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Help me understand my wall wart

Discussion in 'General Electronics Discussion' started by bobbyrae, Jun 2, 2014.

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  1. bobbyrae

    bobbyrae

    5
    1
    May 11, 2014
    My USB cell phone charger takes about 5.5 hours to charge my phone, so obviously there is a problem. I have been able to charge the phone off my computer's USB port in 2 hours, so it CANNOT be the cable or the phone, and as most of you already know, these cheap little chargers are the first thing to go. It is at least 5 years old.

    YES, of course the simple thing is to just buy a new one, but I want to understand how the thing works and see if I can fix it. The first thought is that replacing the cheap Chinese electrolytic caps would probably do it. But then there is that optocoupler, and I have heard people saying they are quite problematic and NOT like other semiconductors in terms of reliability.

    My first question is: how does it handle both 120 and 240 volts? It has two primary windings on the transformer, so the guess would be one is for each voltage, BUT, how is one selected over the other?

    I went to the trouble to draw the schematic in order to help with this. Hope I did well at that. Let me know!

    comet_charger2.JPG

    top2.JPG tracings3.JPG
     
  2. shumifan50

    shumifan50

    548
    56
    Jan 16, 2014
    The wall power supply just supplies 5V to the cable to the phone. Inside the phone is where there is circuitry to limit the current at which the phone is charged. The phones typically have Lithium batteries and these have a limit on how much current can be used to charge them. Different manufacturers and even different batteries from the same manufacturer can have different current limits. The circuitry in the phone will err on the side of safety allowing charging only at the lowest current for the batteries they use with a safety margin. Typically Lithium batteries can be fully charged in about an hour, but this is not guaranteed. If the battery can be charged at a higher current than your power supply can provide, then the charge time will go up, which is what your problem seems like.
    Because of this relationship between the battery and the phone circuitry it is not a good idea to buy pirate batteries as the might have a lower rating than the original and at worst this could cause a fire while charging. The same thing does not apply to the power supply, provided it outputs 5V.
     
  3. davenn

    davenn Moderator

    13,668
    1,891
    Sep 5, 2009
    That plugpack is just a very basic SMPS Switch Mode Power Supply

    AC in is rectified by the bridge rectifier, the hi voltage DC gets switched at maybe 30kHz or more by the oscillator, Q1,Q2
    That is inducted from the primary side of T1 to the secondary to produce approx. 5V out
    The output voltage is fed back to the primary side of the system via the opto-coupler. This feedback controls the frequency of the oscillator
    lower the freq drops the voltage and increase the freq. increases the voltage

    This is probably a form of PWM ( Pulse Width Modulation) where the ratio of the "ON' (conduction time) to "OFF" time of Q2 is controlled

    Dave
     
  4. davenn

    davenn Moderator

    13,668
    1,891
    Sep 5, 2009
    Is the plugpack putting out 5V ?

    Is there a current rating for the 5V rail on the plugpack label ?

    Does the 5V rail drop to say 3 - 4V when the phone is plugged in ?
    you will have to have a test meter to test that?

    Dave
     
  5. bobbyrae

    bobbyrae

    5
    1
    May 11, 2014
    OK, I guess I need to clarify. Sorry!

    Thanks for the responses so far, but I have been googling and found out about SMPS and know what the purpose of a charger is. I know that it takes 120V, rectifies it, then uses an oscillator and higher frequencies to increase the efficiency of a small transformer and then send that current to the phone battery. If I could understand how this specific circuit is doing all that, it may help to diagnose the problem.

    Maybe if we could figure out the purpose of the two primary windings. How does it handle 120 and 240 volts?

    Here are the symptoms:

    My assumptions is that there should be no charging going (and therefore no LED on) if the charger is plugged in but not connected to the phone, BUT the LED comes on anyway!

    The charging was originally like this:
    first 2.5 hours: 2-17% charged
    next hour: 17-45% (+28%)
    next hour: 45-74% (+29%)
    next hour: 74-97% (+23%)

    Then I replaced C3. Now it is still really slow in the first hour or two, but there is a half hour right after that where it goes from 15% to 48%. Then it gets slower again (20-30% per hour).

    Also, as a test, I jumpered the secondary pins of the optocoupler. This should turn the charger on full all the time, right? It didn't seem to make any difference. That should eliminate it as the main problem.

    Here is a slightly better version of the schematic:
    comet_charger2.gif
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    Automatic handling of 115V/230V can be handled in different ways. The simplest way is by designing the circuit to operate at 115V AC (often, down to 100V AC). If the supply voltage is higher, the circuit's normal regulation action will reduce the duty cycle of the signal into the transformer to ensure the right output voltage.

    The output voltage is not simply determined by the turns ratio of the transformer, the way it is with transformers that operate at mains frequency. It is controlled by the width of the pulses that are fed into the primary of the transformer, and these in turn are controlled by the feedback from the output voltage monitoring circuit, via the optocoupler. In other words, regulation.

    The charging of the battery is all implemented inside the cellphone. The mains adapter just provides a DC voltage, typically 5.0V. Current limiting, voltage monitoring, energy level estimation, charge phase sequence and all other "clever" features are implemented inside the phone.

    The phone may use the USB data signals to determine the current rating of the charger. This is discussed in some of the links I gave you in post #5. If the phone thinks the charger cannot supply enough current, you could get the symptoms you describe.

    Was that charger supplied with that phone?
    Is that the battery that originally came with the phone?
    If not, is it marked exactly the same as the original? And did you buy it through an authorised agent or is it possible it's counterfeit?
    How old is the battery?
    Have you tried a new battery?

    Monitor the DC voltage from the charger during one complete charging cycle to make sure it does not change significantly.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    My guess is that it's the cable, or the connector it plugs in to.

    It is quite likely that the phone detects the source of power and draws the appropriate current (e.g. it won't try to over-draw from a USB connection).

    It is possible that this information is not getting to the phone from your plugpack. This may be due to a broken wire in the cable connecting the charger to the phone. If the charger has a USB socket, it may be due to a resistor becoming disconnected from the USB socket in the charger.

    The charger handles voltages from maybe 85V to 265V (a not uncommon range) by varying the proportion of time that the rectified mains is allowed to flow through the primary of the small transformer (edit: NOT transistor). The current flowing through the transformer is proportional to the voltage and the time, so with half the voltage the time is doubled. For a given primary current a certain amount of power is stored in the magnetic field and can be transferred to the secondary. The charger doesn't know what the input voltage is, it measures the output voltage. When the output voltage falls, the ON time is decreased. When it falls, the ON time is increased. At the minimum input voltage, the ON time is at the maximum (or close to it) when supplying the maximum rated power. The maximum input voltage is often determined by the voltage rating of various components, but also to a lesser degree by the minimum time the voltage can be allowed to flow through the transistor under very low load conditions without causing unacceptable voltage spikes on the output.
     
    Last edited: Jun 5, 2014
    KrisBlueNZ likes this.
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