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     I bought a 12v, 8 amp power supply for my project to power three servos (moving randomly) and three dc 12v motors (moving in one direction).  The dc motors are powered directly from the 12v power supply, the servos, from dc to dc converter at 6v. The servos run as planned but only two of the motors at a time will move very weakly. When all three are on line, the mosfets (520) become hot.
    The project needs an addional two steppers, some LED's, and a speaker. Does this mean I have grossly underestimated the total current necessary?

Keith
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Does this mean I have grossly underestimated the total current necessary?

You haven't told us what the current draw of the motors is...check their data sheets for stall current, add it all up, and you'll be able to answer your own question.

Also check that the 12-6V DC-DC converter can handle the servo current.
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the mosfets (520) become hot.

Are the mosfets rated for 'logic level' service. That is will +5vdc gate voltage fully turn on the mosfets? If they are not logic level mosfets then that could be your problem by itself.

Lefty
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Sorry, I am inexperienced and don't know exactly what parameter to look for in this data sheet http://www.datasheetcatalog.org/datasheet/fairchild/IRF520.pdf. Would you please advise whether it is a "logic level" mosfet?   The motors were salvaged from office equipment. I will try to locate a data sheet. 
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Sorry, I am inexperienced and don't know exactly what parameter to look for in this data sheet http://www.datasheetcatalog.org/datasheet/fairchild/IRF520.pdf. Would you please advise whether it is a "logic level" mosfet?   The motors were salvaged from office equipment. I will try to locate a data sheet. 

It is not. The specification I look for is at what gate voltage is the minimum Ron specified at:

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Drain to Source On Resistance (Note 2) rDS(ON) I D = 5.6A, VGS = 10V (Figure 8, 9) - 0.25 0.27 Ω

Yours uses 10 volt gate/source voltage for minimum Ron value which an arduino output pin cannot provide.

A true 'logic level" N-channel power mosfet will have at rating like this:

http://www.sparkfun.com/datasheets/Components/General/RFP30N06LE.pdf

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Drain to Source On Resistance (Note 2) rDS(ON) I D = 30A, VGS = 5V, Figure 9 - - 0.047 Ω

Showing minimum Ron value can be reached with 5 vdc of gate/source voltage applied.

Lefty
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Thanks for the quick response. That must be the problem. The motor is drawing .17 amps at max efficiency and 1.06, stall so an 8 amp supply would seem more than sufficient to run three, plus the servos, and steppers? Do you think a tip31http://www.datasheetcatalog.org/datasheet/boca/TIP32C.pdf would be a better choice?
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Thanks for the quick response. That must be the problem. The motor is drawing .17 amps at max efficiency and 1.06, stall so an 8 amp supply would seem more than sufficient to run three, plus the servos, and steppers? Do you think a tip31http://www.datasheetcatalog.org/datasheet/boca/TIP32C.pdf would be a better choice?

Should work, but being a NPN bipolar transistor you will require a series base resistor from the arduino output pin and the base of the transistor. 200 ohms should work.

Lefty
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IRF520 is rather underspec'd for current and way over-spec'd for voltage...  These days devices for this
sort of load can be such a low Rds(on) that heatsink is unnecessary (much more convenient) - for instance
a 20 or 30V device with Rds(on) of 5--20 milliohms that's logic-level would be a far better choice.  100V MOSFETs
have a lot higher on-resistance than 30V devices, all else being equal.

Tip: never choose a MOSFET by its current rating, always choose by Rds(on) and calculate the power
dissipation - and decide if a heatsink is needed.  The max current rating is a thermal limit almost always,
it'll be running extremely hot if you go near that value, even with a heatsink.
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