Circuit question


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  1. #1
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    Default Circuit question

    I have A circuit that looks like this and all works as expected.

    But Im not sure if the BS250 can handle the two capacitors i the long time, I assume that the BS250 will be shorted for a short time when the button is pressed and the C1 and C2 are empty?

    C1 - 1uF
    C2 - 2,2uF
    Power source - 9v battery

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  2. #2
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    Default Re: Circuit question

    Pushing the button sends the gate low (BS250), but I don’t see how it directly shorts
    C1,C2, or anything else. R? prevents a direct short in any case as far as I can see.

  3. #3

    Default Re: Circuit question

    looks to me like the pushbutton 'closes' the P channel fet , charges the caps and putting PortA1 high keeps circuit going until PortA1 goes low, BS250 opens and the caps drain.... ??

  4. #4
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    Default Re: Circuit question

    I went here to see how much in-rush current you can expect and understand your concern about that BS250 part limited at 180mA. The data sheet doesn't even provide pulse current parameters.

    I would upgrade to a another P-CH FET with a much lower ON resistance in the low 100's of milli-Ohms that can easily handle the short burst for peace of mind.
    Louie

  5. #5
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    Default Re: Circuit question

    Quote Originally Posted by LinkMTech View Post
    I went here to see how much in-rush current you can expect and understand your concern about that BS250 part limited at 180mA. The data sheet doesn't even provide pulse current parameters.

    I would upgrade to a another P-CH FET with a much lower ON resistance in the low 100's of milli-Ohms that can easily handle the short burst for peace of mind.
    In this datasheet from the place I bought the part http://www.electrokit.com/productFile/download/4175
    it says something about "Pulsed Drain Current IDM =-3 A

    But I don't understand what they meaning with -3A?

  6. #6
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    Default Re: Circuit question

    In short, it's the current leaving or supplied through the Drain as the case with the P channel. Electron flow verses Hole flow theory, depending on your upbringing.

    So the 3 Amp rating in the 300us pulse test is more than adequate since the initial cap charge of ~640mA is over very quickly, less than 40us, then tapers off from there.
    Louie

  7. #7
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    Thumbs up Re: Circuit question

    LinkMTech: Thank you very much!
    I'm a little wiser now

  8. #8
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    Default Re: Circuit question

    I would not use a BS250 since it has high leak current.

    Better choice would be a PNP transistor like the BC857 with a BC847 pair to turn it on.

    Resistor would also be dimensioned lower, say c-b at 10K to keep the PNP off and another 10K to turn it on by the other NPN transistor.

    I use such a selection for a commercial product working on a 9V battery. My caps are 10uF and charge through PNP internal resistor with no problem.

    Ioannis
    Last edited by Ioannis; - 12th July 2015 at 13:36.

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