# How Do The Electric Trailer Brakes Work?



## folsom_five

Hi all-

Sorry if there is already an old topic on this... I couldn't find one...

I am looking for some help from someone who can give me a crash course on how the TT brakes work.
I "think" I understand that the brake controller will send power back through the truck wiring and into the trailer harness, which will then power the magnets at each wheel, but I'm looking for a little more details (electric detail) so I can have a logical discussion with my dealer when I go to talk to them about my brake issues.

Or does anybody have a readily available link that they can point me to to explain all of this?

Any help would be greatly appreciated.

--Greg


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## NobleEagle

I don't know if any of these links will help but it does give some useful information on the operation of electric braking systems. Good Luck!

Link 1

Link 2


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## folsom_five

NobleEagle said:


> I don't know if any of these links will help but it does give some useful information on the operation of electric braking systems. Good Luck!
> 
> Link 1
> 
> Link 2


Thanks. They do have some info that should help me out.

Another questions... when proportional power is sent to the trailer brakes, is it the voltage that is variable? Meaning at 100% I would expect 12VDC to be supplied to the TT brakes, but at 50% is only 6VDC sent to the brakes? Or is it the current (amperage) that is variable?

I read that each magnet should draw 3amps at full power. Is this correct? so at 50% power, do the brakes only draw 1.5amps?

--Greg


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## W4DRR

folsom_five said:


> I don't know if any of these links will help but it does give some useful information on the operation of electric braking systems. Good Luck!
> 
> Link 1
> 
> Link 2


Thanks. They do have some info that should help me out.

Another questions... when proportional power is sent to the trailer brakes, is it the voltage that is variable? Meaning at 100% I would expect 12VDC to be supplied to the TT brakes, but at 50% is only 6VDC sent to the brakes? Or is it the current (amperage) that is variable?

I read that each magnet should draw 3amps at full power. Is this correct? so at 50% power, do the brakes only draw 1.5amps?

--Greg
[/quote]

Yes, you have it correct. At 100%, 12V is applied to all the brakes, and they will each draw approximately 3A for a total of 12A for 2-axle trailers. At 50%, 6V is applied. The current to the brakes will be proportional to the voltage applied by virtue of Ohm's Law. i.e. 50% = 6V = 1.5A for each brake.
Hope this helps.

Bob


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## webeopelas

I'm not 100%, but I think the amps are variable. Especially since controllers like the prodigy display amps not volts. Amps are also more easily controlled with resistance than volts since the relationship is linear ie Volts =amps/resistance (curved) so amps = volts*resistance(linear).


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## folsom_five

I found this link during my searches. It has some good info on brakes.

Dexter Axle FAQ


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## BoaterDan

webeopelas said:


> I'm not 100%, but I think the amps are variable. Especially since controllers like the prodigy display amps not volts. Amps are also more easily controlled with resistance than volts since the relationship is linear ie Volts =amps/resistance (curved) so amps = volts*resistance(linear).


No no. Read your manual or the Tekonsha site. The display is voltage according to them, and they ought to know.









The amperage is going to be static, 3 amps per brake.

From the Dexter axle site:

BRAKES - How do I measure brake current or amperage?
Dexter 7" brakes draw about *2.5 amps per brake and all other brake magnets draw about 3 amps per brake*. The total system amperage is calculated by multiplying this number by the number of brakes/magnets connected to the brake system. To measure the brake current the engine of the towing vehicle should be running. Disconnect the wire at the point that you wish to measure the current draw and put the ammeter in series with this line. Make sure your ammeter has sufficient capacity and note polarity to prevent damaging your ammeter. NOTE: If a resistor is used in the brake system, it must be set to zero or bypassed completely to obtain the maximum amperage reading. The amount of current draw will depend on what point you are measuring. If the ammeter is at the plug, you will get the total current draw from all magnets. If the ammeter is connected at one of the magnets, you will measure the current draw through that magnet only.
Back to Top

BRAKES - How do I measure brake voltage?
System voltage is measured at the magnets by connecting a voltmeter to the two magnet lead wires at any brake. This may be accomplished by using a pin probe inserted through the insulation of the wires dropping down from the chassis or by cutting the wires. The engine of the towing vehicle should be running when checking the voltage so that a low battery will not affect the readings. *Voltage in the system should begin at 0 volts and, as the controller bar is slowly actuated, should gradually increase to about 12 volts*. This is referred to as modulation. No modulation means that when the controller begins to apply voltage to the brakes it applies an immediate high voltage, which causes the brakes to apply instantaneous maximum power. The threshold voltage of a controller is the voltage applied to the brakes when the controller first turns on. The lower the threshold voltage the smoother the brakes will operate. Too high of a threshold voltage (in excess of 2 volts as quite often found in heavy duty controllers) can cause grabby, harsh


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## W4DRR

webeopelas said:


> I'm not 100%, but I think the amps are variable. Especially since controllers like the prodigy display amps not volts. Amps are also more easily controlled with resistance than volts since the relationship is linear ie Volts =amps/resistance (curved) so amps = volts*resistance(linear).


The braking level displayed on the Prodigy is the approximate VOLTAGE applied to the brakes. This is per the Prodigy manual.

The Prodigy provides proportional braking by virtue of PWM (Pulse Width Modulation). For all practical purposes, this provides a variable voltage. The 12V is turned on and off at a fast rate, and the duty cycle determines the average voltage. 50% on / 50% off = 6V, 25% on / 75% off = 3V, etc. It is the same technique that has been used in light dimmers for years.

