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Thread: The difference between a Power Supply and a Battery Charger

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    Default The difference between a Power Supply and a Battery Charger

    I want to make an extremely basic Battery Charger for 4 Nicads in series.

    I have a Transformer and a Bridge rectifier.
    I am hooking the Output of the Bridge Rec. to an LM317 configured as a Constant Current Regulator providing 120 milliamps.
    This CC circuit is simply a 10 Ohm resistor between the Adjust Pin and the Output Pin.
    The 4.8 Volt battery pack connects betweeen the adjust Pin and earth.

    Should I include an Electrolytic capacitor for Filtering anywhere or do Battery Chargers work better unfiltered?



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    I don't think that your batteries will care if you have or not a Cap in there, if you 've got one, it won't hurt.

    But, you'll have to limit the charge to 1.44V per cell (5.76 V total) to avoid overcharging your cells and cooking them.

    You could put the above mentioned current source in series with a voltage regulator (another 317 using 2 resistors) that has been set to provide 5.76V to power the batteries.

    The voltage reg will never allow more than the required voltage to the batteries and the current source will maintain the charging at a limit for the batteries.

    Transf > Bridge> Cap> Current Source > Voltage Reg > Batteries.

    Hope that helps

    Cheers

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    The battery is the capacitor
    Reality is an invention of my imagination.
    ಠ_ಠ

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    Quote Originally Posted by Fernbay View Post
    The battery is the capacitor
    In this situation the ripple will interfere with the operation of the LM317's.
    A filter cap after the bridge is essential.

    In the case of battery chargers for normal lead acid batteries you are correct.

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    Quote Originally Posted by moeee View Post
    I want to make an extremely basic Battery Charger.....
    Quote Originally Posted by Reschs View Post
    In this situation the ripple will interfere with the operation of the LM317's.
    A filter cap after the bridge is essential.

    In the case of battery chargers for normal lead acid batteries you are correct.

    From "An optional output capacitor
    can be added to improve transient response."

    Not trying to nit pick, but moeee is typically somewhat reluctant to go splurging on unnecessary luxuries, such as filter caps
    Reality is an invention of my imagination.
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    Quote Originally Posted by Fernbay View Post
    Not trying to nit pick, but moeee is typically somewhat reluctant to go splurging on unnecessary luxuries, such as filter caps
    True , but in this case that is not the reason.

    I was figuring that the output would be more in the Form of pulses without a capacitor , and for battery charging that it would be more beneficial than a steady flow of current.

    In most every car battery Charger I have seen they ain't used.

    And the member that said to be careful about over-voltage , yes I will be doing something.
    It will possibly limmiting the Voltage available to the LM317 , perhaps with a chunky Zener.
    The output from the Bridge rectifier will already be close to what I need.

    I ain't messed with electronics for years now.
    Is like I'm born again.

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    Quote Originally Posted by Fernbay View Post
    From "An optional output capacitor
    can be added to improve transient response."

    Not trying to nit pick, but moeee is typically somewhat reluctant to go splurging on unnecessary luxuries, such as filter caps
    The capacitor in question, from the original post, was the output of the bridge rectifier and before the LM317.
    The LM317 will not behave correctly with volts of 100Hz ripple on its input.
    Last edited by Reschs; 18-04-12 at 01:23 AM.

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    Quote Originally Posted by Reschs View Post
    The capacitor in question, from the original post, was the output of the bridge rectifier and before the LM317.
    The LM317 will not behave correctly with volts of 100Hz ripple on its input.
    Sure Resches, but I am looking at things from a different angle than you.
    Personally, I would 'splurge' on a few caps myself, but in this case I am replying to the thread in the context of the OP.

    One of moeee's key requests was "extremely basic" - OK, now that we are passed that......
    Lets it also be said that battery charging does not require .0001% accuracy. They tend to have a reasonably amount of tolerance to play with

    Again from the data sheet....


    EXTERNAL CAPACITORS
    An input bypass capacitor is recommended [ Note- the word is recommended, not required]. A 0.1μF disc or
    1μF solid tantalum on the input is suitable input bypassing for
    almost all applications. The device is more sensitive to the
    absence of input bypassing when adjustment or output capacitors
    are
    used but the above values will eliminate the
    possibility
    of problems.
    The adjustment terminal can be bypassed to ground on the
    LM117 to improve ripple rejection. This bypass capacitor prevents
    ripple from being amplified as the output voltage is
    increased.
    With a 10 μF bypass capacitor 80dB ripple rejection
    is obtainable at any output level. Increases over 10 μF
    do not appreciably improve the ripple rejection at frequencies
    above 120Hz. If the bypass capacitor is used, it is sometimes
    necessary to include protection diodes to prevent the capacitor
    from discharging through internal low current paths and
    damaging the device. In general, the best type of capacitors to use is solid tantalum.
    Solid tantalum capacitors have low impedance even at high
    frequencies. Depending upon capacitor construction, it takes
    about 25 μF in aluminum electrolytic to equal 1μF solid tantalum
    at high frequencies. Ceramic capacitors are also good
    at high frequencies; but some types have a large decrease in
    capacitance at frequencies around 0.5 MHz. For this reason,
    0.01 μF disc may seem to work better than a 0.1 μF
    disc as a bypass. Although the LM117 is stable with no output capacitors, like any feedback circuit, certain values of external capacitance
    can cause excessive ringing. This occurs with values between
    500 pF and 5000 pF. A 1 μF solid tantalum (or 25 μF
    aluminum electrolytic) on the output swamps this effect and
    insures stability. Any increase of the load capacitance larger
    than 10 μF will merely improve the loop stability and output
    impedance.
    Reality is an invention of my imagination.
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    Quote Originally Posted by Fernbay View Post
    Sure Resches, but I am looking at things from a different angle than you.
    Personally, I would 'splurge' on a few caps myself, but in this case I am replying to the thread in the context of the OP.

    One of moeee's key requests was "extremely basic" - OK, now that we are passed that......
    Lets it also be said that battery charging does not require .0001% accuracy. They tend to have a reasonably amount of tolerance to play with
    I was answering the original question, but now that we have moved on I personally would not use the LM317 or any other regulator without the additional bypassing which must be located as physically close to the regulator as possible.

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