Circuit question

[Fredrick]

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

7880

[Art]

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.

[amgen]

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.... ??

[LinkMTech]

I went here (http://mustcalculate.com/electronics/capacitorchargeanddischarge.php?vfrom=0&vto=9&vs=9&c=.000001&r=14&time=.001)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.

[Fredrick]

I went here (http://mustcalculate.com/electronics/capacitorchargeanddischarge.php?vfrom=0&vto=9&vs=9&c=.000001&r=14&time=.001)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 say´s something about "Pulsed Drain Current IDM =-3 A

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

[LinkMTech]

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.

[Fredrick]

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

[Ioannis]

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