Breakaway Switch Confusion.

AJR wrote:
Holy cow! As I remember it the breakaway switch was a simple device that applied battery power to the TT brakes when the plug was pulled. I also remember there was a concern if you pulled the plug by accident the trailer brake coils could cook.

Well, sort of "simple".

Yes, the breakaway applies full 12V to the brake magnets when the pin is pulled.

NO, the brake magnets will not "cook", they will get warm to hot depending on how long full voltage is applied but not "cook". That is a Internet Myth.

The coils are designed to withstand full battery voltage for a minimum of 15 minutes which is the time specified by dept of transportation rules which in a nutshell require your breakaway system to be able to stop and hold a trailer which has broken from the tow vehicle for 15 minutes.

The coils are designed to draw about 3A each at 12V.

Some brake controllers warn you to disconnect the trailer BEFORE pulling the pin for testing purposes and failure to do that may destroy that controller in the process. Adding that relay prevents the breakaway switch from feeding 12V back into the brake controller and in the case of IBC controllers preventing a rather expensive dealer repair.

If you had a Cargo or flatbed trailer they often have a self contained gel cel battery box, inside that box is a circuit board which uses several diodes to prevent the breakaway system from feeding 12V back into the brake controller and vehicle charge line.

Diodes while they provide isolation also have a .7V loss across them which means your battery doesn't charge as well and your brakes will get less current which means less braking effect.

Relay removes that .7V loss..



RV manufacturers are cheap, they do not include or make provisions like a diode or relay to prevent potential damage to your brake controller nor do they care.

Yes, adding in a few items like a relay or diode adds a bit more complexity but it is the details that counts to me.