Proper Flyback Current Protection Circuit for Variable Voltage Solenoid Driver

Question

I am using a darlington buffered op-amp to drive a solenoid motor at variable voltages (for variable speeds of actuation). Here is my circuit as it is now:

I am wondering which of the two flyback diodes (from coil to GND and coil to VCC) are necessary to drain flyback current from the solenoid, or if perhaps a resistor across the solenoid is necessary.

The diode from the op-amp output to the coil is to prevent flyback from interacting with the op-amp output. This one I am certain is necessary from hands-on testing.

Normally I see this as a flyback circuit:

But this is notably different from my case, sense it works by connecting and disconnecting the solenoid from ground, whereas my circuit provides a variable voltage at the input. I am wondering what I need to change in the flyback protection setup to account for this difference. Thanks!

Answer 1

You don't need change anything from default flyback protection. It is used to protect circuit from huge voltage spike that is induced by the coil. Inductors don't like fast current changes, generally speaking:

$$V = L\dfrac{di}{dt}$$

So, when the coil switch occurs, voltage in midpoint goes up to huge value. In order to avoid that, you need to add a path, in which current will be suppressed (snubber). Diode to the left is doing all the job, and diode to the right is useless (as voltage in midpoint is negative).

However, your circuit will change speed in which magnetic field change vill occur, but not the maximum magnetic flux. I would recommend you to check current controled coil instead of voltage controled, as with that you could limit the actual strength of a field. Sample circuit:

^{simulate this circuit – Schematic created using CircuitLab}