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[Reply #15 on:]

Personally I prefer to use a FET - no resistor needed at all then  

Me too. My favorite $1 transistor:
http://www.sparkfun.com/products/10213

All though it is a good safety idea to add a 10k resistor between gate and source, in case arduin power is turn off but device power is still on, that will ensure mosfet stays off.

Lefty

[Reply #16 on:]

Personally I prefer to use a FET - no resistor needed at all then

Sometimes true... but they can also be a lot more finicky when working with TTL logic levels.

Poor transistors... they get no love anymore.  If you talk to them nicely (bias and buffer them them nicely with resistors) they are really quite sweet.

Some good advice: I feel you can never have too many 0.1uF Caps or  1K, 10K, 4.7K resistors, or 2N2222A, 2N2907, 2N7000's in your parts bin.

N-Channel MOSFET 60V 30A

is overkill for nearly everything I do.  A 2N2222A with one or two resistors can accomplish so much and do it cheaply.

[Reply #17 on:]

use a FET - no resistor needed at all then

Except of course to protect the arduino from over current when it charges the discharged capacitor and drives into an effective short circuit for a small fraction of the time. 100R should be enough to keep it safe.

[Reply #18 on:]

What is the gate capacitance of a mosfet gate?
If its less then say, 0.01uf, i would not worry about the spike too much.

[Reply #19 on:]

What is the gate capacitance of a mosfet gate?

Depends on the specific mosfet and is given in the device datasheet. Larger current ratings have larger gate capacitance.

Lefty

[Reply #20 on:]

Even if a gate is 1000pf, it will not blow the pin unless you drive it REALLY fast.

[Reply #21 on:]

IRF510 is rated at 100V 5A and has a gate capacitance <200pF.

Rise and fall times are a few tens of nS.

I've driven these directly from an output pin with not the slightest hint of a problem.

[Reply #22 on:]

I've driven these directly from an output pin with not the slightest hint of a problem.

You won't have until they fail. A short circuit is a short circuit no matter how brefley it occours and it is exceeds the absolute maximum in the data sheets.

[Reply #23 on:]

You definitely need a resistor there in case the MOSFET fails. Because they very often fail with the gate short-circuiting to one of the other pins. The result is that your high-power motor or light or heater could be effectively wired directly from high voltage to the Arduino's pin, unless you have a protective resistor.

I've also found I need a gate-source resistor like Retrolefty suggested. Even the brief time a pin is high-impedance during a reset cycle was enough to fry one of my MOSFETS And they switch on so much current so quickly it induces spiky voltages all over the place, I had to add some capacitance just to slow the things down.

I'm very jealous of all you people who have them working wired directly!  :-[ Mine need at least two resistors and a capacitor...