Mosfets...WHOA

OK, Thank you for reading first and foremost. I am a newbie at all of this electronics and arudino, but I am very interested and have a lot to learn.

I am trying to use a mosfet for a project, 12v light bar...another post if your interested.

I am having difficulty understanding how I go about determining what mosfet I need to use.

I know there is drain, source, and gate. My arudino would be connected to the gate, to turn the mosfet on and off. Source is battery, drain is the light....

my question is this. Arudio pin has 40mA out and 5v max.

My Lightbar draws about 2A per light at 12v.

I have read that even though the Arudino has 40mA/5v the mosfet/light could potentially make it provide more and damage it? Draw more then it can handle? I surmize this is controlled by a resistor on the pin.

so my questions.

how do i take all of this information and figure out my numbers?

  1. Protect the arduino (already fried one), how do i determine the right resistor to use? to little and its moot, to much and the light will suffer or mosfet won't trigger?

  2. Mosfet, how do i determine what mosfter I need? how much does the gate need to activate?

My general idea is thus:

gate needs to be around 3v (don't wanna make it max at 5v) and less then 40mA? source needs to handle 12v as that is what power source is at.

is this correct? any ideas? WHERE CAN I GO LEARN IT.

I am struggling online trying to understand, i think I may be trying to run before I learn to crawl. I have done some ohms law on it, but my numbers resulted in my arudino frying.

Thanks for the read.

You need to use a N-channel "Logic level" FET. The source is grounded, the drain to the negative of the light bar and the positive of the light bar to your +12 V.

The gate is fed from an Arduino output by a 470 Ohm resistor. You put a 10k resistor between the same Arduino output and ground to keep the FET switched off until the Arduino boots and defines its output.

The gate is fed from an Arduino output by a 470 Ohm resistor

Why ? It's a fet. It doesn't need a gate resistor. I've never used one and I've used fets hundreds of times.

raschemmel:
Why ? It’s a fet. It doesn’t need a gate resistor.

It is protecting the Arduino pin against excessive current.

The resistor can be lower value than 470 ohm - as low as 100 should work (and will work a little better when PWMing it - you want the lowest value resistor you can, if you're PWMing it, since you want it to switch as quickly as possible, and minimize the time it spends between on and off), just barely keeping the current in spec (thanks to the ~40 ohm internal resistance of the pin driver).

I've never actually heard of someone blowing an output pin like that though - and as noted above, lots of people omit the gate series resistors. It is best practice to include them, though, particularly if the fet is larger. The gate acts like a capacitor, so the peak current can be higher than you'd think, even though there's no current flow when not switching.

raschemmel: Why ? It's a fet. It doesn't need a gate resistor.

Further discussion on that very subject, here, by all the usual suspects :)

Thanks for the link.

raschemmel: Why ? It's a fet. It doesn't need a gate resistor. I've never used one and I've used fets hundreds of times.

So you regularly pulse 100mA through your pins rated at a maximum of 40mA then... power MOSFETs are a large capacitive load, not what a logic output is rated to drive direct.

I have a 50 mohm shunt. I'll measure the gate current. I probably don't use power mosfets the size you ard talking about. The largest I've used at home were rated for 30 A. What swiitching speed causes thd high gate current pulses ? (or does it matter?)

What swiitching speed causes the high gate current pulses

You are charging a capacitor from some current source (In this case an Arduino digital output).

At the instant you start charging, the capacitor voltage is ZERO.

SOMETHING limits the current, either the internal driver inside the Arduino, or some resistor you add.

HERE is: an example of driving THIS: Power NFET with an Arduino and a 220 ohm series resistor.

It takes about 500 ns to turn the FET on. Not a problem with slow PWM. If you were going to switch a LOT of current in a bigger FET you would need a gate-driver IC after the Arduino

The high current pulses will happen each time the gate is charged and discharged. I would assume its more destructive the frequency and duration its done (the gate capacitance of the mosfet).

Even if you don't use a gate resistor to protect the arduino a low value gate resistor can reduce ringing. They can be turned on/off too fast. Or with Fets in parallel.

That being said, I have many circuits with low gate resistors (5-50 Ohm) and used those with no issues even though it out of spec with the pin current max. Those mosfets have a fairly low gate Charge (less than 35nC) and switch at less than 16Khz. Even the cheapest chinese brushless motor controller uses gate resistors. They would not put those on if they didn't have to.

For small mosfets with gate charge less than 10nC I have had no issues omitting the gate resistor.

Just to make sure I understand,

Given ChargeGate = 10nC

AND Q =C*V

V = 5V (arduino output voltage)

then ,

Cgate = 10e-9/5v = 0.000000002 F (2 E-9 F) = .002 uF (2 nF)

Correct ?

If so then what scenario (C = ? V = ? ) would cause this:

So you regularly pulse 100mA through your pins rated at a maximum of 40mA then… power MOSFETs
are a large capacitive load, not what a logic output is rated to drive direct.

Since we are talking about an arduino GPIO then V is Given as 5V

If I is Given (by MarkT) as 100 mA (0.100 A)
then RGate = V/I = 50 ohms

Correct ?

@Terryking228,
Can you explain how you created that simulation from this datasheet ? (which parameters did you use ?)

I see Input Capacitance = 1350 pF
Where did you get the 10k from ? (I didn’t see Input Resistance)

Look at the datasheet for a MOSFET driver. Those things drive a staggering number of amps into the gates, but only for a nanosecond.

This only draws 1.5A

Paul__B:
You need to use a N-channel “Logic level” FET.

To expand on the “logic level” part you’re looking for a mosfet that reports a VGS(th), the threshold voltage at which it starts to turn on, around 2V. The datasheet should also show a graph of current versus VGS, typically labelled “transfer characteristics”, and at 5V on that graph the current should be well beyond whatever your requirement is.

Another way to look at that is that the datasheet should report an rDS(on) (on resistance) in the tens of milliohms for a voltage at 5V or lower. The lower that resistance the better. Some datasheets will show a graph of that rDS(on) vs VGS, and that graph is very helpful, but that graph tends to be less common.

Lower resistance isn't always better though - as lower on resistances mean higher gate capacitance, which might mean slower switching (if not using a high current gate driver). Typically its worth looking at the switching speed when using PWM (if not, its not really an issue).

If using PWM, especially at a high frequency, then switching speed and switching losses can come to dominate - its a balance between on-resistance losses and switching losses in that case.

the threshold voltage at which it starts to turn on, around 2V

Again I'll repeat, threshold voltage is not about turning on, that happens at the plateau voltage, not the threshold voltage. The threshold voltage is when the device is basically transitioning from fully off to small leakage currents flowing.

Typical non-logic-level MOSFET has:

Vth = 2--4V (lots of variation in these values, note) plateau voltage ~ 6V Vgs quoted for fully on = 10V. gate drive voltage normally used = 12V

Note that as the gate voltage rises a fraction of a volt through the plateau voltage, the load current increases 10% to 90% or so and the channel and gate charge build up considerably - hence the plateau in the gate charge v. voltage graph.

Its normal to drive the gate at about twice the plateau voltage, hence normal MOSFETs have about 6V plateau, logic-level have about 2.5V plateau. Remember these voltages vary a lot between devices.

Datasheets show a graph of "typical" gate charge v. voltage, indicating the nominal plateau, that's only a guideline, the Rds(on) rating is where to find the gate drive voltage.