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Thread: Need technical help in some electronic circuity

  1. #21
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    Default Re: Need technical help in some electronic circuity

    Using a TL431 with a transistor could be a viable alternative.

    It's not possible to use the device and draw power off a USB port anyway. Even LDO regulators wouldn't work when there's virtually no input/ output voltage differential.

    Try using smallish Sealed Lead Acid batteries. They generally offer much more power storage and have lower discharge rates.

  2. #22

    Default Re: Need technical help in some electronic circuity

    Quote Originally Posted by LittleWolf
    You can pair the 7805 with a PNP power transistor to boost the current. Whether it's worth the hassle compared to getting a higher current regulator integrated as a small 3 pin device is another question.
    Quote Originally Posted by yanyewkay
    chey.. say so earlier lah.. you have 9v supply.. parallel 2 pieces of 7805 will solve your problem. It's a cheapskate way, not highly recommended by many, but I use it all the time.

    I was under the impression that you wanted 5V 2A to be from the USB port

    Else under the 7805 spec sheet there's an example of adding a power transistor like what LittleWolf said to get 2A.
    Yes, I was thinking of using Power Transistor TIP2955 as the one as mentioned here
    http://www.southwest.com.au/~jfuller...regulators.htm

    But I got one question. How does the current drawn work? The Mp3 device like my Creative Vision:M needs a wall plug of 5V, 2.4A which takes 2 hrs for a full charge. As for charging from USB port from a computer, it takes 6hrs, so naturally it uses a smaller current. So My presumption of the device is 1A is not enough to trigger the device to charge but need more than 1A to trigger the charging unit. Am I right?
    Last edited by tokrot; 13th March 2006 at 10:21 PM.

  3. #23
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    Default Re: Need technical help in some electronic circuity

    2A is a guide that the MAX current supply is 2A. It doesn't mean it supplies 5V 2A constant. if you have load that requires only 100mA then the power supply will only output 5V 100mA but 2A is the max it will output even if your devices wants more current.

    What we have is a voltage supply. Not a current supply.
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  4. #24
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    Default Re: Need technical help in some electronic circuity

    Quote Originally Posted by tokrot
    Yes, I was thinking of using Power Transistor TIP2955 as the one as mentioned here
    http://www.southwest.com.au/~jfuller...regulators.htm

    But I got one question. How does the current drawn work? The Mp3 device like my Creative Vision:M needs a wall plug of 5V, 2.4A which takes 2 hrs for a full charge. As for charging from USB port from a computer, it takes 6hrs, so naturally it uses a smaller current. So My presumption of the device is 1A is not enough to trigger the device to charge but need more than 1A to trigger the charging unit. Am I right?
    A properly designed USB device will start in low-power mode (drawing less than 100mA) and communicate with the host. If there is insufficient power on the port, the host will notify the device and the device will not switch to high power mode (drawing up to 500mA).

    Since your battery pack has no way of telling/ tricking the Vision:M that it's a USB host, the Vision will not switch to slow charge mode (drawing up to 500mA max). It will attempt to draw more than 2A and will not charge if it is unable to get the power it needs.

  5. #25

    Default Re: Need technical help in some electronic circuity

    There are many smart battery charger ICs available nowadays that detect whether a wall plug or USB port is used. The charger will switch its mode accordingly to charge the portable device.

  6. #26
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    Default Re: Need technical help in some electronic circuity

    2.4A is hell lot of current to charge a MP3 device. The device is most likely drawing significantly lower than what the wall adapter is rated for. You most likely don't have to design for such high current.

    If it takes 2.4A to charge the battery in 2 hours, the battery capacity should in the region of 2400mAH. I doubt that your MP3 has such a large battery.

    You might be better off measuring the actual current draw during charging for each device and re-specify your DIY charger accordingly.

    Most likely the battery has a capacity of lower than 1000mAH. If I remember correctly my Zen Xtra's battery is a 780mAH (3.6V). Most manufacturers will use a 1C charging current for lithium Ion batteries so a 1A regulator should be enough for my Zen Xtra.
    As complexity rises, precise statements lose meaning and meaningful statements lose precision.

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