# solenoid control valve

What is the purpose of the diode in the solenoid controlled valve circuit? (From what I read, without the diode, the MOSFET will break down but I don't know why)

• Flywheel/Flyback diode serves as a protection to prevent a spike in voltage. If you look at WhatRoughBeast's equation, you see why. I had an interview question one time that asked me about the purpose of the flywheel diode :) – KingDuken May 4 '18 at 2:35

For what it's worth, although it's cheap and easy, it gives the worst performance in terms of speed. This is precisely because it clamps the voltage at Vcc. The appropriate equation is $$V =L \frac{di}{dt}$$ where di/dt is the rate of change of the solenoid current. Since what you want to do is change the current to zero, the lower the clamping voltage the slower the response. While this is almost certainly not a problem in your case, there are other approaches available which will give better performance.