pm
content: Quote from: Alex Trofimov on May 16, 2016, 11:04:04 amThanks for the reply.To be clear/specific (I made an error in my post and referred to the Apple power adapter as a charger), the device is the basic Apple 5V/1A power adapter:http://www.apple.com/shop/product/MD810LL/A/apple-5w-usb-power-adapterThere doesn't seem to be a user replaceable battery in the headset; perhaps it is a Li-ion.So, is the Apple power adapter (that's what Apple calls it) is simply an AC to DC converter with no regulation circuit inside? It converts AC to DC but in no way governs or regulates how much current can be drawn by a device? It will put out whatever current is drawn up to 1A? So any current regulator must be inside the headphone circuit? When the battery in the headphone circuit is fully charged it will quit drawing current, or maybe take just a trickle?If I had used the linear Power Supply and set it for 5V/2A - do you think the headset would have regulated itself down to only consume 1A? And same as with the Apple power adapter - the headset would have quit drawing current when the battery reached full charge?Thanks

author: sokoloff
author_rank: Super Contributor
timestamp: May 17, 2016, 03:10:25 am
content: Basically, yes. The USB power source is providing ~5V and "up to" a certain amperage, but the device is regulating what power is actually be drawn.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 17, 2016, 07:37:26 am
content: Quote from: sokoloff on May 17, 2016, 03:10:25 amHi sokoloff. Thanks for netting that out, I appreciate it.So if I had connected the headset to the linear Power Supply with the PS set for 5V and 2A the headset would have drawn only the current it needed (1A or less as the headset battery was charging) and then the headset would have quit drawing current (other than maybe a trickle?) once the battery was fully charged. So, it would have been safe/reliable/effective to use the linear Power Supply? Yes?Thanks again. EF

author: sokoloff
author_rank: Super Contributor
timestamp: May 17, 2016, 10:06:33 am
content: Yes. Doing so would be safe, and if your supply has an ammeter built-in, you'd see the current profile. Matter of fact, I'd do exactly that as an experiment/experiential learning. Just make sure you get the polarity right.Also note that some devices will not charge from just the +5V and GND pins on the USB, but will "read" the current capabilities advertised on the data pins. It will be safe still, but just might not charge. I'd imagine most BLE headsets will charge from just the power pins.See this App Note for a good description of how the power capabilities of a USB 1.2 port are "advertised" by the port:https://www.maximintegrated.com/en/app-notes/index.mvp/id/5801

author: Alex Trofimov
author_rank: Contributor
timestamp: May 17, 2016, 11:03:08 am
content: Quote from: Electro Fan on May 16, 2016, 05:25:29 pmI believe it is. It have DC regulation (to voltage remain 5V in any case and current do not exceed 1A), but no charge regulation. I governs only maximum current limit. Yes, it will. Yes, the charge control circuit must be inside the headset. Generic charge controller works this way: When the battery is charged, it stops charging, then starts evaluating battery voltage. If it drops, it starts charging procedure again.Quote from: Electro Fan on May 16, 2016, 05:25:29 pmYes, it must regulate. As it is designed for use with 1A supply, it must never exceed this limit. Power supply limit is a safety feature, it is not intended for normal use. So the headset consumption must be bang on 1A or less, otherwise it is broken.

author: Alex Trofimov
author_rank: Contributor
timestamp: May 17, 2016, 11:07:55 am
content: Quote from: sokoloff on May 17, 2016, 10:06:33 amThanks for the link, sokoloff! I didn't know this, very interesting.)

author: Electro Fan
author_rank: Super Contributor
timestamp: May 19, 2016, 06:02:59 am
content: Quote from: sokoloff on May 17, 2016, 10:06:33 amThanks for the article.After reading the article maybe connecting the headset directly to a linear Power Supply set for 5V and 1A would have worked, or maybe the headset charging circuit would have been expecting a more "intelligent" "advertised" handshaking arrangement? Perhaps the headset would have expected an initial lower setting on the current and then expected the current to ramp up based on communications between the headset charge management circuit and the Power Supply (which of course would have had no communications path to the headset)? So maybe it was good/lucky that I stuck with the USB power adapter?

