As applied to a concrete circuit relay. Transistor VT is high-speed, high- voltage switch is. Switch voltage requirement is 5-10 times the rated voltage solenoid valve. Fast re
sponse time of the circuit is most beans. Varistor Rv can also be used instead of the diode VD (shown in dashed lines in Figure). But will relay diode (or receive plot, a solenoid) current decay slows down, extending the armature release time.