It just shuts off the drive to the P channel power FET.
(Yanks the gate high.)

  *
  |  __O    Thomas C. Sefranek  WA1RHP@ARRL.NET
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> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Mark Perri
> Sent: Wednesday, June 23, 2004 5:10 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [EE:] How does this current limiting circuit work?
>
>
> I am trying to build a motor controller to drive a robot from a
> PIC using an H-bridge, so I'm
> reading up on FETs now in The Art of Electronics.  They have a
> circuit which limits the current
> draw through a mosfet, which I'm trying to understand.
>
> It's Figure 3.72c, with a PNP transistor before the source of the
> FET with a 0.5 ohm resistor from
> emitter to base with the base wired to the source, and a 100k
> resistor from their +12V source to
> the collector of the PNP and also to the gate of the MOSFET.
> Originally when I was looking at
> this circuit I thought the purpose of the transistor was to shut
> off when the voltage drop across
> the 0.5 ohm resistor became more than 0.6V so that the gate was
> too far below the emitter.  But I
> can't understand why this would stop the load from being powered.
>  It seems the voltage drop
> across the 0.5 ohm resistor would lower the source voltage of the
> FET anyway, causing it to supply
> less and less current.  The explanation for the circuit is left
> to the reader as exercise 3.21..
> maybe I should try and find a copy of the answers somewhere.
>
> If anyone has any insight, I would appreciate it, I think it
> would help out my understanding of
> these transistors which I can't seem to wrap my head around.
>
> Mark
>
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