Trailer Dolly Brake Control - Fiberglass RV


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Old 03-28-2012, 07:45 PM   #1
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Trailer Dolly Brake Control

I have added some hardware to my trailer dolly. Before I actually connect it to my trailer, I would appreciate some input.

The operation of the unit is as follows. The trailer is mounted to the ball on the dolly, the trailer is plugged into the dolly. The selector switch, which should be in the off position to start, is turned to the desired power source. This would be either the trailer battery itself, or a break away battery on the dolly. This should energize the brakes. The green light on the push button lights, indicating that the trailer breaks are on. If the light is dim, then the selected battery is low on charge. When the button is pushed, the circuit to the brakes is broken. The button light goes off and the trailer brakes release. Now the trailer can be moved. If the operator falls, or whatever, then the button is released and the brakes are activated. Basically a dead man switch for the trailer brakes.

The part that I am conserned with is the trailer plug. In the drawing, the plug is shown from the terminal side, so the actual plug that the trailer would plug into is reversed. I am using the ground connection on the trailer connector to complete the circuit for the dolly battery, but I am assuming that the negative of the battery on the trailer would also connect to it. This means that there is no obvious negative return when the trailer battery is used.

If anyone can see a problem with the design, I would really like to know. I don't have much experience with the trailer connection, and my assumptions may be wrong.
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File Type: pdf Trailer Dolly Brake Control.PDF (7.5 KB, 67 views)
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Old 03-28-2012, 10:46 PM   #2
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Pictures

Here are some pictures of the dolly
Attached Thumbnails
Trailer Dolly Brakes - Before.jpg   Trailer Dolly Brakes - Control Wiring.jpg  

Trailer Dolly Brakes - Control Wiring 2.jpg   Trailer Dolly Brakes - Control Wiring 3.jpg  

Trailer Dolly Brakes - Plug and Battery.jpg   Trailer Dolly Brakes - Sequence - Off.jpg  

Trailer Dolly Brakes - Sequence - On.jpg   Trailer Dolly Brakes - Sequence - Lets Go.jpg  

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Old 03-28-2012, 11:08 PM   #3
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Quote:
Originally Posted by David Tilston View Post
If anyone can see a problem with the design, I would really like to know. I don't have much experience with the trailer connection, and my assumptions may be wrong.
Some of us have built similar boxes only using the pigtail that normally plugs into the tow vehicle, as in this thread:
Emergency flasher

When I built mine, I only had to wire the positive side of a circuit from the "Charge Line pin" of the receptacle through a switch to the pin for the circuit I wanted to activate. The negative side took care of itself inside the trailer.

I would prefer to use a rheostat for the brake circuit instead of a switch, for smoother operation. But I like your fail-safe circuit adaptation.
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Old 03-28-2012, 11:34 PM   #4
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Originally Posted by Frederick L. Simson View Post
I would prefer to use a rheostat for the brake circuit instead of a switch, for smoother operation. But I like your fail-safe circuit adaptation.
I am still considering a large resister, (high wattage) and/or possibly a coil in line with the brakes to limit the current/voltage. I also need a fuse on the dolly battery and the trailer power supply. I was thinking 20A.
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Old 03-29-2012, 06:18 AM   #5
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This sounds like one of those projects that you have to build and test to get working properly. However, when I added a breakway switch to my Casita, the manufacturer's instructions clearly stated "The breakway switch should not be used as a parking brake." To me, that means that the the trailer brake coils are going to overheat or there will be other problems if you connect a battery directly to them for too long.

The maximum power of my Tekonsha is usually set at 50% of what is available and from watching my brake controller in action, I seldom need more than a few amps going to the brakes when actually stopping. A big resistor will limit the power to the coils but generate a lot of heat. I tried to find a brake controller schematic but these don't seem to be available online. I believe that they're a lot more complicated than a simple switch, using some sort of solid state switching and are possibly pulsing the brakes.

In short, I think using a battery for brakes on a trailer dolly is a good idea, but only with very limited current and for very short periods of time.
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Old 03-29-2012, 07:49 AM   #6
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looks good. ad a 12 volt motor and pulley drive to the wheels now,,,,with forward and revers....hmmmm robo trailer parker.
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Old 03-29-2012, 11:33 AM   #7
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Originally Posted by john warren View Post
looks good. ad a 12 volt motor and pulley drive to the wheels now,,,,with forward and revers....hmmmm robo trailer parker.
Oh believe me, I thought about it!
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Old 03-29-2012, 11:36 AM   #8
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Modern brake controllers use a form of yaw sensor, sensing the rate of acceleration or deceleration to set the rate of braking to the trailer.
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Old 03-29-2012, 12:23 PM   #9
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Quote:
Originally Posted by Terry G View Post
"The breakway switch should not be used as a parking brake." To me, that means that the the trailer brake coils are going to overheat or there will be other problems if you connect a battery directly to them for too long.

