Hi All,
There seems to be conflicting replies to people on this forum.
Do you need a gate resistor or not.
I plan on using a BSS138-7-F Enhancement mode MOSFET. It has a threshold voltage of 1.2V, Rds of 1.4 ohms. It will just switching a 2K7 resistor into circuit so no large currents or voltages.
There was, not long ago, a multipage thread on the subject. Try the nifty new forum search tool to see if you can find it. I use a resistor.
The use of the resistor has nothing to do with the voltage or current being switched. It has to do with limiting the high current spike from the Arduino output while charging the gate capacitance.
It's always "good practice" to include the resistor and "bad practice" to leave it out.
Since the current flows when switching, if you are not switching the MOSFET on & off frequently you can probably get-away without a resistor.
There was, not long ago, a multipage thread on the subject. Try the nifty new forum search tool to see if you can find it. I use a resistor.
Fifteen pages, to be precise. Myth Busters 3 – Myth: “You must have a gate resistor” - General Electronics - Arduino Forum
You left out an important part of the equation. What processor are you using to drive the mosfet?
Given the low gate capacitance of the BSS138 and driving it with an AVR processor, you don’t need a resistor. Read the thread. You may or may not come to the same conclusion. But hey, it’s only a resistor. If no one can give a good reason to use one that can be quantified, it’s a very hollow argument. Hand waving and spouting theories isn’t engineering.
But, that’s just my opinion and everybody’s entitled to at least one.
I am using an ATTiny3217. It will be switched once per power cycle. Every time the bike is turned on it will switch a resistor to the ECU. Only 1 of 3 resistors will be switched depending on previous settings. I am limited in board space but if I can use 0603 resistors I might be able to get away with it. I need to do some power calcs.
To answer your question.... No you do not need a resistor for basic functionally.
But there is nothing wrong about leaving the two above resistors out. As you have found there is some people who are emotional about this topic. Me, I've been using MosFets for a long time and recommend at least the series resistor but it only increases my comfort level in the event of a failure.
Well the finished product will be encapsulated in epoxy so if the MOSFET fails its a bin job.
If I did use one what sort of value would you recommend.
I use 180 to 220 Ohm's for a 5V output. The object is to limit the current to around the recommended max (and well under the absolute max) current from a pin while charging the gate capacitance as quickly as possible. For an Uno the recommended max is 20mA (absolute max 40mA). 5V / 0.02A = 250 Ohm's. The next lowest standard value is 220 Ohm's. 180 gives 27mA, a bit over recommended, but still under absolute max.
Check the data sheet for your processor to find the recommended and absolute max currents.
As speed is not a constraint I might even go for 270R or 330R.
It is better to turn the MOSFET as fast as you can and still keep the current in limits. The less time that the MOSFET is in the active region (where Rds(on) is higher) the less it will heat up.
All of this is discussed in the thread that I mentioned and WattsThat found the link for.
DVDdoug:
It's always "good practice" to include the resistor and "bad practice" to leave it out.Since the current flows when switching, if you are not switching the MOSFET on & off frequently you can probably get-away without a resistor.
For switching frequently (i. e. PWM) it is essential to have as strong gate drive as possible. For occasional switching you can probably get-away with a resistor.