Handy Tips - Logic Level FET's
(N-Channel)
Have you considered using a
FET to switch on and off a target device rather then a transistor?
If you haven't, then perhaps you should have a read, as FET's have
many advantages over transistors. The pin-out of a FET is much the
same as a transistor by analogy, consider;
| Transistor |
FET |
|
Collector |
Drain |
| Base |
Gate |
|
Emitter |
Source |
Transistors heat up when
driving large loads because they have a
voltage dropped over them (Vce), and Heat (Watts) = Voltage *
Current. This leads to thermal runaway within the transistor,
eventually driving the device to destruction if not handled
carefully.
FET's are like digital switches, capable of turning on and off
between the Drain and
Source
via a voltage
potential at the Gate. When a FET is on, it usually has a
resistance of less than 0.01 ohm, and when off, its like an
open circuit. Because of the low resistance during the FET's on
state, it can allow large amounts of current to pass through it
without heating up.
FET's turn on by an electric
field, not an electric
current, and in return they have a very high input impedance. With
this in mind, you only need a voltage to turn them on, perfect for
digital electronics.
Similar to transistors, there
are 2 types of FET's, N-Channel, and P-Channel. Depending on your
application, you will need to choose which one suites you. N-Channel
FET's are best used when the FET is switching the earth, as no
drive circuit is required - even if the target supply is greater
then the logic voltages at the Gate.
If you want to control the supply voltage of the target device, have
a look at the
P-Channel MOSFET guide. Consider the below
circuits; (note that only a voltage is required at the
Gate, not current like
transistors);
The above circuits are
examples of how to drive a motor with either a N-Channel FET. The reverse biased diode in parallel with the motor should be used
when ever you are driving inductive loads, but is not required with
purely resistive loads.
One of the FET's greatest
upsides is its massive input
impedance, but this must be treated carefully. The Gate can
and will float high if not
tied down to earth. This isn't an issue while the micro controller
is turned on and the output
is configured as an output in either one of two states (high, 5V or
low, 0V) - its a major issue when the micro is switched off or
starting up. In the diagram below, the switch isolates the
Gate similarly to what would
happen if the control pin was made an input with many Meg-Ohm's of
impedance (note the FET does not
turn off);
To rectify this, a
resistor
(from 100K to 1M) is placed at the Gate, tying the FET to earth;
I haven't delved into any
particular models of FET's as yet, just covering the basics first.
There are different types of FET's available to you out there, but
most require a high Vgs voltages to operate. For micro controllers,
the best type of FET to use are
Logic Level MOSFET's, as
no driving circuit
is required to switch high voltages. They can be directly driven by
5 volts, and some as low as 3
From the hundreds of different
N-Channel Logic Level MOSFET's out there, I use the following;
Finally, keep in mind that
FET's are very sensitive to static, so handle with care. I am yet to
damage one while "hobby-handling" though.
 | Site Tutorial Index |
|  | 16F PIC Examples |
| | 18F PIC Examples |
|  | Handy Tips |
| |  | High Power Switching |
| |  | Serial Comms |