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Anon 02-13-2005 12:01 PM

Great schematic, john hd!

Originally Posted by john hd

here is my attachment for what it is worth.


If you ever need a job drawing electrical diagrams look me up!
Good job, buddy. IEEE would be proud of you.

What was this thread about anyway? Man, it has gone on and on and on...

I guess I am just adding to it.

Inland RV Center, In 02-13-2005 01:06 PM


Your suggestion has nothing to do with brake magnets that are wired in parallel, as you now have a series parallel circuit. Your suggested set up, is only a single series circuit.

All wires have some resistance. The larger the wire, the smaller the resistance.

Additionally, as any wire heats up, it's resistance increases, which will make the initial current drain reduce. Same as a light bulb. As the element becomes hot, it's resistance increases, therefore the current decreases

Shorts in electric brake systems seldom come from the wiring or breakaway switches.

Shorts do happen, "very often" in the brake magnets.

Normally at 12 volts DC a 12 volt magnet will draw 3 amps, but at 13.5 volts DC (when towing) that same magnet now draws 3.37 amps. So for the single axle owner, not bad, as that trailer will only draw 6.74 amps. On a tandem trailer the draw will be 13.48 amps. Since fuses should not be used more than 75 percent of their capacity (for safety) thats getting very close to the maximum that a 20 amp fuse should be used at.

Now lets talk about a tri-axle owner. He will be at 20.22 amps, at maximum.
He does not have a chance with a 20 amp fuse. He also will be very close to the limit of drawing no more than 75 percent through a 30 amp fuse, which is 22.5 amps.

These are examples that have not considered any resistance from the wires or connections.

Owners usually are not aware that a brake magnet contains a coil of wire.
When the magnet has been in service way after it's design limit, a short will develop when the magnet is energized and firmly touches the armature plate.

Therefore the short will not be zero resistance, but can be low enough to cause a fuse or circuit breaker to open.

Tests have been made, many years ago, duplicating what I just described.
On any trailer, that has two or more brakes, if one magnet shorts out (having a lower resistance) because of wear, the other wheels will still operate almost at normal. Simple series parallel circuit set up is the reason that

Practical experience, is far removed from theories, thoughts, ought-to-be's, and the like. Regardless of documentation, there will always be those that will disagree, with most anything.

My sole purpose is to be helpful. If someone choses otherwise, so be it. My only suggestion, is to prove otherwise, not by theories or I believe's, but go do the tests.

They have the trailers, so what's stopping them from doing a test themselves? Is it easier to argue than it is to set up and run tests? Perhaps so.

But the bottom line is that many newcomers come to this site to learn, since they are new to Rving.

Lets give them a shot at learning the right stuff, so that they too can join the rest of us who enjoy "safe Rving".

Differences of opinions, are always welcome. But facts can only be refuted with other facts, not by opinions.

It is in that context that, as time permits, I offer answers that came from factual experience, not theories, or guesses.

Hopefully, the next post on this subject will be from someone that has duplicated the setup and has run the tests.

Whoever that may be, make sure you use one magnet that clearly has the wire coil of the magnet exposed.

But the bottom line that dooms all the theories, as well as practical experience with electric trailer brakes, is "disc brakes".

Wow, no brake current drain. No worn magnets to cause problems. State of the "art" in braking systems? You bet. Superior to electric brakes? Hands down. And another "wow" is that a rotor is miuch closer to being in balance than a hub and drum. Therefore if you balanced only the tire and wheel, you most likely will be in great shape.

But, there are those that will disagree and will still feel that there is no real motive to change from electric brakes.

Airstream has always been the innovator. Why then, have they chosen to make disc brakes standard on all the classic models? Surely, we can't accuse them of being "dumb. They want the best, for the best owner/customer in the RV industry, namely the Airstream owner.

New or used, that's you.


