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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;

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;

• N-Channel Logic Level MOSFET
• STP36NF06L

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.

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Comments (3)

• 2010-02-14 11:40:49 | jotsna  - thanks

  many many thanks! been looking for this!

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• 2010-02-27 21:03:30 | mischike

  Hi! Thanks for the great summary!

  I want to use a pic18f452 to switch a coil at 5 kHz. The mentioned FET would do the trick, as I need 15 V and 5 A. My question is: will it handle the 5 kHz switching frequency, or will I need a FET driver for it (if so, which would you recommend?) and what diode will I need connected parallel with the load?

  cheers,
  Michael.

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• 2010-02-28 11:09:07 | Graham

  Hi Mischike,

  If your using a N-Channel Logic Level FET then it should be fine :)

  Depending on the FET's properties (on-time, on-resistance etc), it may generate some heat. If it does, then put a heat-sink on it if possible - or get a faster/better FET

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