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Author Topic: Series resistor to gate mosfet needed ?  (Read 1204 times)
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I'm pulsing an IR led at 1 A with PWM for a short period of time. To do this I use an N-channel (power) mosfet

(http://nl.farnell.com/jsp/search/productdetail.jsp?SKU=1017789).

My setup:



At the moment I use the setup in figure A, but I was wondering if the setup in figure B suffices. My train of thought: a gate doesn't draw any current, right? So no delimiting resistor is necessary. And since the Arduino already pulls down the gate nicely to ground, I don't need the pulldown resistor as well. Is this correct ?
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A gate does draw current.  Just not all the time.

A gate is effectively a capacitor.  When you transition from LOW to HIGH, current will flow into that capacitor to charge it up.  That is called the "inrush" current.

Similarly, when you transition from HIGH to LOW the current will flow out of the capacitor into the pin you are driving it with.

You need to limit the flow of that current during the inrush / outrush times of the capacitor charge/discharge to a level that is safe for the pin to source / sink.

100Ω is a good value, as it limits the current enough, and doesn't slow down the charge / discharge of the gate enough to really worry about.
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My train of thought: a gate doesn't draw any current, right?

A MOSFET gate has capacitance so it can draw a LOT of current for a short period of time when you start to charge it up (or drain it).

So...you need about 150 Ohms between Arduino and gate to prevent too much current coming out of the Arduino pin during that time.

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100Ω is a good value, as it limits the current enough, and doesn't slow down the charge / discharge of the gate enough to really worry about.

100Ω allows 50mA to pass at 5V. That's too much for an Arduino pin.

125Ω is the absolute limit, 150Ω is a better/safer choice.

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100Ω is a good value, as it limits the current enough, and doesn't slow down the charge / discharge of the gate enough to really worry about.

100Ω allows 50mA to pass at 5V. That's too much for an Arduino pin.

125Ω is the absolute limit, 150Ω is a better/safer choice.
True... I have become so used to working at 3.3V these days, I forget that little extra voltage on the Arduino.
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Never knew the input functions as a capacitor, thank you for the explanation, I'll change to 150 Ohms.

Do I still need the pulldown resistor ?
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Pulldown is good, yes, as it stops the gate floating when the Arduino's IO pin is set to input (which it defaults to at power up), or is disconnected / powered off in any way.
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Thanks, so I'll keep them smiley
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100Ω allows 50mA to pass at 5V. That's too much for an Arduino pin.

A 100 ohm series resistor plus the internal resistance of the output pin (about 25 ohms) limits the peak current to about 40mA.
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Never knew the input functions as a capacitor, thank you for the explanation, I'll change to 150 Ohms.

Its not exactly like a capacitor - the charge is partly capacitive, and partly used to balance the
channel region forming between source and drain.  Higher drain currents require larger amounts
of charge on the gate.  In fact the gate voltage has a plateau as the drain current increases
during the turn-on process - charge is needed to enlarge the active channel.

For fast switching of large currents a MOSFET driver chip is usually used and these provide between
100mA and several amps depending on the size of device you intend to drive!
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