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power mosfet

Former Member

Hi,

 

I am designing a heating control system using a PIC, using a number of thermistors and coils of nicrome wire to heat up various parts of a machine.

The PIC will switch the nicrome on and off accordingly. It has been suggested that I used a power mosfet to do this, is this correct?

The nichrome wire that I want to heat up is approx 5ohms, and will be powered by a 5v supply.

 

Thanks in advance for any help,

 

Edd

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  • in reply to kemmi

    Please don't use the circuit suggested by Kemmi !

     

    The MOSFET type suggested is OK but:

     

    The MOSFET is wired as a source follower so there will be a voltage drop between gate and source. From the data sheet this will be between 1.3 and 1.7V for a drain source current of 1A. If your PIC runs off 5V the MOSFET will fry (about 800mW in the MOSFET wiha 5R load).

     

    Connect the MOSFET source to ground and put the load between the drain and +5V.

     

    Now if your PIC runs off between 3V and 5V you should get an ON resistance for your MOSFET of less than 0.06R which will give you the full (nearly) 5V across the load and only 60mW to heat up the MOSFET.