Curious Find Inside AC to USB Adapter- A Coin Cell

Discussion in 'Power Electronics' started by PGB1, Jul 13, 2016.

  1. PGB1

    Thread Starter Member

    Jan 15, 2013
    58
    3
    Hi All!
    I hope everyone is well & enjoying today!

    I bought a 'charger' for my cell phone. It is 120 vac input & DC 5v out in the form of a USB connector. I plug in the included USB to Micro USB cable to charge the phone. About an hour into the first use the unit's blue indicator light went off & the phone showed that charging stopped. It would not work again.

    The vendor sent me another. It did the same thing. All together, I have received 6 of these that all charged for about an hour & died. So I took one apart to look for a fuse or other obvious sign of the problem. I was quite surprised to find a 3.6 volt lithium ion coin cell inside. The cell is showing 0.03 vdc when removed & tested. (In the attached photo, the circuit board with the battery is shown inverted. It plugs into the underside of the larger board.)

    I am quite curious what the coin cell is used for in these chargers. I've got very limited electronics knowledge, but am reasonably familiar with power supplies of many types. I've never seen a cell inside one, so I searched around the internet for some clues. I came up empty. Could it have been for initial programming? If so, that seems like planned obsolescence- The programming would be lost when the battery is exhausted. (I am guessing...)

    Do you all know what the coin cell is used for in these units?
    Thanks For Your Knowledge!
    Paul

    IMG_1468.JPG
     
  2. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
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    It's probably there for programming during manufacture as you guessed. A program would be customized for each OEM customer's devices to negotiate efficient fast charging.
     
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  3. SLK001

    Well-Known Member

    Nov 29, 2011
    820
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    I doubt that it is there for programming. A cheap charger like that wouldn't waste the extra money for batteries. At first, I thought that it might be used as a stable reference, but since it discharged within a hour or so, that can't be it. Maybe the whole thing is a DC-DC converter with the cell being the source. Better photos of both sides of both boards could help with this mystery.

    Do all six have this same cell?
     
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  4. PGB1

    Thread Starter Member

    Jan 15, 2013
    58
    3
    Thanks KJ6EAD for taking time to reply. That's interesting, indeed.
    This information makes me think the programming was specific to a different device than mine (despite the vendor's information) and that's why the units are self-destructing when used with my phone.
    Thanks Again!
    Paul
     
  5. PGB1

    Thread Starter Member

    Jan 15, 2013
    58
    3
    Sorry SLK001 for being rude & not mentioning your reply. It wasn't showing when I typed mine. Perfect timing, huh?
    Thank You for taking time to reply with ideas.

    All six have matching cells. I don't know if they discharged while in use or if they were already discharged. (I didn't open the cases until they failed.) But, all of the cells are discharged.

    Attached are some photos of the board. On the photo with the "A" on the transformer, the strips on the right attach to the blades that are the wall plug for the 120 volt input.

    Thanks Again,
    Paul
    IMG_1469.JPG IMG_1470.JPG IMG_1471.JPG IMG_1472.JPG IMG_1473.JPG
     
  6. SLK001

    Well-Known Member

    Nov 29, 2011
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    The output is not galvanically isolated from the 120V that I can see. Looks like a pretty dangerous charger to use. It says that it is a 5V 1000mA capable charger, but I don't see any switching transistor capable of that.

    Remove all the black heat shrink and tape from the unit and post the picks again. Try to get as large a photo as you can so that the parts can be read.
     
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  7. DickCappels

    Moderator

    Aug 21, 2008
    2,658
    632
    How much of a charge did your phone get before the charger stopped working?

    It might that the charger is a fake or was designed for a different use and the coin cell peovides the power.

    It has happened before.
     
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  8. Kjeldgaard

    Member

    Apr 7, 2016
    73
    17
    It looks like something with mains communications:
    A uController, a communications IC, a serial flash and a small white transformer in heatshrink sleeving.
     
