Hello and welcome to AirsoftTech.dk
In this episode I will be showing some of the math that goes into selecting what mosfet to use.
And how much power it needs to dissipate.
To do math we need of cause, a pen, and some paper.
In the past i have been using
The IRL7833 N-Channel power MOSFET
It has some characteristics...
It can handle up to 30 volts
It has an internal resistance of 3.8 milli ohms
It can handle up to 140W of power
This is milli ohms, this is just ohms.
Airsoft motors
Has some characteristics
Current draw, peak value of up to 75 Amps
I will be showing you how i got this value, in another video later.
Average value: It draws 15 to 25 amps.
Right now you just have to trust my words. The peak value is just when the motor starts spinning
We can draw a curve
At time zero the trigger is pressed
What happens is that immediately we get a very high current spike, right when the motor starts spinning.
That spike quickly drops down.
the current then fluctuates a bit
Then at some point in time ...
... we get a shot. And the bb comes out the barrel.
The spring releases and then there is no load on the motor and the current drops almost to zero.
Some times even below zero
Then the motor starts spinning up
doing down, and then we have the same cycle again
untill a new shot accures
And then again we have the same behaviour
And for each shot we have a bit less of a spike
untill we reach a point where the cycle just reppeats it self ...
over and over and over for each shot ..
The initial peak value will be roughly 40-75 amps.
Then we take the average value of all the values in this area...
You will get an average that will be:
15-75 Amps
Teaser: This graph is what I will be generating in another video later on.
We have the values from the mosfet
Now what we want to know is how many watts does the mosfet have to handle
The mosfet has an internal value of: Lets round up to say 4 milli ohms
To calculate this we use the formula saying:
Recistance times current squared equals power. R*I^2=W
The resistance is 0.004 ohms
Because mosfet's almost never get's to the absolute lowest value it's okay to always round up.
Times the current squared of the peak, that is 75 amps
If my calculation is correct that is 22.5 Watt of power
For the average value, let's say 25 amps to be safe.
because these values will of cause depend on what airsoft gun you use.
When we use the 25A value, the calculation is basicly the same. 0.004 times..
25 amps squared ...
Equals 2.5 Watts of power ...
So the mosfet alone will have to disapate 2.5W of power
And power equals heat .
The peak value is just initialy
So it's only for a very short amount of time that we need to handle that much
And we are talking milli seconds here
So the most correct thing to do is to use the average value ...
... And assume that we need ti handle that ammount of power.
Then what we do is to go to our mosfet datasheet
For the IRL7833 we have another value listed
And that is the "Junction to ambient temperature "
that value is listed as 62 deg centius per watt
What this means is that:
This mosfet will rise 62 deg C per watt of power
To estimate how hot the mosfet will get
We calculate the avreage heat it has to disipate and calculate the...
2.5 watt of power
Times 62 deg C per watt
equals 155 deg Celsius
So that is very hot
But as i just showed you on my curve:
When running it will in average dissipate that much
But we also have periods with very low values and ...
.. each time you stop shooting you will have periods where you don't touch the trigger ...
... and then the heat can be dissipated..
But is you just pull the trigger and continue shooting...
the mosfet will get very very hot.
One thing that you can then do is to add a heat sink.
it is basicly a peace of metal that you attact to the mosfet that will then dissipate some of the heat.
I have found a heatsink where the specification say that it can dissipate heat down to 18 deg C per watt
that means that instead of the 62 deg it will lower it to 18 deg C per wat
using that value instead you can calculate that the average value will be:
2.5w times 18 deg C
Equals 50 deg C
That is hot to the touch but still acceptable...
So with the heatsink we should have no problem running the gun continuesly ...
for a verry long time, and it will not be the mosfet that gives up ...
If we do not have a heatsink we need another way of getting the heat away...
One thing that we can do is to add thick wires to the mosfet and have that dissipate some of the energy.
Another thing that we do is:
When we make the mosfet it looks kinda like this:
We have a mounting hole up here and the 3 legs down here ...
we remove the middel leg so that there is some more spacing between the 2 legs we need ...
and we use this up here to add the wire
so we have the incomming wire up here, and the outgoing wire down here.
We add a huge fat connector here with a screw on it, that we bolt to it.
As you may have seen in my other videos this screw connection here
This screw and the hole plate here will transfer the heat to the wire
The same will happen at the source pin
The wire is connected with a big fat lump of solder to help transter som of the heat to the wires instead of the mosfet.
so all of this area and the wires as well will be used to dissipate the heat
that of cause helps allot
so that we will not get to 150 deg, but ...
... instead we will get a lower temperature of ...
... perhaps we will get down to 40-50 deg per watt...
... so we will get perhaps 100-120 deg max ...
... and the mosfet should be able to handle that ...
... But the best thing to do is of cause to add a heat sink...
And with a heat sink on you will have no problem what so ever :-)
For more infomation >> Ethnique Punch - Dravdan (Official Video) - Duration: 3:46. 
For more infomation >> Aprender Colores y Números con el Cráneo - Video Educativo para Niños! - Duration: 2:09. 
For more infomation >> Video: Crisp day, cooler along coast - Duration: 2:20. 
Không có nhận xét nào:
Đăng nhận xét