 If you've ever put together a circuit to drive a solenoid, you may have noticed there's two essential parts. A big transistor that can drive the large amount of current, and a diode. The transistor makes sense, but why do we need the diode? Well, if you look inside of a solenoid, you can see that it's made of many turns of coiled wire. Just like an inductor, which is also made of many turns of coiled wire. And when you cut off current to an inductor, it throws a fit and generates a spike in voltage, which can be useful when constructing a voltage booster, but a spike like that from our solenoid could easily damage the driver and connected circuitry. That's where the diode comes into play. Adding a diode across the solenoid's leads prevents the voltage from getting too high, directing it away from the power rails, keeping our circuit nice and safe.