regulated relay latch

Lets say i have a relay that is powered by the ignition wire, however when the ignition is turned off, I want that relay to stay closed by a regulated 12v source. This source will come from a power supply unit for a CarPC. To protect the electronics on the +12v rail of the power supply, I should use a diode correct? I imagined a circuit like the picture. Will this work correct, and will I have to worry about any electronics on the +12 V rail of the PSU being damaged from the larger/smaller voltages of the ignition line?

The diodes you show will isolate between the different supply voltages, but if you're at all worried about voltage spikes damaging your PSU drive circuit, you should add a diode across the coil, in addition to the other two.