Protect those expensive 12V electronics with a CROWBAR!!!!!!!

[Mogman]

Whether you are using a voltage reducing regulator or tapping off the middle of the batteries this can save you a ton of money.

If you are using a "tap" on the batteries and for some reason you lose the ground connection to the low side battery (some suggest ONLY putting a battery switch on the ground side) you have just given your 12V electronics a reverse polarity condition and that can blow your radios etc., OR if that cheap Chicom regulator all of a sudden decides to start putting out 24V, again disaster.

This is a CHEAP way to protect the 12V electronics from these disasters, a ZENER diode, the most simple way is to just install a Zener in the 12V line to ground.
In a high current system say a 30A regulator for example, in the event of a over voltage condition (18V or higher) the Zener could be destroyed (will become a permanent short) but it will blow the fuse,, again cheap insurance.

I use a 50W 18V, part number 1N3317B.

The more complex circuit would be a Crowbar using a SCR, this would be able to blow the fuse with no danger of killing the Zener.

DO NOT FORGET TO FUSE THE SUPPLY SIDE OF THE CIRCUIT!!!

In the simple circuit the Zener also acts like a regular diode for reverse polarity protection, for the SCR crowbar circuit I would add a large "regular" diode (10A10) across the load in the same polarity as the Zener for reverse polarity protection, make sure if buying a stud mount Zener to get one with the case being the anode and bolt the stud to ground.

Forgive me if this has been covered, I did a search and came up with nothing.

 

[Mogman]

Here is an example of how the 12V equipment can be subjected to a reverse polarity condition if the ground is removed from the low side battery, there needs to be a 24V load, in this example the headlights are turned on but it can be any load, ign. ect.
Notice the current is forced backwards through the 12V electronics (radio in this example) when the ground is removed.

Forgive me I am not an artist.

 

[papakb]

I used a couple SD-51 Schotky diodes I had laying around from my days at Varian.

 

[Mogman]

I used a couple SD-51 Schotky diodes I had laying around from my days at Varian.

Those make great reverse polarity protectors, very low forward voltage drop, would be perfect to protect folks using the "tap" method to get 12V, no over-voltage protection.