MOSFET recomendation for use as switch

Hello, I'd like to get a TTL level MOSFET for using as switches w/ Arduino. I've never built a circuit using one.

IRLU024: $0.65

IRL2203: $1.50

The more expensive one has a Id=116A, the cheaper one has Id=17A. The 17A rating is more than enough for the stuff I'm doing. I'm just switching current to relays, so this should be plenty.

If there's a better/cheaper MOSFET that I can use, please let me know :)

Would the IRLU2203 be appropriate to use for PWD application on a 12V computer fan?

td(on) Turn-On Delay Time 7.1ns tr Rise Time 74ns td(off) Turn-Off Delay Time 20ns tf Fall Time 29ns

Both are okay.

They are n-channel, logic-level mosfets. Even the small one has a Rds(on) of 0.065 ohm. A low value is good. It means that almost no heat is created inside the mosfet. When I need a mosfet I search on Ebay and buy the cheapest ones that meets my demands.

Don't hesitate to buy a 100A mosfet to switch a 20mA led. That might seem odd, but it is normal.

Sparkfun has selected an mosfet that is cheap and has good specs:

The current rating of a MOSFET is often nonsense, here the TO220 package can’t handle more than about 50A(30A is a more conservative rating) so 115A is utter fiction.

You always select a MOSFET by choosing the on-resistance given the max power-dissipation
you can allow. You also check that the max on-current times the on-resistance is a small
voltage (much less than the gate drive voltage).

The max current rating is almost always a way of saying the max heat dissipation of the
die given an infinite heatsink, which is never useful. See footnote 5 in that datasheet.

So worked example:

Want to switch 10A without needing more than a small heatsink. Thus power
dissipation is limited to 1W or so. Therefore I-squared-R <= 1, therefore R <= 0.01 ohm