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Based on the circuit from this page called transconductance amplifier, I decided to build an adjstable constant current source for up to 1.6A:My circuit

Current is set by the voltage divider connected to the op-amp input: Iout=Vdiv/Rlim

On Multisim it works fine, however, in real life the current keeps drifting from this value, and the 5k pot does nothing.
I tried swapping the op-amp inputs and then the current starts high, slowly going down to the desired value, but in both cases the FET is heating too much. Is this design correct?

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  • \$\begingroup\$ Did you put the MOSFET on a heatsink? \$\endgroup\$ – Claudio Avi Chami Feb 17 '17 at 11:07
  • \$\begingroup\$ Clearly you haven't even looked at the TL082 datasheet, let alone read any of it. RTFM! \$\endgroup\$ – Olin Lathrop Feb 17 '17 at 12:03
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This won't work with the TL082 due to (a) not having enough power supply voltage and (b) the inputs don't like to work down towards the negative power rail (0 volts on your diagram).

10 volts is the recommended minimum supply voltage and the input common mode range doesn't cover the range 0 volts to 2 volts. Try picking a rail-to-rail op-amp or one that can handle such a low supply voltage along with the ability for its inputs to work down to 0 volts (LM324 might be worth considering).

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  • \$\begingroup\$ An MCP601 opamp might also be a viable alternative, it is nice in that it has rail-to-rail output capability. \$\endgroup\$ – Bimpelrekkie Feb 17 '17 at 11:19
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in real life the current keeps drifting from this value, and the 5k pot does nothing.

check out the common mode input voltage spec in the datasheet.

in short, the input voltage has to be sufficiently high from the negative rail (=ground in this case) or sufficiently low from the positive rail for the output to be meaningful.

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