On Monday 01 September 2003 09:27, Michael Rigby-Jones scribbled:
> If you check the schematic you will see that RB0 and RB1 are driving MO=
SFET
> gates.  I suspect the original intent is that pressing the switches dos=
en't
> affect this gate drive.

Correct.


> It should be fairly obvious why the circuit dosen't work as you expect:
> With 5 volts applied to the base of the transistor, a current will be
> flowing out of the emitter irrespective of the voltage on the collector=
=2E

Yes, but I expect that that should be quite small.  Roughly (5 - 0.7)/1k =
=3D=20
4.3mA.  And with the extra diode in place, there would still be this curr=
ent.

Also, with the transistor switched on and the pushbutton pressed, when RB=
1=20
goes High (~5V), the 1N4148 gets forward-biased and the essentially=20
reverse-biases the B-E junction of the transistor, eliminating that curre=
nt=20
=2E.. right?


>  A
> further potential problem is that with RB0 driven low, the BC junction =
will
> be forward biased.

Hmmm..... if this happens, then there should be some side-effect to the 7=
-seg=20
displays (the columns of which are driven by the mosfets).  But there are=
 no=20
side-effects ... at least no noticeable ones.


>   This application either wants a mosfet or a PNP bipolar
> configured as a common emitter circuit rather than an emitter follower
> (which then inverts the switch polarity).  By putting an extra diode at
> point (D), you are reducing the voltage by 0.6 volts which is obviously
> enough to fall below the logic threshold of the PIC.  You could also ju=
st
> lower the value of R2 to acheive the same effect.

Lowering R2 from 10k to 1k doesn't help, but raising R1 from 1k to 10k do=
es. =20
What's interesting is that in the other (first) circuit, I have a 1k base=
=20
resistor as well, and it works.  I even swapped transistors, with no=20
resulting difference in operation of both circuits.  Odd.


> Mike

Thanks,
-Neil.


---
[This E-mail scanned for viruses]

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.