# transistor/mosfet help???

#### mhastie1234

Joined Feb 10, 2012
29
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:

#### Audioguru

Joined Dec 20, 2007
11,251
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

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

#### dumle29

Joined Jun 26, 2011
45
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 )

#### mhastie1234

Joined Feb 10, 2012
29
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:

#### mhastie1234

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