OK,  Here's my two cents...

24V----|----|----------|
       |    | LM317    |    ------------    ------------
      ---   | TO-3     |---| 4ohm 1/2 W |--| 10 ohm POT |---- Iout
   1uf---   |----------|    ------------    ------------   |
       |         |                                         |
      ---         -----------------------------------------
       -
Iout ~= 1.25/(4+R(POT))
Iout ~= 300mA when R(POT) = 0
Note the 4 ohm is use to limit max current when the POT approaches the zero
resistance stop.  7.2 Watts when max current is applied to an output short.
The TO-3 LM317 will have no problem handling this power @ 3 C/W theta JC
(Unless, of course, you are shredding trees at an ambient of greater than
130C).

<facious comment please disregard> Remember to use a PIC16C72A running a PWM
@  10% D.C. to illuminate an LED to show that this circuit is ON.  This is,
after all, the PICLIST.

Tim Crist

-----Original Message-----
From: Arthur [mailto:artb@CABLEINET.CO.UK]
Sent: Thursday, April 20, 2000 11:28 AM
Subject: Re: [OT] 180mA to300mA circuit


The circiut i have in mind would moniter and close down if short ciriut or
over current
will do a circiut and post need to install some software first but will post
Saturday at latest.this will also include a spelling checker.
Besides the pic I see opamp a few resistors  a pot and two transistors
I am waiting for my new provider to give me details for my web site and when
he does it will make posting easyer.

----- Original Message -----
From: Chris Eddy <ceddy@NB.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, April 20, 2000 4:02 PM
Subject: Re: [OT] 180mA to300mA circuit

Art

> Arthur;
> This surely will work, but at what cost?  Let's say worst case there is a
> short circuit and 300mA.  The pass transistor must consume 24*.3 or 7.2W.
I
> am sure there is a safer way.
>
> One could use a PWM circuit.  Setup a Pchannel Mosfet to drive +24 to the
> load.  Bring the load back through a sense resistor, say 1 ohm.
> (0.3*0.3*10=reasonable).  Now add an opamp to drive the FET (possibly
through
> a buffer to drive gate capacitance).  Tie one input of the opamp to the
> feedback resistor.  Drive the other input from the pot, divided down to
match
> the range of the feedback signal (0-3000mV).  This circuit would normally
run
> linear, and give the same results as the original idea.  ( if the opamp
output
> slams to a rail, the inputs are backwards)  Now add an R and C in the
current
> sense feedback path.  The opamp will break into oscillation.  The circuit
will
> now modulate around the target setpoint, and the FET will be driven on and
> off.  The circuit will (assuming no parts fail) limit current even during
a
> short circuit of the load.
>
> Chris Eddy
>
> Arthur wrote:
>
> > pic with ladder output driving buffer amp or d/a amp to control a Fet or
> > Transistor emmiter follower ctt.
> >
> > Art
> > ----- Original Message -----
> > From: Graham North <graham.north@LANDINST.COM>
> > To: <PICLIST@MITVMA.MIT.EDU>
> > Sent: Thursday, April 20, 2000 11:53 AM
> > Subject: [OT] 180mA to300mA circuit
> >
> > > Hi
> > >
> > > One of my friends works on a tree harvester and asked me to make
> > > him a circuit with a 24V output with constant adjustable current
between
> > > 180mA and 300mA to control a solenoid.  He would like to control the
> > circuit
> > > with a pot.
> > >
> > > I do have an idea how to do it, but thought I would ask the experts.
> > >
> > > BTW the harvester uses a 24 V battery, so it is only a matter of
> > controlling
> > > the current.
> > >
> > > Thanks
> > >
> > > Graham