Hello All,

I want to build a manual control for trailer electric brakes for use when moving the trailer around on it's own with no tow vehicle connected. Basically, I want to be able to connect a 12v battery to the trailer 7 pin plug with some sort of manual control in between so that I can vary the braking force from zero to maximum. Ideally an on/off volume control type thing such that the knob clicks to On and then increases braking for from there to max.

Is this as simple as putting an appropriate rheostat (not a voltage divider / potentiometer) in between the battery and the brakes or more complicated? The brakes draw a maximum of about 8 amps at 12 volts DC. I believe brake controllers work by adjusting the voltage rather than the current, but my grasp of such things is quite tenuous. Your answers will be greatly appreciated.

Thanks and please pre-forgive my newbieness!

Thad

 

Assuming your brakes are a friction arrangement and the brake solenoid is sprung loaded to hold the brakes on when not powered then braking force can be controlled by switching the power to the brakes rapidly on and off around 20 - 100 times/sec, the ratio between on and off controls the effective braking force - this is known as Pulse Width Modulation (PWM) and is a common solution to such problems (inc dimming lights, controlling motor speed, and creating variable voltages).

The difficulty is that, depending on the mechanical arrangement, the braking force is likely to be very non-linear and a simple rotary control is unlikely to give a useful result (based on my experience of wheelchair brakes which are very simple friction clutches with an on/off sprung toggle).

 

There are quite a few 12V DC motor speed control boards on Ebay that may suit your application.


Buy one that has more current capacity than you need, like 40A, not just a 10A one.

 

Trailer brakes work much differently, at east the drum brakes do. An electromagnet pulls an attached ever against the inner side of the drum, and then the lever operates the brake shoes against the ID of the drum. The older hydraulic trailer brake control wasanot more that a high power variable resistor. I have an arrangement that uses a pedal operated variable resistor to apply the brakes. Those may still be available at some trailer supply shops.