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  9. PGB1

    Thread Starter Member

    Jan 15, 2013
    58
    3
    Hi Everyone!
    Thanks Very Much for your help on this mystery.
    Attached are more photos with better views. I couldn't get very much of the tan colored glue off the unit to show more component labels. It's like a hard plastic material. (Harder than my razor knife blade)

    Looking at the battery holder's terminals, it looks like they go nowhere. (A photo is attached of the underside of the board that has the battery.) But, taking the holder off showed that the positive tab is connected to the circuit board. There's a small trace on the non-battery side for the negative.

    I'd have to guess about how much charge I got before they died, Dick. Probably (just guessing based on my habits) that the phone's battery icon showed about 1/4 left when I plugged it in for charging. I do recall one of the chargers quitting when there was still less than half charge on the phone. Each charger lasted about an hour before dying.

    I also failed to mention in my original question that, with one of the chargers, I heard a very faint 'click' sound, looked up and saw the blue indicator light on the charger quickly fading to off. That was the end of that charger's life.

    I hope the photos are readable. I have very poor eyesight, so I took them with the super-duper magnifier lens & have to guess they look clear to others. I reached the maximum allowed photo attachments for this reply, so the rest will be on a quick follow up reply.

    Thanks Again. This is getting really interesting (and I'm learning quite a bit along the way).

    Paul

    C-2.JPG A.JPG B.JPG C.JPG C-2.JPG C3.JPG
     
  10. PGB1

    Thread Starter Member

    Jan 15, 2013
    58
    3
    Here are the rest of the photos. They are of the smaller circuit board.

    D.JPG D1.JPG E.JPG F.JPG G-2.JPG G.JPG
     
  11. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,039
    1,667
    Yep I do.

    If you look at the devices specs tag on the bottom it says it uses zero watts at idle. The coin cell runs a watchdog circuit that shuts the device off completely when no load is present on the USB output and wakes it up when it sees a load by activating the tiny 3-volt relay (grey block next to the 4 pin connector.
    Thus if the battery goes dead it never knows when to wake up.

    My guess it that either it has a design flaw in the watchdog circuit that draws too much power and runs the batteries dead in a short time or these devices were manufactured some time ago and have ran the batteries dead by normal standby power draw from sitting for a few years and likely have no built in recharging ability once they are put into active mode.

    If it was me I would replace the battery on one unit and let it sit for a while and see how fast the battery drains down. if it's dead in a week its a design flaw. If it only down a few millivolts in a month these devices sat for some time before being sold.

    BTW if you solder the four connections together on the circuit board under the 4 pin (CON 1) connector (AC power supply point to and from the control board) together the charger will more than likely power up and work just fine.



    Look closer. The tiny yellow block is the HF transformer and the TO-92 size device (Q1) next to it is the switching device that drives it.
     
    Last edited: Jul 14, 2016
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  12. SLK001

    Well-Known Member

    Nov 29, 2011
    820
    229
    Yes, I saw the transformer. That by itself does not galvanically isolate the supply. I don't see any opto, or other feedback device that bridges the cold side to the hot. As far as I remember, a TO-92 is not capable of switching an amp or more.

    As for your possible age comment, I do see a 1033 date code. If real, your theory that the battery has ran out of juice could be correct.
     
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  13. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Many small SMPS now do not need any low side to high side connection to control their output voltage. They use an independent and isolated extra secondary winding as a voltage drop sensing point for their feedback loop.

    And how many amps is 5 - 6 watts at ~160 DC? o_O
     
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  14. SLK001

    Well-Known Member

    Nov 29, 2011
    820
    229
    But there are only two wires on the secondary side (the only side you can see).

    I guess that the primary doesn't really need to switch 1+ amps to get 5-6 watts out.
     
  15. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,039
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    It's hard to tell butit may use a simple current sensing method for basic output regulation.

    Ther are a number of ways to make a simple isolated SMPS that has fair output regulation without using conventional feedback methods.

    Do some research and see what you can learn. You may be surprised at what some designs can dow with minimal parts.

