I am working on a project where I'd like to deliver variable DC control voltages to a continuous solenoid motor in order to achieve different velocities of actuation.
To buffer my DC control voltage for high current to drive the solenoid I tried using a Darlington buffered LM358 op-amp like so: For the Darlington I am using a TIP120 which is the following circuit in a TO-220 package. As you can see it includes a flyback diode:
I had thought the internal TIP120 diode between the collector and emitter would deal with flyback current. However I'm experiencing an issue where if I give the solenoid DC for too long I get massive flyback current when the op-amp input swings low again (I can see the flyback going crazy on my scope) and the op-amp ends up blowing itself up.
What is causing this to happen? Is there a way I can modify my circuit to protect again flyback current?