DC Pump PWM with SSR, EMF protection

Discussion in 'The Projects Forum' started by Josh33, Aug 15, 2012.

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  1. Josh33

    Thread Starter New Member

    Aug 15, 2012
    Hi all,

    Apologies for the wall of text, but wanted to give as much relevant information as I could.

    After some help regarding a project of mine for my car. I am fitting the car with water methanol injection, and want to run the setup roughly proportional to fuel (as this has been proven to be more effective and less troublesome with these systems)

    I have a nozzle sized to 15% of the total fuel injector size, and a 12 volt 10 amp (rated) DC pump that I wish to PWM off the fuel injector earth signal from the ECU. As a result, as the injector signal increases (anywhere from 0-100% and 0-60hz as an example) the pump's duty cycle and pressure will roughly match, giving a more proportional flow than an on-off system.

    To do this, and after some reading, I have sourced a "Mager" 40amp DC-DC solid state relay that I think should do the job. The system also uses a pressure switch to power the relay, so the relay will turn only about 10% of the time the car is running, and the switching signal and load will be "disconnected" the remaining 90%.

    Some points I need help with:

    -I intend to supply 12V/battery power from the pressure switch to the positive switching side of the relay, and the injector earth signal to the negative, after using an oscilloscope to view the signal on the earth (worrying about the quality of the signal), I established the following:
    1.The earth has a battery voltage supply most of the time, equivalent to the input or positive side.
    2. The PCM then applies earth to open the injector, dropping the voltage near 0volts, (this is the bit I need to "copy") time at 0 volts varies with injector opening time.
    3. Once the earth is released, the injector causes what I assume is EMF, a spike lasting less than 2ms up to 26 volts, then returning to battery voltage before the next opening.

    My questions:
    1. Will I need to protect the input side of the relay with a diode against the 26V spike? If so, what size/type etc. diode should I use and where? Or will the duration of the spike being within the relays operating range (3-32V) not cause problems?
    2. Will the 10 amp DC pump on the output side cause any issues for the relay as it is an inductive load? Or will the relays 40 amp rating withstand this?
    3. Are there any other problems that I may encounter with this design? Such as an effect on the injector opening time or PCM operation? Or damage else where?

    If you read through the whole post, thank-you for your time and any input you have to offer. :)
  2. wayneh


    Sep 9, 2010
    So sorry, but that's all the text you needed to write. Auto mods are forbidden here, as you'll read about soon when this thread is closed.
  3. bertus


    Apr 5, 2008

    I am closing this thread as it violates AAC policy and/or safety issues.

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