Quote:
Originally Posted by rcheshire
The freetronics smd mosfet runs a little warmer than the IRF2804 I have, although freetronics advertise it as logic level Vgs 4.5v - I think the rds on is not as low as the IRF.
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Well I have another gotchya for you. Firstly remember the 100 and 750 ohm resistors mean you never actually produce the logic level voltages at the gate You get up to 4.41V at the gate. Secondly the spec in the datasheet for Vgs_thresh is usually specified under the condition of Vgs = Vds. I.e. the value for Rds_on is only true if you're above the threshold voltage and you're putting the same voltage through the gate as across the MOSFET.
The IRF2804 has a fairly well defined model. A spice simulation shows that when you put 5V into your circuit Rds = 0.075ohm when Vds=5V which is good, BUT Rds = 0.474ohm when Vds=12V as in your case. At steady state with your circuit if the MOSFET were IRF2804 you'd burn about 480mW into the MOSFET. If you removed the 100 ohm series resistor the power in the MOSFET drops to 170mW.
If you're aiming to reduce power consumption you may want to look for a MOSFET which saturates more easily. Or another option is switch the gate with 12V instead of 5V using a gate driver like the one I attached. That reduces your 470mW to 30mW (actually 70mW given the 3 other components which now dissipate power too). Just be aware if you do do something like this that it inverts your output (i.e. MOSFET is ON when Arduino output is OFF).