# transistor/mosfet help???

Discussion in 'The Projects Forum' started by mhastie1234, Feb 22, 2012.

1. ### mhastie1234 Thread Starter New Member

Feb 10, 2012
29
2
I am building an h bridge running a 12v 5amp motor. What are the equations for determining to proper transitor or mosfet to use. Just wanted to know the math behind choosing one vs the other. I have looked at this link http://www.mcmanis.com/chuck/robotics/projects/esc2/FET-power.html but i find it hard to follow. Could someone simplify it for me. I was thinking of using tip147/142. I would like to know how much current they can handle in free air. Thanks

Last edited: Feb 22, 2012
2. ### Audioguru New Member

Dec 20, 2007
9,411
896
Each TIP147/TIP142 has a maximum saturation voltage loss of 2V at 5A. Then each one heats with 10W and will need a fairly large heatsink.
The motor will get only 8V.

The "typical" saturation voltage is only 0.9V for each one. Then the heating is less and the motor gets 10.2V.

mhastie1234 likes this.
3. ### mhastie1234 Thread Starter New Member

Feb 10, 2012
29
2
Thanks Audioguru. Could you point me in the right direction and tell me what to look for when choosing a transitor / mosfet

4. ### dumle29 Member

Jun 26, 2011
45
0
ive just whent through this, and recived a lot of help from audioguru (thanks! )

The link you reffered to is hard to follow, but its REALLY nice.

It took me a weeken of reading it every day, to understand it completely.

and if you can fiqure it out, you will never blow a mosfett due to overload (unless thats your intention )

Sep 30, 2009
4,096
1,690
6. ### mhastie1234 Thread Starter New Member

Feb 10, 2012
29
2
In the link that i provided. I don't understand where he got 11.3mOhms from. Should it not be 11.9mOhms in the last equation and how do you get 214.16=I2 from 2.42W=I2*11.3mOhms. I understand the rest of the calculations just not this part.

Last edited: Feb 23, 2012
7. ### mhastie1234 Thread Starter New Member

Feb 10, 2012
29
2
Can someone help me out?

8. ### mhastie1234 Thread Starter New Member

Feb 10, 2012
29
2
figured it out.