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Please advise a good substitute potentiometer

Discussion in 'General Electronics Discussion' started by hotrodjohn71, Jul 16, 2019.

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  1. hotrodjohn71

    hotrodjohn71

    28
    1
    Jul 22, 2017
    Hello group. If I have posted this question in the wrong class, please advise and I will make it right.
    There is a component that I wanted to obtain but found out it is obsolete. It is a voltage control that you would install in a vehicle to control the amount of power going to the trailer brakes. Please bear with me as I am not sure of the correct terminology whether it is a potentiometer, variable resistor, or rheostat. It is meant to reduced power, I'm not sure if that means voltage or current or both, to the trailer brakes. I would like to find out the specifications of this variable resistor and buy one and install it to accomplish what the unobtainable obsolete version would do. Here are a couple of photos from the specification sheet. Which variable resistor do you think I could have came from an electronic store that would accomplish this? Thank you so much
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,826
    2,757
    Nov 17, 2011
    As this is for a safety relevant function (brakes) I highly recommend you do not use any replacement that seems to have similar parameters. Only replace by a product specified/certified explicitly for use in this application.

    A note on the side: why would you want to control the power going to the trailer brakes? I'd expect the trailer and its components to be designed such that the parameters match. Reducing the brake power imho is a risky adventure.
     
  3. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    I guess that the trailer brake power is needed to be adjusted to give appropriate retardation with a certain load.

    If a variable resistance cannot be found, then five 1 ohm resisters could be connected in series and a six pole switch could be used to select the required resistance.
     
  4. Bluejets

    Bluejets

    5,167
    1,081
    Oct 5, 2014
    That would be some power resistor combination.
    Given worst case, i.e. 4 brakes @3R a piece in parallel is 0.75R.
    With the 5R dialed in this would be around 2A flowing through the controller. (12v/5.75 =2.08A) and wattage would be 21.63W at the controller.
    With 1R dialed in this would then be around (12V/1.75 = 6.8A) and wattage (6.8*6.8*1) = approx 47W at the controller.

    Modern day controllers make use of electronic control elements such as mosfets to avoid such dissipation.
    Might well be time to upgrade.

    Edit:- included the full pdf below.
     

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    Harald Kapp likes this.
  5. Audioguru

    Audioguru

    3,440
    738
    Sep 24, 2016
    Electric brakes on an antique trailer? Usually the truck that pulls the trailer has the hydraulic brakes.
    Of course the old rheostat is obsolete.
     
  6. dave9

    dave9

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    311
    Mar 5, 2017
    So if you wanted alternate ideas, it seems you did not need that specific controller, either did not have that make/model originally or did and you found no place to purchase it?

    There are other ready made proportional brake controllers out there, which would make setting this up as safe as it can possibly be, safer than just using a rheostat to proportion based on weight because many also have a decceleromometer (lol is that a real word? accelerometer) to sense tow vehicle deceleration and actively adjust the trailer in addition to the proportioning based on weight.

    I don't know which are best, which are worth the money or other quality issues but keeping in mind that a merchant would like to make as much profit as possible, I suggest you contact one of them with your trailer specs and see what they recommend. For example:

    https://www.etrailer.com/dept-pg-Brake_Controller-sf-Proportional_Controller.aspx

    While you could try to rig something out of separate power resistors, that is more a question for a towing/trailer forum, to be answered by those who have experience with DIY as far as resistances, current/wattage, issues in use, etc. It might not be legal (might vary by state/other location) if there are codes about some kind of compliance or certification?
     
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