2N3904 vs BC850

fonik

below you see the clock and reset circuit i use for the 4024. i built it with 2N3904s and everything works fine.

then i built it in SMT using BC850 NPNs and not the manual reset button (SW1) works just once! i replaced Q2 and it worked again until i pushed the reset button again.

i don't want to replace the transistor after every manual reset, so i am looking for a solution.

i compared the maximum ratings of both transitors. they don't spec a max current for the base, but for the collector onöy (200mA for the 2N3904 vs 100mA for the BC850).

anyways, i wonder: should i place a resistor (say 47k) between the switch and the base of Q2? or can the problem be found elsewhere?

clkrst.gif

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Blue Hell · Posted: Mon Dec 07, 2009 10:00 am Post subject:

Add a base resistor for the reset switch as well Wink

fonik · Posted: Mon Dec 07, 2009 10:08 am Post subject:

good idea, jan. would be good practice, i guess.

anyways, the 47k between switch and 15V limiting the current solved the problem.

funny that the 2n3904 has a high tolerance concerning my ignorance, whereas the SMT version is a little bit more sensible...

Blue Hell · Posted: Mon Dec 07, 2009 10:15 am Post subject:

Maybe your power supply can't source enough to blow the 2n9304, but something will have to give in when you short your supply with two PN junctions ... either one of the juctions or the supply.