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MOSFETS and information

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strokedmaro

New Member
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!!!
 

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Papabravo

Well-Known Member
I can think of at least two reasons
  1. Bipolar Transistors are less sensitive to static discharge
  2. Base CURRENT controls Collector CURRENT
 
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MikeMl

Well-Known Member
Most Helpful Member
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...
 
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MikeMl

Well-Known Member
Most Helpful Member
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.
 
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smanches

New Member
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

Well-Known Member
Most Helpful Member
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.
 
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