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[Reply #15 on:]

P=IV = 6W, heatsink recommended I think!

Yes, I agree with that. Same time it's applicable to any BJT, Darlingtons just have saturation little bit higher than others.
 So the trade is TIP120 + heatsink  or MOSFET + TC426. 

No, the trade for medium current (a small number of amps) is TIP120 + heatsink or logic level mosfet + series resistor. You only need a mosfet driver if you are switching much larger currents than the TIP120 can handle, or are using fast PWM (which the TIP120 wouldn't handle well anyway).

[Reply #16 on:]

You only need a mosfet driver if you are switching much larger currents than the TIP120 can handle, or are using fast PWM

Not quite. Logic level P-channel MOSFET has high Gate Threshold Voltage (example, SFE FQP27P06) and again you need a driver IC (charge pump voltage multiplier) for 3.3V uCPU.
 Darlington TIP127 has no trouble with voltage as low, as 2V at the base.

[Reply #17 on:]

Darlington TIP127 has no trouble with voltage as low, as 2V at the base.

Actually 2.5V max @ Ic=3A according to the datasheet. If I was only switching 3A, I would probably use one of these http://www.farnell.com/datasheets/1648222.pdf which only needs 1.8V.

[Reply #18 on:]

Actually 2.5V max @ Ic=3A according to the datasheet. If I was only switching 3A, I would probably use one of these http://www.farnell.com/datasheets/1648222.pdf which only needs 1.8V.

I was referring to P-channel ,  when I need high side switch in low voltage (< 3.3V) design.

[Reply #19 on:]

I was referring to P-channel ,  when I need high side switch in low voltage (< 3.3V) design.

What, switch 3.3V @ ~3A high side using a PNP darlington? Even if you did have a high current 3.3V device, the Vce(sat) of the darlington would render it pretty much useless at that supply voltage.

[Reply #20 on:]

Figure 2   http://datasheet.octopart.com/TIP127-Fairchild-datasheet-21899.pdf
shows saturation voltage below 1.5V for current less than 2.5A. That may be more than enough for DC motor, to start moving, and when it gets speed current 'd drops, so saturation would not be a problem at all. 

[Reply #21 on:]

"Saturation voltage" definitely not a problem if I use http://www.nxp.com/documents/data_sheet/PMN34UP.pdf instead.

[Reply #22 on:]

I wish I had equipment to solder SMD components, microscope, re-flow oven etc. Or I had a job with all this stuff at my convenience. But than again,  'd I have time to play with arduino and visit this forum? 
 
 

[Reply #23 on:]

Yes, it's a shame that a lot of the best components are only available in SMD packages. I bought a batch of SMD-to-DIP adapters (readily available from Sparkfun, Futurelec, eBay etc.) a while ago so that I can use them with breadboards and stripboard for prototyping. I don't use a reflow oven to solder them, just a hotplate. The larger SMD devices can be soldered using a fine-tipped soldering iron.

[Reply #24 on:]

I wish I had equipment to solder SMD components

Most smd parts (lqfp, qfn, soic, etc.) are fairly easy to hand solder.