Staying in Control: Electric Brake Controllers
It’s a fact that most caravans are equipped with electric brakes which means that the tow vehicle needs to be fitted with an electric brake controller.
As we explained in the previous issue of this magazine, electric trailer brakes use a variable voltage to control the amount of trailer braking. This variable ‘control signal’ is required by Australian Design Rule 38/02 so that the trailer brakes can progressively increased and decreased from the towing vehicle. The maximum level of this ‘control signal’ is usually varied by means of a thumbwheel, sliding or rotary potentiometer or digitally via a menu system.
No matter what type is fitted, the controller must have a manual override control for operating the trailer brakes independently of the tow vehicle brakes. This device must be accessible from the normal driving position.
On most brake controllers, this manual override control allows the driver to vary the amount of trailer braking depending on how far the control is moved. Some limit the manual override power to the preset braking level but others can vary the trailer braking anywhere from zero to full power, independent of the preset level.
Most controllers also have a visual feedback system such as a LED which changes colour or brightness as the amount of trailer braking changes. Some that are equipped with LCD or LED dot matrix displays can also signal error conditions such as loss of trailer brakes, short circuits or other overload.
Solving these requirements has lead to a number of different solutions which generally sees electric brake controllers grouped as either fixed gain, time delay or proportional brake controllers.
The simplest type of electric brake controller is what is known as a ‘fixed output’ controller which produces the same voltage on the ‘control signal’ for each brake event. As with all electric brake controllers, the amount of braking can be altered by changing the maximum output level. However, with this type of brake controller, as soon as the brake pedal is touched the trailer brakes are applied at the preset level. This means that the trailer will be braking to the same level whether gently rolling to a stop at the traffic lights or in a full emergency stop.
One advantage of this type of controller is that is can be mounted anywhere at any angle because it cannot sense vehicle deceleration. This also makes it easy to split the unit into a small remote control panel and the main electronics unit.
The control panel is often so small that it can be mounted in an unused dash switch by drilling out the switch blank. The main electronics unit is hidden behind the dash wherever is convenient.
Braking is usually activated either by pressing an OFF-ON pushbutton (for trailer brakes only) or by application of the vehicle foot brake. The amount of braking performed by the trailer is usually controlled via a rotary potentiometer.
A LED is also usually provided for visual feedback. Typically this LED will be OFF with no trailer connected and then turn green when a trailer is connected. It then changes to red when the brakes are applied with the brightness of the LED varying according to the preset brake level.
HOW THE CONTROL SIGNAL IS VARIED
Electric brake controllers can vary the voltage on the ‘control signal’ anywhere from zero to the full battery voltage. Instead of producing a steady DC voltage via a high power resistive divider, these controllers rapidly switch the voltage on and off from 12V to GND at around 300 times per second. The ratio between the ‘on’ and ‘off’ times determines the average voltage, a process known as Pulse Width Modulation (PWM).
For example, if the ‘on’ time equals the ‘off’ time the output voltage is half the battery voltage. Similarly if the ‘on’ time is twice the ‘off’ time the output voltage is two-thirds the battery voltage.
Incidentally, the hum produced by electric brakes is due to the rapidly switched PWM signal making the magnets in the drums vibrate.
HOW TO WIRE UP A BRAKE CONTROLLER
Electric brakes are relatively simple to wire into your tow vehicle. No matter which type is being installed, only four wires are needed:
1) Black - +12V power,
2) White - GND,
3) Red - ‘brake-on’ and
4) Blue - ‘control signal’ output.
Note that all manufacturers appear to have adopted the same colour code for these wires.
The ‘brake-on’ active signal is usually connected to the active side of the stop-light switch. Alternatively, the ‘brake-on’ signal needs to be wired to the active side of the brake lights. In fact this second method may be the only way to trigger the brake controller in modern vehicles equipped with CAN bus which do not have a separate stop light switch.
Time delay controllers are a little like fixed output controllers but have the added feature of being able to adjust the time it takes for the ‘control signal’ to reach its maximum output.
The time delay is usually adjusted by the ‘sync’ control which is most often a slider or rotary dial.
When activated, a time delay controller initially outputs a small voltage which then ramps over time to the maximum output level set by the user. The sync control simply adjusts the time it takes for the brake controller reach the maximum setting.
The main advantage of the time delay controller over the fixed output controller is that the aggressiveness of the trailer brakes can be controlled to some degree by changing the ramp time. The longer the brake pedal is depressed, the harder the trailer brakes will be applied.
When you know this fact, smooth stops when rolling up to a set of traffic lights can be achieved by a series of short brake applications rather than one long one.
A proportional brake controller varies its output signal according to how hard the vehicle is decelerating. If the vehicle is only braking gently, the trailer brakes will also be applied gently. In a full panic stop the trailer brakes will be applied to the fullest extent. In other words, the trailer brakes application is proportional to the severity of vehicle braking.
Proportional brake controllers fall into two categories:
1) Pendulum and;
2) Solid state.
Pendulum brake controllers use a small pendulum that is free to swing as the vehicle decelerates. Electronics, either optical or hall-effect, are used to determine the amount of pendulum movement and translate this into a variable output PWM voltage. The harder the vehicle decelerates, the further the pendulum moves forward and the harder the trailer brakes are applied.
Another advantage of the pendulum controller is that they automatically provide more braking power going downhill (when you need it) compared to going uphill. This is because gravity is pulling the pendulum past the zero position and into the braking zone so that the brakes are immediately applied as soon as the stop lights come on. The steeper the hill the harder the trailer brakes will be applied.
The problem with pendulum proportional controllers is that they are restricted in how they are mounted as follows; 1) the back of the controller must face the front of the vehicle, 2) the controller must be level side-to-side, and 3) the controller is restricted as to how far it can be tilted upwards or downwards. Even when mounted correctly they must be calibrated to ensure that the pendulum is hanging straight down when the vehicle is on a level surface.
Solid state proportional controllers overcome these limitations by employing solid state accelerometers to measure the deceleration. Those with 3-axis accelerometers can be mounted in any orientation, including backwards and upside down.
Some of the more up-market proportional controllers also have a ‘boost’ feature which, similar to the time delay units, immediately outputs a preset voltage as soon as the vehicle brakes are applied. After a time delay this ‘boost’ level is replaced with the normal proportional output.
This ‘boost’ feature is particularly attractive when towing heavy trailers as the trailer will begin braking before the tow vehicle, thus keeping everything in line, even in a panic stop.
WHICH ONE SHOULD YOU GET?
The type of controller you install is obviously a personal choice but for my money the only type to get when you’re towing a heavy trailer is a proportional controller, preferably one with a solid state accelerometer and the boost feature. The time delay type is OK for lighter trailers but I would advise against a fixed output type except perhaps for those who want to be able to dial up a continuous fixed level of braking, such as when descending step declines off-road with a heavy camper trailer behind.