Picking solenoids "kickback" diodes

Over the years, I've done several projects using relays or smallish solenoids, that used dozens to hundreds of mA. So I always just put a 1N400x diode across them, called it "good", and never had a problem.

Now I'm doing one that uses hefty solenoids that draw 4-6A. I'm guessing that their coils will store a lot more energy to be dissipated, but I don't really know, and I'm wondering whether I need to use a diode with a higher current rating. Is there a formula for this?



I would recommend a diode with a peak current rating of twice the peak current in the coil. As long as you don't switch the coil off at a high frequency, the average power dissipated in the diode will not be a concern, just the peak current. The theoretical energy stored in the coil is E=0.5*L*I^2, but I'm not sure why that would matter.

You can also speed up the discharge of the coil by using a TVS in series with the diode (cathode-to-cathode) to increase the coil voltage thus increase the rate of current decay. Downside: your switching transistor has to tolerate a higher voltage stress.