Re: PMOS in parallel with NMOS

On May 8, 4:20 am, rickman <gnu...@xxxxxxxxx> wrote:

Before I bother to address the design issues you present, I want you
to go back and reread the last couple of messages you posted and tell
me exactly what circuit you have described.  I only see described a
pair of transistors with their B-E junctions connected directly to the
power supply.  I have no idea if you are talking about an emitter
follower configuration or a common emitter configuration.


Take a PNP bi-polar transistor.
Connect 0 volts directly to the base.
Connect 5 volts directly to the emitter.
Connect the anode of the LED to the collector, and the cathode of the
LED to ground.

Now obviously, in everyday life, you put a resistor going into the
base, and also a resistor in series with the LED. Why? To stop:

1) Damage to the transistor from having too much base current
2) Damage to the LED from having too much current thru it

Regarding the LED, well a friend of mine has told me of experiments
where people flashed a normal LED with as much as an entire ampere,
and it worked fine because the duty cycle and the pulse width were
sufficiently low that the LED didn't get damaged. This is quite easy
to see if you take a green diode; if there's too much current, it will
glow yellow. On my own board, my green LED's stay green.

It seems quite conceivable to me that I'm experiencing the same thing
with the transistor, i.e. I'm putting a massive current thru it but
it's OK because the pulse width and duty cycle are low enough that it
won't get damaged.

Another thing I'll look into is the current limit on the
microcontroller pins -- specifically, what happens if you try to draw
too much current. Maybe the microcontroller will die, or maybe I'll
just get an output voltage less than 5 volts. Who knows? I'll look
into it.


However, if you
think the output capacitance of the MCU pin is limited the current to
the transistor, whew! you have a lot to learn.


I'm sure I could find a sufficiently low pulse width and duty cycle
that would make ths happen. Whether this pulse width and duty cycle is
within the MCU's capability, I don't know.


 BTW, what is the speed
of your multiplexing?  Is it above 100 MHz?


There's 16 columns. Each column stays lit for 220 microseconds. The
shifting time is neglibile compared to the 220 microseconds, so it
takes about 3.5 milliseconds to perform one full flash. That gives a
frequency of about 284 Hz.


 The rise time of a
typical I/O pin on an MCU is on the order of a few ns.  So the
multiplexer would have to be running seriously fast for the output
capacitance to have any effect on it.


Noted, that answers my question above ^


Have you tried reading some books on embedded system design?  Or are
you planning to learn it all on your own from first principles?


None specifically on embedded systems, just more generic elecronics.


If you go back to the MOSFET parts, this circuit will work ok.  The
gate does not need current drive.  When the MCU is not driving the
gates can be pulled to the mid voltage point with a pair of resistors
and you can pick MOSFETs with high enough threshold voltage that there
will be no chance of either LED being on, as long as you use an
emitter follower circuit.  You do still need a current limiting
resistor in the drain leg.


I'll look into it, thanks.


If you think all of this is unneeded, then please explain why the MCU
I/O pin burned out when the LED didn't have a current limiting
resistor?  And no BS.  Either figure it out so that you understand it
and can explain it, or don't bother replying.  The MCU is not burning
out why you think it is


Each LED gets flashed as follows:
Pulse width = 200 microseconds
Duty Cycle = 1/16

The board works perfectly when set like this.

However, if I increase the pulse width to 300 microseconds, the board
dies. Perhaps it's the microcontroller that's being killed, perhaps
it's the shift register that's being killed. Maybe even the
transistors, I don't know.

Thinking about it logically, what exactly changes when I increase the
pulse width? Well, current flows for a longer amount of time. The
significance of this? Well, current flow produces heat, so maybe too
much heat is building up for the duty cycle to compensate for. I don't
know, I'll look into it.
.

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