Attached is how I would do it. I moved the resistor to the input of the diode bridge. I think it helps protect the diodes from lightning. I used a low wattage zener and a power transistor. The transistor will get hot but you can attach a heat sink to it. Some people use only two diodes. In this case I would use the 4-diode bridge to get more power. Set C1 size so that at low voltage with all relays on you just make 12 (11) volts. Then at high voltage the transistor will have to burn up the extra power.
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