    I know I have a bunch of tiny 100 - 250 VAC universal voltage input to 5 volt DC 500 ma - 2 amp output power packs that use a number of different switch mode control and voltage regulation methods and some are almost bewilderingly simple in the number of components they use yet are able to achieve good regulation characteristics in a fully isolated package.
     
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  16. drvlas

    New Member

    Oct 16, 2015
    26
    5
    I do think the charger is fake. It is rather idiotic to design AC-DC converter with a cell.
     
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  17. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    1,147
    204
    All you have to do is be a no-name company and get it out the door. It might last 3 years and after that you forget about it. It broke.

    TCMTECH is onto something.
     
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  18. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Unfortunately, you may be seeing a lot more devices using this method to achieve zero power usage when plugged in but not in active use.

    I think it's a clever design and has a good potential for mass implementation but having the device designed without an easily replaceable battery and or a way to recharge its own battery should it run down is a poor concept.

    Although realistically given the cost of a typical 2032 type coil cell at $1 Vs the device having an ultra-low standby power mode like a number of similar devices now have that barely break the .01 watt standby mode $1 in electrical power at a pricey 20 cents a KWH would run one of those devices in standby mode for something like 500,000 hours or roughly 67 years. :rolleyes:
     
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  19. PGB1

    Thread Starter Member

    Jan 15, 2013
    58
    3
    This thread is very interesting and it really increased my curiosity, which lead me to do lots of reading about power supplies, their components and circuit arrangements. The best information I found was right here at All About Circuits. I like the Weekly Update e-mails, too. Each edition leads to more study & learning.

    I looked closely at & read labels on several obsolete power supplies I have in the 'stuff' box looking for a zero power consumption when in stand-by mode unit to dissect & study in order to compare how it is designed versus these phone chargers. No luck finding one (so far anyway- It's garage sale season so who knows what I'll find).

    Learning about the device being charged sending a signal to the charger solved my curiosity of why all four pins on the USB output were in use. I was surprised when I saw that, expecting only the power pins to be in use.

    Actually, it would be nice if every PSU would have low or no power consumption at idle. It is amazing to me that the little digital converter boxes on our TV sets draw between 11 and 16 watts when the set is turned off. And, of course, there are dozens of power supplies in every house. Even our basic model washing machine & clothes dryer have power consumption when turned off.

    You weren't kidding when you said "...without an easily replaceable battery", Tcmtech. I wanted to test out the battery drain and see if the cell is being charged by replacing the cell or even by connecting a power supply & meter instead of a battery- but no luck. I could not get one of the cells out without basically wrecking stuff. (Planned obsolescence?)

    I did, however, bridge the 4 <Con 1> pins on a couple and, as you suggested, they power on & have a good, stable output voltage. Thanks for that great idea.
    I got brave & tested on on the phone (it's a cheap $29.00 phone). The unit charged the phone and the phone itself posted a notification to unplug the charger (never saw that with the OEM charger). I was not brave enough to leave it unattended.

    On one unit, there was green powder inside so I wrecked that one to look at the coin cell. It was bulged & split, so I suspect it did receive a charging voltage at some point (or a short circuit damaged it).

    One thing I wonder about is that since the cells are still inside the unit, by bridging the <Con 1> pins, is the battery receiving voltage when the device is plugged in? I could not get test probes near the cell's connectors due to the design.
    I wonder if the cell would end up being re-charged to the point of self-destruction.

    Thanks Again Everyone for sharing your knowledge and sparking my interest to learn more. Any day I can learn new stuff is automatically a good day!

    Enjoy This Day!
    Paul
     
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  20. tcmtech

    Well-Known Member

    Nov 4, 2013
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    If I was to design a unit such as this I would use a rechargeable battery and add a reset button of sorts to it so that if it goes stone dead it has manual switch that bypasses the relay and manually turns it on so it can then start recharging its battery while providing the 5 volts to whatever else. ;)
     
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