The relationship between volts, amps, and ohms is always linear. Volts = Amps x Ohms. Amps = Volts / Ohms. Ohms = Volts / Amps.

Bob


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## webeopelas

W4DRR said:


> I'm not 100%, but I think the amps are variable. Especially since controllers like the prodigy display amps not volts. Amps are also more easily controlled with resistance than volts since the relationship is linear ie Volts =amps/resistance (curved) so amps = volts*resistance(linear).


The braking level displayed on the Prodigy is the approximate VOLTAGE applied to the brakes. This is per the Prodigy manual.

The Prodigy provides proportional braking by virtue of PWM (Pulse Width Modulation). For all practical purposes, this provides a variable voltage. The 12V is turned on and off at a fast rate, and the duty cycle determines the average voltage. 50% on / 50% off = 6V, 25% on / 75% off = 3V, etc. It is the same technique that has been used in light dimmers for years.

The relationship between volts, amps, and ohms is always linear. Volts = Amps x Ohms. Amps = Volts / Ohms. Ohms = Volts / Amps.

Bob
[/quote]

Okay, I stand corrected on the amps vs volts question, and I even got Ohm's law wrong,(been a long time since college) but the relationships are not all linear. If you graph V=I*R it is linear, Graph I=V/R and it is a curve, so the rate of change in amps for a given change in resistance is not constant. And now I will hang my head in shame for posting without verifying my facts


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## Northern Wind

Huh?


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## W4DRR

webeopelas said:


> I'm not 100%, but I think the amps are variable. Especially since controllers like the prodigy display amps not volts. Amps are also more easily controlled with resistance than volts since the relationship is linear ie Volts =amps/resistance (curved) so amps = volts*resistance(linear).


The braking level displayed on the Prodigy is the approximate VOLTAGE applied to the brakes. This is per the Prodigy manual.

The Prodigy provides proportional braking by virtue of PWM (Pulse Width Modulation). For all practical purposes, this provides a variable voltage. The 12V is turned on and off at a fast rate, and the duty cycle determines the average voltage. 50% on / 50% off = 6V, 25% on / 75% off = 3V, etc. It is the same technique that has been used in light dimmers for years.

The relationship between volts, amps, and ohms is always linear. Volts = Amps x Ohms. Amps = Volts / Ohms. Ohms = Volts / Amps.

Bob
[/quote]

Okay, I stand corrected on the amps vs volts question, and I even got Ohm's law wrong,(been a long time since college) but the relationships are not all linear. If you graph V=I*R it is linear, Graph I=V/R and it is a curve, so the rate of change in amps for a given change in resistance is not constant. And now I will hang my head in shame for posting without verifying my facts








[/quote]

If the applied Voltage is held constant, current will always be inversely proportional to resistance. In other words, if resistance is doubled, current will be halved, if resistance is tripled, current will be 1/3, if quadrupled, current will be 1/4, etc. etc. It is a linear relationship.
If you set up your graph with actual numeric values of resistance equally spaced...1 ohm, 2 ohms, 3 ohms, etc. you will get a curve. This is because the change from 1 ohm to 2 ohms is doubling, but 2 to 3 is only a 50% increase, and 3 to 4 is a 33% increase, and so on. Try graphing on a logarithmic scale.

Bob


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## BoaterDan

Right Bob.

Since this is a stretch for me remembering college physics <cough cough> years ago, verify if this is right:

The ultimate point here in terms of braking is the amount of Power applied to the magnet. The more power applied, the stronger they grab.

Power (watts) = V * I (volts x current)

That's why the current remains constant at 3 amps and varying the voltage varies the power and thus the grabbing force.

Yes, no, maybe so?


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## W4DRR

BoaterDan said:


> Right Bob.
> 
> Since this is a stretch for me remembering college physics <cough cough> years ago, verify if this is right:
> 
> The ultimate point here in terms of braking is the amount of Power applied to the magnet. The more power applied, the stronger they grab.
> 
> Power (watts) = V * I (volts x current)
> 
> That's why the current remains constant at 3 amps and varying the voltage varies the power and thus the grabbing force.
> 
> Yes, no, maybe so?


To vary the voltage, and maintain a constant current would imply the resistance is changing. Ohm's Law again. But the resistance of the brake solenoids remain constant at approximately 4 ohms each. We get a variable braking force by virtue of the voltage being varied, and the current through the solenoids is proportional to the applied voltage. Current = Voltage / Resistance. Ohm's Law again.
And FYI, the actually braking force is a function of the current through the solenoids producing a magnetic field. Most of the power (voltage x current) dissipated by the solenoid does nothing to contribute to the braking force. This power, dissipated as heat, is mostly the result of the DC resistance of the solenoid. It is an unfortunate fact that wire has some resistance. What makes it work is the current creates a magnetic field, and it is possible to create a magnetic field without dissipating any power, at least in theory. But when we ask that magnetic field to do work, such as move the permanent magnet that moves the brake shoes against the drum, an impedance to current flow is created that is the equivalent to resistance, and some power will be dissipated. But that is a whole other topic.

Bob


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## Rip

So back to the ???? Does the magnet when energize ride out on a arm and try to attach to the rotating brake drum which intern pulls the brakes out into the drums ?????


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## W4DRR

Here is an animation from AL-KO that shows how the brakes activate....
AL-KO Link


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