66Overlander 02-13-2005 01:29 PM


Originally Posted by pattersontoo
A fuse is considered the "weak link" in an electrical circuit. It is designed to protect equipment from damage or personnel from a dangerous electrcial shock. The NFPA and NEC considers anything over 50V, AC or DC, to be a hazardous voltage. I do not see how your system would have hazardous voltage at the levels we are dealing with in a 12VDC system. This rules out using a fuse to protect personnel.
Now, to use a fuse to protect the switch. These are just plain switches. When the insulator, (the part that is hooked to the tether), is pulled out, (when the trailer goes walk-about), then the two conductors touch. Two simple little pieces of metal hooked to two simple little pieces of wire. The alternative path to ground is completed and power is passed from the trailer's battery to the trailer brake circuit. What would you need to protect from damage in this circuit at that time?
Remember, there are only two reasons you use a fuse in a circuit:
1. To protect personnel from injury or death.
2. To protect equipment from damage.
I teach these in the Electrician Apprenticeship program I do for a number of automotive plants.

A fuse or circuit brakers IS a "designed in" weak link, done on purpose to make sure that a circuit fails in a "safe" manner if a short circuit to ground occurs.

At least in automotive (i.e. 12V) applications, a fuse or curcuit breaker is not used to protect personnel which, in any case, are of too high a resistance to trip such a circuit protection device. And in most cases in automotive applications (there may be few exceptions), the sole purpose of the fuse or circuit breaker is to protect the wiring, nothing more, nothing less. A shorted wire that is not protected by a fuse or circuit breaker may overheat with all of the negative side effects that can come from such overheating.

In the case of a short circuit in the wiring the wire resistance which can be much less than one ohm (depending upon the nature of the short) represents the only resistance to current flow. As an example only, let's say the short is 1/10th of an ohm: 12 volts divided by 0.1 ohm = 120 Amps. A typical brake control circuit will not last very long at this current level.

This is my final word on this message string (really). I will no longer point out incorrect theories about the need for (or purposes of) circuit protection devices. Each person that reads this message string can decide which "facts" they choose to beleive and wire their tow vehicle or trailer in a manner consistent with those choices.

john hd 02-13-2005 04:10 PM

"All wires have some resistance. The larger the wire, the smaller the resistance.

Additionally, as any wire heats up, it's resistance increases, which will make the initial current drain reduce. Same as a light bulb. As the element becomes hot, it's resistance increases, therefore the current decreases "


you are correct in the strictest sense on this one. it is a series parallel circuit when you consider the resistance of the wires and connections.

that is my lineman training showing through, we are taught to consider only the resistance of devices. and not the wires connected to circuits because of the extremely high voltages i work with AND because the conductors are usually the size of your arm (not kidding!)

however, if you were to consider only the magnets as you suggest in a pure parallel circuit. there would be the same voltage drop across each magnet with only the current varying with the resistance.

the end result is the same, shorted magnet produces no braking. the remaining magnets get "some" current.

obviously the entire circuit would be in a state of overload at this point, AND adding a fuse would be a recipie for disaster if you were screaming up to a crosswalk full of kids!


gsymes 02-13-2005 06:18 PM

More or less
Maybe, more or less to this than we are seeing. Obviously the technical issues have been covered for the electrically challenged. Have you ever bought a old boat and had to replace all the wiring because of inline splices and corrosion. Not to speak of the PO thinking that speaker wire makes a good system because that’s all he had at a moments notice. Fishing being the priority LOL.
If I built brake controllers any amateur, was going to install in his vehicle, I would definitely recommend a intermittent breaker. Actually I would recommend they take it to Inland RV if they were in California. Smooth HUH Andy.
I have seen some horrible installation jobs locally, for this reason I decided to do it myself. Also breakers do fail, a friend of mine just had this happen.
Did I use a intermittent, the answer is NO. I encased the wiring from the battery to the trailer in flexible conduit. I made sure that this wiring would not see daylight in my time, nor would it see any sharp metal object on my Dodge truck.
After 32 years in aviation I have seen many a burnt wire that never gave a indication by “popping” a circuit breaker. I take that back, smoke in the cockpit or cabin is always a good indicator. Protect the wires properly.
I don’t enter this issue lightly, there is a time and place for “fused” circuits and if you doubt your, or your manufactures installation, I think you should have a intermittent breaker. If you trust the installation then I would rather see smoke, than see my family in the hospital or at the grave yard.

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