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capacitors in single polarity split rail supplies

Bob Edwards
Prodigy 210 points
Other Parts Discussed in Thread: TLE2426, BUF634

Hello all,

A novice question. I want to use a TLE2426 rail splitter in conjunction with a BUF634 op amp buffer to create a split power supply (half my battery supply voltage) for use in testing/prototyping op amp circuits. I have seen numerous circuits like this which incorporate capacitors, eg, a 220 μf across the battery in front of the TLE rail splitter, along with other capacitors downstream of the op amp buffer.

Most references give little information about which capacitors are helpful (or even needed), or how to calculate the sizes of any capacitors  which should be used. Any light someone can shed on these two issues would be greatly appreciated.

Thanks

  • 0
    Guru 21975 points

    Hi Bob,

    Your question has no quick and easy answer. Consider this: If your battery is a good, solid voltage source with low source resistance, there is no need for a capacitor in parallel with it. Such a capacitor is usually added as a counter-measure to a weak battery condition. In this case, the battery impedance is relatively high. The capacitor provides a low source impedance for AC signals. A transient or AC load current is supplied by the capacitor. Without it, the battery voltage would waver in response to the varying load current. Even with it, the average voltage will sag--the capacitor cannot supply continuous current.

    The need for a capacitor and choice of its value depend on the dynamic nature of the load current--how much does it change and at what frequency. The other factors are the maximum source impedance of the battery (weak battery condition) and the acceptable variation in battery voltage due to the AC load variation.

    For a quick estimate, consider that if a constant load current must come solely from the capacitor. The voltage on the capacitor will sag at a rate of I/C volts/second.

    In any event, you should at least have a capacitor to assure a low impedance at very high frequency to assure stability of the rail splitter and BUF634. I'd recommend a 10uF tantalum capacitor.

    Regards,  Bruce.

  • 0 in reply to Bruce Trump
    Intellectual 280 points

    Dear Bruce,

    By "Good battery" do you mean the C rating should be high or AH rating? Which characteristics would make it a "good" battery? Pl explain.

    If the battery is good, then can we say that the current transients are absorbed by the battery itself?

    Regards, Anish

  • 0 in reply to Anish Joshi1
    Guru 21975 points

    Anish,

    In the context of my answer, a "good" battery is one that is nearly new or fully charged. A "bad" battery would be one that is old, nearly expended or needs recharging. A "good" battery has a much lower internal series resistance than a "bad" battery.

    Of course, higher capacity (AH rating) battery has lower resistance than lower capacity batteries, too. Without much specific information in the original posted question, I was keeping the discussion in relative terms.

     

    Regards, Bruce.