> I am just trying to learn to use them, and was thinking that I could
> start by replacing regular transistors with these. I normally drive
> relays or 7 segment LEDs so current is much less than 1amp or maybe
> even 1/4 amp. I understand the heating during changeing but it should
> be better than a regular npn or pnp transistor, right?

Bipolar transistors are easier to switch quickly than FETs, especially at
low load currents like what you have.  I would stick with bipolar
transistors for switching 7 segment LED displays.

There are basically two advantages to FETs over bipolar transistors.
First, high power FETs have such a low on resistance, that the voltage
drop accross them, and therefore the power dissipation when on, can be
less.  Second, it takes no power to maintain a FET on or off since the
gate essentially looks like a capacitor to the driving circuit.

However, FETs have disadvantages too.  First, they usually cost more.
Second, they require significant gate current to switch quickly.

Overall, your application is better served by bipolar transistors.  Figure
200mV C-E voltage drop in saturation.  At 500mA load current, that's only
100mW.  No problem for a TO-92 case.

> The info I found
> on the web says that fets are faster than regular transistors.

Well, yeah, the FET itself turns on almost instantly as the gate on
voltage is applied.  The trouble is that it can be tricky to apply that
voltage quickly.  That's why there are special FET driver ICs out there.


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