Re: PMOS in parallel with NMOS
- From: rickman <gnuarm@xxxxxxxxx>
- Date: Wed, 7 May 2008 05:27:47 -0700 (PDT)
On May 7, 12:31 am, Tomás Ó hÉilidhe <t...@xxxxxxxxxxx> wrote:
On May 7, 5:14 am, "robertwess...@xxxxxxxxx" <robertwess...@xxxxxxxxx>
wrote:
That won't work. There will be some voltage leaking from the high
impedance device, and exactly what that is, and how much current there
is, will dictate exactly what the two drive transistors are going to
do.
With the PIC microcontroller, if you put a volt meter across a high
impedence pin, I think you measure about 3 V. Of course, it's behind a
massive resistance, but I think that 3 V might be enough to turn on an
NMOS transistor.
What are you trying to do? Build a high output tristate device? Are
you trying to drive a multi-color LED or something?
I have a bi-colour LED that has 3 pins. Within the LED package there
are two LED's, and they have a common cathode. Their anodes are
separate.
When my microcontroller pin is high, I want it to be green. When low,
red. When high impedence, off.
To do this, I want to use a p-type and a n-type transistor that have a
common gate/base. When the microcontroller pin is high, the n-type
transistor will be turned on and it will put five volts onto the red
anode. When the microcontroller pin is low, the p-type transistor will
be turned on and it will put five volts onto the green anode.
I might end up doing it with bi-polars instead of mosfets...
Your idea won't work with bi-polar transistors. They turn on at 0.7
volts on the base. There is no voltage you can use that will turn
them both off. You *can* however turn them both ON!
I don't think you will get this to work well with 5 volt drive, but
you might. It depends on the characteristic of the MOSFETs. You need
parts that have a threshold voltage that is *much* higher than 2.5
volts. All you need to do is use two resistors to bias the gates to
2.5 volts when the MCU is not driving the circuit. By leaving it
tristated, you are not controlling the voltage at all and the gates
will float which can be up or down or somewhere in the middle.
There is an easier way. Use an LED that has 4 pins and a 3.3 volt
supply. You can put the two LEDs in series, across the power with
neither of them turning on. Use a current limiting resistor from the
middle connection to the MCU pin. The MCU can pull up to turn one on,
down to turn the other on or tristate to let the voltage float with
both LEDs off. To make this work with 5 volts, you need LEDs that
have a forward voltage much higher than 2.5 volts.
.
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