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12 Volts on the trailer without a truck

Ever wanted to run the 12 volt stuff on your horse trailer or camper without the truck hooked up and running? I wanted to be able to run the 12 volt lighting in the back of my horse trailer and the water pump without draining the batteries on the truck or leaving it running all the time. The truck’s electrical system isn’t really designed for large charge/discharge cycles, so it’s best to keep the batteries charged. If you’re plugged in to AC power though, you can run your 12 volt stuff without a truck if you add a power supply.

I found a cheap 12 volt power supply from allelectronics that is actually a converted PC power supply. Allelectronics has all kinds of surplus electronic stuff, so you can probably find something that will work. Otherwise, you can buy a PC power supply anywhere and convert it yourself. The only other thing you need is a high-current diode to prevent the power supply sinking current when the truck is plugged in. Since the power supply puts out 12 volts and a properly operating automotive electrical system puts out 13-14 volts, having both operating at the same time isn’t a problem as long as you use the diode. If the truck is plugged in while the power supply is operating, the truck will supply current and the diode will be reverse-biased until the voltage drops below 12 volts, and then the diode will forward-bias and the power supply will take over. Make sure to fuse the output of your power supply to the 12 volt bus. Connect the output of the power supply to a fuse, and then the other side of the fuse to the anode of the diode (usually the non-stripe end. check the datasheet). Connect the cathode of the diode (stripe end) to the 12 volt bus.

The only problem you may have is that you will have to convert any strings of high-current 1156 and similar light bulbs to LED or the power supply will “protect” itself by shutting off when it sees the low and slowly rising resistance of the bulbs. Mine works fine with the single bulb of the porch light and fine with the motor of the water pump but dies when I turn on the bank of three bulbs in the back. I haven’t changed them to LED yet.