Geoff Raynak <> wrote:
>Sorry for the confusion.  I meant to say that I tried the circuits
>provided by H&H but they don't seem to be _precision_ in the sense that
>they cannot maintain the load against the resistance.  I am trying to
>sense geometry changes in a tube filled with saline.  Using a resistance
>method, i need a constant current source (very constant) to measure
>resistance-induced voltage changes without any current-induced voltage

Basic circuit
                                           <-- constant current
               |\                   d
  Vref --------|+ \              g |--------- Load ----- Vcc
               |    > -----------| | Q1
            +--|- /                |--+
            |  |/                   s |    transister = n-channel mosfet

This circuit provides an approximatly constant current of Vref/R1

1.A resister (1K) may be required between op-amp output and fet gate to
surpress oscilations.

2. A resister (100K) from the fet gate to ground to insure the fet is
off when power is applied could be useful.

3. Use diff amp to sense load voltage.

Opinions expressed herein are my own and may not represent those of my employer.

Date: 11 Jun 1996 14:14:29 GMT

Original Subject: Re: Constant Current Source (1-5 mA)

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