I am trying to make up a test kit to plug into a vehical’s 7 pin trailer connector and determin if the controler and wiring system is putting out the required 12 Amp 12 Volt DC power.
This trailer is used on 8 different trucks each having controlers in the cabs and all are running off road in a heavy industrial setting.
The trailer has two axles and the coils for the braking require 3 Amps each at maximum braking.
I want to be able to test each one on a regular PM inspection with a dummy load and also for a lioght load for brake drag on decline.
I am hoping to have tis rigger to a male connector and have 2 each lights, one for on (green) and one after some kind of resistor to come on in the area of 12 Amps (red).
Or am I wishing in one hand and filling the other with something else?
I do know that there are test plus that will light up if there is power, but I again am intereted in knowing I have full braking power for steep mine pit operation.
As a dummy load for the brake coils, I’m not sure if “3 Amps each” means 3 amps per axle or three amps per coil (wheel) For each 3 Amp load, you are going to need a 4 ohm, 50W resistor. Use 2 in parallel if it was 3 amps per axle, 4 in parallel if 3 amps per wheel. You may find that 3.9 ohms is a more common value, which is close enough.
Those will be big resistors and need to be mounted to deal with the heat. The aluminum case type need to be bolted to an aluminum or copper plate with heat transfer compound.
You can buy such resistors from Newark or Allied Electronics, maybe Digikey. (the first two are better bets for high power stuff, but the last one has better service IME) Radio Shack* will prove a waste of time.
For an indicator you have options:
-voltmeter across the load.
-amp meter in series with the load.
-12V light bulb across the load.
-LED:
Brake wire LED 9V zener diode 47ohm gnd
---------------->|----------|<-------------////-------
The zener keeps the LED off until you get over 11V or so. Note that the zener and LED are shown with opposite polarity.
You will have to futz with the resistor value to get it the brightness you want. You might need a 7 or 8V zener instead of 9V. All depends on how much voltage a “good” brake controller drops. The 9V zener assumes you’ll still have 11-12V left at the trailer plug.
Way simpler: Just use a headlight as a test load. These draw around 6A, and you can pretty well tell if it is getting full power or not.
*“You’ve got questions…We’ve got batteries!”
Thank You for the information,
No I am not Dale, I am Greg:)
Yes 3 Amps per coil, and 2 coils per axle for a 12 amp max load.
My 1st thought was a head light, then when I discussed it with a couple others they started with the resistors and shunting an Amp meter. But I do like the head light as it could be checked very easy by the driver.
This is a very harsh environment we work in. Any given day I will send one, some times more trucks to the shop for leaking air brake lines/fittings. Last month 2 trucks lost brake lines in traffic and that is 240 ton haul truck traffic.
And I want the KISS concept with this project.
Use two headlamps to get that load. Actually, I’d buy a set of cheap 55W generic after market driving lights…they come with mounting hardware…sealed beam headlamps would be a pain to fabricate a mount for.
If you do go the resistor route, you can safely use wirewound resistors and submerge them in a can of mineral oil for cooling as long as the duty cycle isn’t all that harsh.
That sounds like an interesting idea, however in addition I would like a light that would only illuminate when a full or near full load is applied, and a light to illuminate with a light load.
Thanks
