Solid state relays are very reliable devices for both AC and DC
applications. The reference made to "crossing zero 120 times a second"
has to do with a type of AC (not DC!!) relay known as "zero crossover".
The purpose of this type of relay is to minimize transients by connecting
the load when the AC waveform passes through zero volts. If transients
are a concern (e.g. if there's any kind of amplifier nearby) a zero
crossover relay should be used. I've designed them into equipment where
they turn a 12 Amp. motor ON/OFF every 30 minutes; these devices have
been working outdoors 24 hours/day for over 15 years with no failures.
However, right next to this solid state relay are several mechanical
relays. Why not use all solid state relays?
Mechanical relays offer a much wider assortment of contact configurations
which are required to implement logic functions; such configurations are
generally not available in solid state relays. To sum up, if you just
want to turn something ON/OFF, a solid state relay is the way to go; some
of these devices have ratings of over several hundred Amps.
Some thoughts on using diodes to suppress inductive transients in DC
applications. Diodes will definitely increase the "hold time" of the
coil which means there will be an additional time delay before the relay
contacts open or a solenoid plunger retracts. In many applications this
is not a problem, but if quick release is required (many solenoid and all
rotary stepping relay applications) the diode should not be used. For
quick release you need a device that consumes both voltage and current,
for rapid dissipation of the stored energy; the diode consumes current,
but very little voltage (about 0.7 Volts).
A much better choice is to use a diode in series with a zener diode; if
the breakdown rating of the zener diode is about 70 volts, energy will
now be dissipated 100 times faster than just using the diode alone.
Other good choices would be varistors or MOV (Metal-Oxide-Varistor)