The circuit shown (Adobe PDF file)
provides the necessary drive for a MOSFET bi-directional relay used to switch power to 120vac
loads. A pair of C-MOS schmitt triggers (A1) forms a 100KHz square wave oscillator with
complementary outputs. The two signals are coupled to the FET gates of the relay circuit
through a pair of high voltage 100pF capacitors. A bridge rectifier is used to convert the AC
voltage to DC. Sufficient gate/source capacitance within the FETs eliminates the need for any
additional filtering. The 220K resistor (R1) is picked to discharge the capacitance and turn
the transistors off in about 2 milliseconds, once the drive signal is disabled. Faster
switching times are possible with lower values of R1 and higher drive frequencies. With the
values shown less than 2ma of current is needed from a 12v to 15v supply to drive the circuit.
Also, the small 100pf charge coupling capacitors constitute only a 5 microamp leakage path to
the 120vac line, which is sufficient isolation for many applications. Leakage currents less
than 1 microamp is possible with a 1MHz drive frequency and 12pF coupling capacitors. If
50ua's of leakage current can be tolerated the required drive current from the C-MOS invertors
can be reduced to less than 200ua's by reducing the drive oscillator frequency to about 10KHz
and increasing the coupling capacitance to 1000pf.