Re: L297/L298

A MOSFET does have a region between full-on and full-off where it acts like
a linear device, i.e. source-drain current is proportional to the gate
voltage. If operated in this region it will dissipate as much heat as a
bipolar (silicon junction) transistor. This is why fast switching is so
important with MOSFETS. Actually both types are made of silicon, they just
operate differently. Bipolar transistors are controlled by current and FETs
are controlled by voltage.
--
Phil Mattison
http://www.ohmikron.com/

>
> All:
>
> Yikes! Both John and Ballendo are correct. I was
> using the "techie" terminology for silicon transistors.
> The first transistors that were invented are called
> PNP and NPN silcon transistors (actually they used
> Germanium first; but I digress.) Silicon transistors
> drop .2 volts when they are saturated and consequently
> radiate .2V * (current) watts of heat. For the L298,
> if you have 4 coils turned full on, that 4 (coils) *
> (2 amps) * .2 volts = 16 watts. This is quite hot and
> needs a heat sink.
>
> FET (field effect transistors) are newer and have
> a lower voltage drop when the are "on". Thus, they
> radiate much less heat. I'm not as comfortable talking
> about FET's, since most of my experience is with the
> older silicon NPN/PNP transistor techonology. I'll
> let other more experienced people on the list chime
> in about FET transistors.
>
> -Wayne
>
> P.S. I'm out on vacation, and my internet access is
> kind of spotty; hence, the delay in my responses.
>
>
>
>
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