You may substitute a DC coil (one with a diode) on an electromechanical AC
circuit by clipping off the diode. Otherwise you will likely blow a fuse or
Here's the reason why the diode is needed on a solid state machine. When a
DC coil is switched off the magnetic field in the coil collapses and
generates a spike of current back towards the drive circuitry. This spike is
brief, but can be in the hundreds of volts! The diode blocks the
spike, protecting the drive circuits.