Re: PMOS in parallel with NMOS

Tomás Ó hÉilidhe <toe@xxxxxxxxxxx> writes:

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.


Be aware that leds used like this can easily fail after a few weeks or
months. Just because it works for hours or days does not mean it is
safe. (It's not just theoretical, I have personally seen this).

--

John Devereux
.

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