# How hot will a mosfet get.

In project I will be going over calcutations.
To calculate how much heet / power the mosfet needs to handle you have to do som simple calculations. But before you start you should get the basic statistics of your mosfet. Personaly I often use the IRL7833 mosfet. That mosfet has the folowing charactaristics: Max: 30V, Rds on 3.8m ohms. Those are the info we need for our main calculations.

Now the first thing you need to know before you start is how many amp's your motor draws? If you don't know then don't be to sad :-) Not many people know this. But I can inform you that the typical airsoft gun draws around 15-25 amps of power, and has a peak right when the trigger is pressed at around 40-75 amps. So you can just use these values. I will later on in another project show you how I found these vallues.
So now it's just basic math left:
The formula that you should use is: Resistance times current squared equals power.
R*I^2=W or Ohms * Current * Current = Watts

So for the IRL7833 the calculation will be:
Peak value: 0.004 * 75 * 75 = 22.5 Watts
Average on: 0.004 * 25 * 25 = 2.5 Watts

Thease values meens that right when the trigger is pulled the mosfet wil have to handle 22.5 Watts of power. Then when the motor has speed up the motor draw drops to 25 (or less) amps, and there by only generating 2.5 Watts of power in the mosfet. So remember these vallues..

Then we can go to the mosfet datasheet again and find the "Ambient to junction" temperature. As you can see it is listed as 62 Deg C/W meaning that the mosfet will rise by 62 degrees celtius for each watt of power ..

So knowing this we can calculate the temperature:
Average on: 2.5 W * 62 C/W = 155 Deg. <- Yes that's HOT On the limit of what it can take :-(

So something has to be done.
Basicly that meens that we need to help the mosfet get rit of the heet. And one way of dooing this is by adding a screw to the mosfet connection point. Just like you may have seen me doo in the mosfet page. This will help the mosfet get rid of some of the heet. But an eaven better thing to doo is to add a heet sink.

I have as an example found these heet sink's They are listed to be able to lower the dissipation temperature down to 18 Deg C/W ...
So if you add one of these the calculation would be:
Average on: 2.5 W * 18 C/W = 45 Deg. <- We are good...

A heat sink like that is a bit more than what we need but it's deffenetly reccomended to keep the mosfet BELOW 100 deg C !!!
To prevent damage

That's is .. Now you know how to calculate the power usage :-)

If you want to se the calculations done in a video, watch my channel on youtube.