author: Zbig
author_rank: Frequent Contributor
timestamp: May 20, 2016, 09:55:13 am
content: Quote from: Electro Fan on May 19, 2016, 06:02:59 amNo, you're overthinking that. Absolutely no part of the charging process control happens on the charger side. All it has to do is provide a stable 5V and be capable of reliably providing current ofat least up tothe stated 1A. Plus, as others already mentioned, there's often either a chip or just some simple pullup/pulldown resistor trickery on the USB data lines to tell the charged device what it's dealing with. You won't be able to damage a modern, properly designed USB-charged consumer piece of electronics with "too much current", end of story. It's the actual device that governs the Li-Po charging process. In case of overwhelming majority of consumer gear, a "charger" really is a constant voltage DC power supply. The worst that could possibly happen after connecting the headset to your lab supply with USB data lines floating, it would assume basic/non-compliant charger and fail-safe either by refusing to start charging or (most probably) charging slowly at safe less or equal to 500mA current. Just think about it, how silly and irresponsible on the device engineer part would it be to rely on the actual charger to control the Li-Ion chemistry charging process. It would just be terrible, terrible engineering. There are literally thousands of cheap chargers, power banks, car adapters, you name it, used willy-nilly with thousands different USB-charged devices used by millions people around the world every day. And it's the single isolated incidents of someone unlucky enough to actually get hurt you can read in the press. Another example: your car battery is easily able to provide some 100-300A of current you need to start the engine. Have you ever been afraid that's "too much current" for your car radio or car laptop adapter to handle? Or that the utility power in your house provides "too much current" for the new LCD TV you just got? Of course you haven't - the devices just take what they need. They present an equivalent resistance (according to Thévenin's or Norton's theorem) to the source and it doesn't matter what current the source is theoretically able to provide. You really should stop thinking in terms of a "supply giving too much current" - it's the job of the circuitry inside the device being powered to be able to deal with the supply that is able to provide stated voltage andat least the maximum currentit needs. Of course, as with almost everything in this world, there are exceptions to this rule (some super-cheap power tools, toys, novelty items, etc.). That's why I kept reiterating "properly designed".

author: Electro Fan
author_rank: Super Contributor
timestamp: May 21, 2016, 04:47:50 pm
content: Quote from: Zbig on May 20, 2016, 09:55:13 amZbig, Thanks for your reply. It is very helpful.Here is a quick recap to make sure I have it (at the risk of over-thinking) and a follow-up question.For any device that runs on DC that is properly engineered, the device could be powered by a lab power supply as long as the lab PS supplies a stable voltage as required by the device and as long as the PS is set to current equal to or greater than (including any amount greater than) what is required by the device. In this arrangement the device will draw whatever current it needs, and no more than it needs. I'm pretty sure this is an accurate summary but I will rely heavily on it and want to make sure something wasn't lost in the communications (and based on your very good post above I'm confident it is correct, I think/ I hope- Thanks).Moving on...When a battery operated device is fully charged, what happens to the battery during the charging process? Does the resistance raise to a point (when the battery is fully charged) that the current can no longer flow? Does this mean that a battery with no charge has very little resistance and as the battery charges the resistance either gradually increases, or maybe it remains steady at some charging resistance level from empty to near full and then it significantly increases? (And presumably the PS just keeps happily supplying the set Voltage and little (trickle) or no current?)On a related question, if the battery's resistance changes during charging, what is the role of the charge management circuit in the device?Finally, what is the risk if any of overcharging a battery (assuming an on-going supply of the proper Voltage and minimum required current)? To what extent does this risk vary with types of batteries? (And to what extent does the risk depend on the charge management system (if it is present)? Are some types of batteries more happy to charge in a "dumb" environment (with no charge management or a very limited charge management circuit), and are other types of batteries more reliant upon relatively smarter (more active or IC-enabled) charge management circuits?Thanks again for the very good teaching. EF