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Sorry it's been so long since I responded to this post, just been busy with other stuff...
Look for parts that have Rds(on) specified at 2.5V. this one will have maximum 0.065 ohm resistance with 2.5V gate voltage. beginners make the mistake of looking at the spec for gate threshold voltage, but that is specified at very low current level. this MOSFET will start to turn on at typically 0.9V, but to drive high current you need more gate voltage. a 3.3V GPIO can drive this MOSFET with drain currents over 4A.
I read the article linked by Gordon about MOSFET's and it's actually pointing to Vgs being the specification I should look for... Why would knowing the resistance levels with 2.5V on the gate tell me how much current can be passed through the transistor? Actually, I'm really starting to get confused now after looking at that transistor datasheet you linked, the graph from figure 3 says there would be much much more current than 4A flowing through the transistor with just 2.5 V for Vgs...
Also, for the general purpose BJT, I'm confused as to how much current needs to be applied to the base for the collector/emitter to reach the peak 40V given the saturation levels I highlighted in the OP...
Thanks :-)
user52526-
if we look at the datasheet I linked to, Vgs(th) is the threshold voltage at which the MOSFET starts to conduct current (0.00025A). This threshold is between 0.6V and 1.5V, at room temperature. This is good information in the way that if we have 1.5V on the gate we should be able to run at least 0.00025A in the MOSFET and can expect that the drain voltage will be 1.5V or below. But most of the time our loads are larger than 0.00025A, are they not?
Now, the threshold voltage is not useful information if we want to control a real life load. If the load is for example 2A we will need a lot more gate voltage if we want the transistor to turn on. If we look at figure 3 we could be fooled to expect that we only need 2V gate voltage. But don't forget that this curve is showing only typical behavior and with 5V across the MOSFET. totally useless information, because 2A * 5V = 10W. the MOSFET would expire in a puff of smoke if we tried that.
The only section in the data sheet that gives us any solid promise is the Rds(on) value at 2.5V gate voltage. If we apply 2.5V at the gate and the load current is 2A we know that the resistance will be max 0.065 ohm, if the transistor is at room temperature, because the data sheet says so. the power dissipation then is 0.065 * 2^2 = 0.26W, and we can be sure that the transistor can handle this heat dissipation. the temperature rise will be about 34 degrees.
With bipolar transistors (npn or pnp) the common mistake is to look at the DC current gain value and forget that this is specified at a relatively high voltage drop. for switching loads we have to look at the saturation voltage, and read the values of base current and collector current and the saturation voltage, to get an understanding of what the transistor can do. in most cases an ordinary bipolar transistor is not suited for driving amperes of load current, because the required base current is so high that it is unpractical to drive from a GPIO pin. it can be very difficult to figure out how much current is going into the base with a certain base resistor, because the GPIO output voltage will sag as the pin current increases. and it is many times not well specified what current will be available from the pin. and in the same microcontroller, the drive characteristics for GPIO pins can be different.
tage
you are on the right track. Vce(sat) indicates what voltage drop to expect across collector-emitter with (in this case) 10mA collector current and 1mA base current.
(http://diodes.com/datasheets/ZXMN2F30FH.pdf)
if you want to control a transistor with a GPIO pin, have a look at n-channel MOSFETs such as ZXMN2F30FH. there are dozens of part names to choose between. Look for parts that have Rds(on) specified at 2.5V. this one will have maximum 0.065 ohm resistance with 2.5V gate voltage. beginners make the mistake of looking at the spec for gate threshold voltage, but that is specified at very low current level. this MOSFET will start to turn on at typically 0.9V, but to drive high current you need more gate voltage. a 3.3V GPIO can drive this MOSFET with drain currents over 4A.