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.
opnspaces wrote:Gdetrailer wrote:
Heck even on my TT, I have added a relay so when the pin is pulled it automatically disconnects the brake wiring electrically from the vehicle. The vehicle brake controller will now give you a warning of no connection before you start driving..
Gdetrailer, maybe I just haven't had enough caffeine, but having trouble picturing the wiring of your relay.
I can see a normally closed relay where the the brake signal wire runs through from the tow vehicle. But what are you using for the power and ground on the trigger wire?
Addition of a single pole double throw (SPDT) relay makes this happen, I used a "Bosch" style 40A relay.
Brake coils are connected to the common terminal of the relay, Brake wire from tow vehicle is connected to the NC Normally Closed) terminal of relay.
12V+ goes into one side of the relay coil, other side of relay coil goes to breakaway switch. Other side of breakaway switch goes to ground.
Pull pin, the switch now grounds the negative side of the relay coil, pulling the NO contact to closed position, this now breaks the NC terminal to open which the break controller now sees as no brake connection and alerts you. This same action now applies full battery voltage directly to the brake magnet coils.
You also get a few benefits, the breakway switch no longer must carry the high amperage and the relay coil limits the max current through the switch and any shorts will not burn your camper to the ground.
I did a drawing years ago but in reviewing it, I show a fuse on the relay coil, I don't remember putting one there but the power source is most likely fused or breakered, typically the breakaway system is not fused, but I suspect I used something like 30A or 40A just so the wiring is somewhat protected from major shorts (catastrophic fuse). I would have to take a look at my fuse block to see what I did.
