CCS with no load - op amp warms up

Discussion in 'General Electronics Chat' started by atferrari, May 20, 2015.

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
    I've just completed the smoke test of these two CCS. They work as expected until I leave them unloaded and the voltage control is in any position other than fully turned OFF; in few seconds the respective op amp starts to warm up.

    After some thinking I understand that by not having load, the opamp sinks / sources current from / to its corresponding Darlington trying to balance both inputs what never happens.

    What could be the solution? For a moment I thought to use kind of a bleeding load similar as in voltage regulators but I discarded th idea. If the resistor is too high I will be in a situation similar to what I describe and, if too low, too much of my current wil be going through there.

    For a moment I considered a base resistor but it seems useless as well. Or tell the user (me) to keep them fully turned OFF when not loaded? This last seems typical of a bad design.

    Suggestions or comments appreciated.
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    How about a resistor between the opamp output and the base of the Darlington input transistor? This will limit the max output current that the opamp sources or sinks...

    Or replace the Darlington with a power FET. It is hard to pump much current into the gate of a FET.
  3. crutschow


    Mar 14, 2008
    Mike has two good suggestions.
    A base resistor is not useless in this situation. It is a simple solution to keep the op amp from being overloaded when there is no output load.
  4. ian field

    Distinguished Member

    Oct 27, 2012
    There is one reservation about a MOSFET: Its theoretically possible to run some op-amps from single ended supply up to 30V (although not many people do that). A typical MOSFET has maximum gate voltage around 18 - 20V.

    Its just a small posibility that nonetheless shouldn't be overlooked.
  5. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
    A 10K resistor makes the op amps to take in worst condition 1,2 mA and 860 uA respectively.

    Problem solved.