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Traila Brake Controller 2010-01-05

Here's a controller for 12V electric traila brakes.
I made this up back in 2003 when I built my 2000kg GVM traila & could'nt afford a controller at the time.

There is a control for intensity & a switch for manual activation.
It features soft start & Over Current Protection. (OCP)
With 2 IRFZ44 mosfets, should be able to drive 4 standard magnets (2 axels)
I didnt fit the manual switch as I can do it with the brake pedal (brake lights come on before brakes applied)

There a 2 issues I had with this design...
1. the OCP caused operating problems during commissioning, however bench testing both breadboard proto & PCB with real solenoids worked without fault. At the time I just removed the opamp & thats the extent of trouble shooting this problem (it could have been my external grounding)
I would implement the OCP differently if I did a revision, (like in the megasquirt injector drivers - much simpler)
2. After 5 years of operation the intensity would not adjust down & I was getting brake lockup when empty, I assumed this was either a bad cap or IC fault but ended up being a "no find cause", the fix was to change values of trim resistor attached to RV1 pot. ( something drifted)

The design is fairly simple & intended for cab mounting.
The unit consumes about 20mA when no brake signal, so disconnect +12 if not driving vehicle for month or so.

Operation is as follows.
Brake light signal applied to TB2-1 & inverted by Q1 & applied to IC1 shutdown pin to wake up IC1.
IC1 is a SMPS controller & this configuration was inspired from ?? .
The outputs are nored via diodes D5 & D6 to allow 100% duty cycle
Q2,Q3, Q4, D7, R10., R8. are high side MOSFET driver, this could have been done with high side driver IC but at the time was not easily available in small qty's.

D1,D3, C7 are charge pump to generate MOSFET gate voltage above Vcc.

The OVP circuit senses volt drop on resistor R11 using an opamp IC2A configured as differential amplifier, IC2B configured as a comparator with threshold set by trimpot RV2 The output of amplifier IC2A is proportional to current flowing thru R11.
The Comparator output is short pulses (depending on intensity adjustment)& pulls the soft start(SS) line effectively disabling IC1. The SS Capacitor ensures the line stays low while being pulsed. The OC indication circuit Q6, C11, R19, D10, R18 .... integrates the pulses to steady LED indication.

Other components D9, MV1,2,3, D2 are for circuit protection.

I used clear poly carbonate for the front panel & secured this in place with the control nob of the pot.(lazy but effective)

The 2 mosfets should have individual gate resistors eg 10R but this was overlooked at PCB design time as only 1 mosfet was originally in schematic. With the mosfets used I experience no operation issues.

The PCB is 2 sided & has Vias that can be made with "thru links" when making own PCB using resistor leg offcuts. I optimised the pad sized & tracks for DIY so should be ok for most.
Sorry about the incomplete B.O.M., it is output from protel schematic.

The Box is easy to make if you have a basic workshop, some holes are threaded.
I made the box about 6mm longer at the front than in the mech drawing (see photo),
Use heatsink compound between the 10x10x50 sq bar & the case & silicone insulator under the copper(CU) plate & insulating bushes on the screws securing the CU plate to sq bar.
I use bootlace ferrules on wires to terminal block & wires poke thru snug fit holes in rear panel. This provides reasonable strain relief for wire termination.
I conformal coated PCB for environment protection.

PCB copper prints are 1:1 so should be able to print page as is.
files are in the zip, enjoy
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