The maximum power of my Tekonsha is usually set at 50% of what is available and from watching my brake controller in action, I seldom need more than a few amps going to the brakes when actually stopping. A big resistor will limit the power to the coils but generate a lot of heat. I tried to find a brake controller schematic but these don't seem to be available online. I believe that they're a lot more complicated than a simple switch, using some sort of solid state switching and are possibly pulsing the brakes.

In short, I think using a battery for brakes on a trailer dolly is a good idea, but only with very limited current and for very short periods of time.
I think you are correct. The figure I have heard for current consumption of the brakes is 6A. If this is at 12 VDC, then the brakes are 2 ohms and are consuming 72 watts. If I put a 4 ohm resistor in series, the current should drop to 2A. and the resister should be dissipating 16W with the brakes down to 12W. Trouble is a 16W resistor would require a heat sink.

I would not be using this for a parking brake. I suspect that the break away battery would provide no more then 10 min of power to the brakes.

I guess I should go and measure the resistance of the brake coils.
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Old 03-29-2012, 09:07 PM   #10
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Originally Posted by David Tilston View Post
Oh believe me, I thought about it!
If you're really interested in building a 12 volt power dolly and are a member of the CasitaForum, Jack posted a thorough thread with pictures: 12 volt Power Dolly Build. Like I said, you have to be a member and logged in.
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Old 03-29-2012, 10:15 PM   #11
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Originally Posted by Terry G View Post
...............However, when I added a breakway switch to my Casita, the manufacturer's instructions clearly stated "The breakway switch should not be used as a parking brake." To me, that means that the the trailer brake coils are going to overheat or there will be other problems if you connect a battery directly to them for too long........
Manufacturer is likely correct. Braking action is proportional to current flow through the actuator's electromagnet resulting in friction between the electromagnet and the drum plus the drum rotation. Higher current = higher friction, higher drum’s RPM = higher force on the actuator (edited: due to some increase of coefficient of friction with higher speeds). If the drum is not rotating the current would have to be high enough to lock the electromagnet with the drum. That is most likely why manufacturers don’t recommend trailer brakes being used as parking brake; Braking action is proportional to current and RPM, at least this is my understanding but I have been wrong in the past.

George.
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Old 07-16-2013, 12:14 PM   #12
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I thought this was a great idea when David posted it, and now that another topic has reminded me of it I have a suggestion:

A really cheap ($40 brand new, who knows used) non-proportional timer-based brake control that I wouldn't consider using in a tow vehicle could serve well on the dolly. Power from the trailer battery to the controller's brake light input (usually the red wire) could go through an on/of switch and a normally-closed switch as a "deadman" feature, the normal gain adjustment could be used to limit applied voltage, and the manual lever of the controller would be functional.
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Old 07-16-2013, 12:56 PM   #13
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Well, all I can say is that it works for my purposes. The brakes actually get to lock up in a little more then a second, (inductive impedance?). They release right away. A brake controller might work better, but the cost for this job, (minus the battery) was about $10. I have since found a source for free batteries. Plus it is possible with this design to use the battery in the trailer. Princess Auto sells all the parts for this.

I think that the recommendation not to use as a parking break might be about 100% duty cycle for hours. The brake coils might get hot, especially with none of the air flow that would come along with traveling down the road. I only use the brake for as long as it takes me to put the wheel chocks down.
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Old 07-16-2013, 09:53 PM   #14
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Well, all I can say is that it works for my purposes... A brake controller might work better, but the cost for this job, (minus the battery) was about $10.
I'm sure it does. I was just suggesting the controller as a convenient way to address the resistor addition that was discussed, and to provide a smoother way to manually apply the brakes.

Quote:
Originally Posted by David Tilston View Post
Plus it is possible with this design to use the battery in the trailer.
Same with a controller: the controller would replace only one of the switches - and any potential resistor - in the current design.

I like the idea of using the trailer battery, just because it is the simple approach and it's one less battery to maintain.
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