The Vds on the FET should be a whole lot less than 2.4 Volts at 37 mA - 
actually I would think more like 0,24 Volts.  The 2N-7000 FETs have a
nice low ON resistance when saturated - I would have to say that yours
is not.

RJG
  ----- Original Message ----- 
  From: John Remington 
  To: Microcontroller discussion list - Public. 
  Sent: Friday, April 21, 2006 1:21 PM
  Subject: Re: [EE] JFET 2N7000 as SS Relays


  Yes, I am using this set up as a switch.

  I think I have it covered, I'm now using a 2N2222A to drive the fet's gate.
  10k resistor between the pic pin and the transistor base, then a 10k from
  the 12vdc to the collector, junction collector/resistor to the fet gate,
  transistor emitter to ground.

  Ground to load, load to source on the fet, it's drain to V+.  Works good,
  the fet seems saturated as I have a 37ma load, 2.4 Vds drop, so it looks
  like 98mw or so, well within specs.

  And on boot there is the 12vdc on the fet's gate to force it live.

  Thanks for the input . . .



  On 4/19/06, Forrest Christian <forrestc@imach.com> wrote:
  >
  > Ok, I'm going to restate what I think you said.
  >
  > You are attempting to use 2n7000's (which isn't a JFET) as a switch.
  >
  > You have a 12V supply attached to a 12 V load.  The other side of the
  > load is attached to the Drain of the MOSFET.  The Source of the mosfet
  > is connected to the 0V rail.
  >
  > You also have a 5V supply wired identically to above with a 5V load on
  > another 2n7000.
  >
  > You have a PIC or other TTL/CMOS device wired to the gates.
  >
  > The 0V rails used throughout your circuit.  I.E. it's common on all the
  > supplies.
  >
  > Yes, this is safe to do as long as you don't exceed the ratings of the
  > 2n7000.   The 2n7000 datasheet I use says 200mA continuous, 500mA
  > pulsed.   And 60V Vdss and Vdgr.
  >
  > Just one more thing.
  >
  > John Remington wrote:
  > > I like the FET's cause they fail "on", so my devices should be live in
  > the event of a cold reboot and/or system
  > > failure (on is what I want),
  > A FET may or may not fail "on" depending on your exact circuit.    I'm
  > assuming you mean that the circuit is such that they are on during a
  > cold reboot or system failure.
  >
  > -forrest
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