MOSFETS and information

[strokedmaro]

Just a couple quick ones

1. Why would you ever want to use a transistor over a MOSFET?? If MOSFETS require less current to operate and can handle several times the load why would you ever want to use hotter, current hungry transistor? Is it price?

2. I want to play with a linear actuator I bought. Its a just for fun project. The actuator is 12vdc, 6 inch stroke with a 10k pot feedback. I bought a round, paper thin, ipod like pot (also 10k) and will use a PIC of some sort (I have many to play with). Id like to use a relay to swap the power and ground for extension and retraction and a MOSFET to actually apply the ground to operate the actuator.

I have a IRF3711z MOSFET and want to make sure that I'm reading the data sheet correctly so it doesn't pop when I try to use it. The actuator draws 4A at max load and the MOSFET is (as I read it) capable of 66A @ 100 degrees C. Is this correct or am I interpreting something wrong? This should be more than capable correct? THANKS!!!

 

[Papabravo]

I can think of at least two reasons

1. Bipolar Transistors are less sensitive to static discharge
2. Base CURRENT controls Collector CURRENT

 

[strokedmaro]

I drew this up with components I have available. Any issues you guys see?

 

[MikeMl]

The 66A rating of your MosFet could be misleading you. That rating is contingent on the FET being bolted to (with a very low thermal resistance) to an INFINITE Heat Sink.

Also, you didn't specify if your circuit drives the FET as a switch, or as a gate-voltage dependent variable resistor. If used as a switch, your unheatsinked FET might switch your 4A (or it may not)! If you are using it to vary the current through the actuator in an analog way (non-PWM), then you WILL have to bolt it to a heatsink.

Also, be sure that the 0 to Vcc voltage swing on the PIC port pin will fully turn on the FET. Some FETs require 10V on their gate to fully turn them on.

OOps, I was posting as you were. Your circuit will turn on the FET hard enough. Why not just use four FETs in a H-Bridge, and get rid of the relay...

 

[MikeMl]

Link to circuit that may work for you. It should work fine with your actuator and 12Vdc. You can replace the power transistors with NFETs and PFETs. The OP I was responding to was trying to do something similar to running your actuator.

 

[smanches]

There is a note on the continuous current stating the package itself can only handle 30A. The 66A figure is based on junction temperature only. That's a terrible caveat to put in a footnote.

 

[crutschow]

There is a note on the continuous current stating the package itself can only handle 30A. The 66A figure is based on junction temperature only. That's a terrible caveat to put in a footnote.

As they say in the legal profession, the Big Print giveth and the Fine Print taketh away.