EJA van Es <vanes@ilink.nis.za> wrote:
>Anyone know how to build a constant current source using Op-amps. I know
>how to do it with transistors, but I want to try it with op-amps.

Hi Eric,

Try this, from a National Semiconductor Ap Note:


                          R3
                    -----/\/\/\----
                  |                |
                  |                |
           R1     |    | \         |
  E_in ---/\/\/\-------| - \       |      I = E_in * R3
                       |    \------          -----------
                   ----|+   /      |           R1 * R5
                  |    | /         |
                  |                |          R3 = R4 + R5
                  |                \          R1 = R2
                  |                /   R5
                  |                \
                  |                /
                  |       R4       |
                  |-----/\/\/\-----|
                  |                |  <---Constant current = I
                  |                |
                  \              -----
                  /             |     |
             R4   \             |     |  Load
                  /             |     |
                  |              -----
                  |                |
                 GND              GND


--
Bob Fehrenbach    Wauwatosa, WI     bfehrenb@execpc.com