Quote Originally Posted by Rob Withey View Post
12v doesn't always come from the battery. You'll get up to 14v out of the battery when the alternator is charging it.
On the flip side of this, 12v ac, when full wave bridge rectified and smoothed will give you about 1.4 times that voltage (this is because ac voltage is specified using rms).

So the 12v ac input turns into nearly 15v (taking 1.4v off the ac figure for the two diode drops) : 1.4 * (12 - (0.7 * 2)) = 14.84.

Given that this is smoothed with a 1000uF electrolytic, only used to drive the relays, and there is a dropper resistor in parallel with the indicator LEDs on the relay circuit I would feel quite happy putting unregulated car voltage into this circuit.