In case of you want to use Voltage 28V sizes for apply to Relay 28V. But you have just DC power supply Regulator 12V. Will do ? good! The exit is good. Be build DC to DC converter be 12VDC to 28VDC converter. By I uses the integrated circuit number LM2585 be pillar equipment for this work. Not difficult tell a lot of invite you see in circuit picture better.