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Help selecting a n-channel mosfet and resister Answered

Hey guys!

Been a while. . .

Anyways I was looking for some guidance on selecting an N-channel mosfet, and also what resistor to use when I'm applying voltage to the gate. So I'm working some long LED strips, and I need the mosfet to be able to handle at least 7 amps, so I was hoping for like a 10 amp mosfet or so. I just didn't want to guy buy the first mosfet I found on digikey, I was just hoping for some cheap good options or popular ones. I needs to be able to do PWM.

Also I don't know where in the datasheets to find the information on what resistor to use on the gate. For instance I see people using anything from a 1K to a 5K resister with a TIP120. How do you select that information? I will be using an 5v output from my Arduino. 

So after searching ebay I found the IRFZ44N mosfet, 55v and 49A. I know this is way over my needed specs but I can get 50 of them for 10 bucks. Steal. Since it's so overrated what problems might I run into if any?

Thanks guys!

What I've found so far:
IRF740 - 400v 10A

Discussions

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HavocRC
HavocRC

4 years ago

Alright gmoon I'm only working with 12v and 7A max. I need help finding a resistor to use on the gate.

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Downunder35m
Downunder35m

Reply 4 years ago

You did not check the circuit I linked, did you? ;)

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HavocRC
HavocRC

Reply 4 years ago

No I did! I read over it 4 times, and didn't see anything about selecting a resister for your mosfet!

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Downunder35m
Downunder35m

Reply 4 years ago

Exactly! ;)
Depending on how you drive it you can use the PWM signal directly.
The resistor just limits the current to the gate and for most applications you actually want the gate to go fully open and close as fast as possible.
If you check the datasheet for y mosfet you can see the required gate voltage.
Often +-20V.
But it is not like a normal transistor as the mosfet operates the gate with current more than voltage.
If the source can provide enough current for the gate the mosfet will not fully open and waste a lot energy in the form of heat.
That is why for some applications people use gate transformers to make sure there is enough current.

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HavocRC
HavocRC

Reply 4 years ago

Now I'm even more confused.

I'm not just using a PWM, I'm also using a HIGH output of 5v. I think you know a lot about the subject and I'm not there yet so you're overcomplicating it for me :D.

You said, "If the source can provide enough current for the gate the mosfet will not fully open." Do you mean if the source can't?

And this is exactly what I'm trying to figure out, where in the datasheet do I found how much current is need to turn the mosfet fully on? If it's dependent on the load then where do I find the formula. I'd just like to have a better reason for using a 1k resister than everybody else does.

Thanks!

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Downunder35m
Downunder35m

Reply 4 years ago

A resistor of 150 - 180 Ohm is usually used in such cases.
It basically limits the current for the logic circuit, like a NE555 timer for example.

Put it this way:
It a mosfet should switch very fast from on to off and back, like with a PWM signal, then you want these changes to happen as fast as possible.
To that the resistor is selected to be quite low or in some cases no resistor at all is used, like for timer outputs.
The signal you provide would be a square wave and the mosfet needs some power to switch.
Depending on the circuit this sudden load will cause the signal to break in, resulting in a not so perfect square wave.
You can check this happening best with an oscilloscope but for the normal hobby projects stick with resistors between 150 and 180 Ohm and you should be fine.

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HavocRC
HavocRC

Reply 4 years ago

Alrighty thanks a lot!

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Downunder35m
Downunder35m

4 years ago

If you only need to switch them on but have no need for dimming: SSR
A SSR (Solid State Relay) only needs a few volt and next to no amps to switch.
So a simple transistor output with a SSR behind can switch 40amps with ease.
In case you want to use a mosfet I always use the IRFP260 when I need serious power levels.
For even higher power the IRF2203 is a good choice.
As for the dimming part:
You can use a cheap LED dimmer as offered with many LED strips and connect the mosfet to it.
Or if you want to go old school use a circuit like this one.

I hope to see an Instructable of it ;)

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gmoon
gmoon

4 years ago

Only to check the max power dissipation for each device. You might think the IRFZ44N can handle both voltage and amperage at max specs, but max power is 94W (not the 2695W you'd get by multiplying V*A).