This seems very simple, and so just the job!

Thanks I will give it a try.

Graham

        ----------
        From:  Jinx [SMTP:joecolquitt@CLEAR.NET.NZ]
        Sent:  21 April 2000 13:08
        To:  PICLIST@MITVMA.MIT.EDU
        Subject:  Re: [OT] 180mA to300mA circuit

        The list helped me out a couple of weeks ago with a constant current
        charger for a bank of large-ish caps that worked very well. It was
simply
        an LM317T.

        24V to In, resistor between Adjust and Out, the output being taken
from
        the Adjust terminal. The 317T is rated for 37V in and 1.5A out with
a
        voltage drop stated as 3V, but I found it settled to 1.2V. If you're
using
        a nominal 24V battery (which could be > 27V when charged) would this
        drop-out matter ? Should be OK for a solenoid. BTW, why use variable
        current - is it a valve of some sort ?

        If you try it, the current out is determined by 1.2/R, where R is in
ohms.
        So for 150mA it would be 8 ohms, for 300mA 4 ohms. A 1/4W 10 ohm
        pot with 3R9 resistor in series would do.


        > From: Graham North <graham.north@LANDINST.COM>

        > 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