Re: PMOS in parallel with NMOS

On May 7, 10:38 am, Tomás Ó hÉilidhe <t...@xxxxxxxxxxx> wrote:
On May 7, 3:24 pm, rickman <gnu...@xxxxxxxxx> wrote:

I put 5 V on the base and 0 V on the emitter for the NPN.
I put 0 V on the base and 5 V on the emitter for the PNP.

I have no idea what you are talking about. Does your circuit only
consist of the transistors and a power supply??? What you have just
described will blow the transistors. The B-E junction will only
support about 0.7 volts. Trying to put 5 volts on this junction will
burn up the transistor.

Of course normally one puts a resistor on the base of a bi-polar
transistor in order to limit the base current, but because the
transistor is being used to power a multiplexed LED with a one
sixteenth duty cycle, I tried taking out the resistor and it worked.
If I increase the duty cycle though, the microcontroller dies. Perhaps
it's the output capacitance of the microcontroller pin that prevents
the current from rising too high before the display multiplexer moves
on to the next LED.. ?

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.

Either circuit has current gain as the transistor provides. So I have
no idea how the MCU can limit the current in the LED. More likely the
LED current is limited by the gain of the transistor. 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. BTW, what is the speed
of your multiplexing? Is it above 100 MHz? 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.

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

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.

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.
.

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