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I have A circuit that looks like this and all works as expected.
But I´m 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
Attachment 7880
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.
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.... ??
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/4175Quote:
Originally Posted by LinkMTech
it say´s something about "Pulsed Drain Current IDM =-3 A
But I don't understand what they meaning with -3A?
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. :wink:
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.
LinkMTech: Thank you very much!
I'm a little wiser now :D
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