Grounding multiple supplies

Discussion in 'The Projects Forum' started by farso, Mar 2, 2010.

  1. farso

    Thread Starter New Member

    Mar 2, 2010
    2
    0
    Hi everyone

    I am at university and have been tasked with designing a controller for a SRM. We have two supplies, a 15V @ 10A supply to power the windings of the SRM though a mosfet. We also have a 5V @ 1A supply to power a logic circuit which detects when the current goes over a certain amount.

    The problem we are having is the current is measured by comparing a reference voltage with the voltage drop across the mosfet through a 0.1Ω resistor. The problem with this is because the two are at different grounds we cannot get the desired reading.

    I was thinking, would connecting together the grounds be a good idea? Would it solve our problem? I have talked to several PHD students, none of which seem to know.

    If anyone here could help me, that would be great thanks.

    Additionally, if anyone is interested with the rest of the project I could post more details.

    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Connecting the grounds might solve one problem, and create yet more problems.

    Posting a schematic is a good starting place. .png format is preferred; those types of image files work best with line graphics like schematics.
     
  3. farso

    Thread Starter New Member

    Mar 2, 2010
    2
    0
    Hi,

    Thanks for the response. I have (tried to) attach a circuit diagram.


    Description
    The circuit compares the voltage dropped over the 0.1Ω resister just below the mosfet on the right hand side of the page with the input voltage.

    If the current (voltage over resistor) is above a certain amount, kill the inputs from the dsk for a period of time determined by the 555 timer using logic.

    If everything else is normal, allow the dsk to control the mosfet through an amplifying transistor powering a push pull.

    Hope all is clear.

    Thanks
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You don't seem to have any MOSFETs in that schematic. However, I'll assume that you meant the transistor sinking current from the primary side of the transformer is supposed to be a MOSFET.

    The way things are now, you will have to have a common ground between the two, as you won't be able to control the MOSFET gate without it.

    If you want them to be completely isolated, you will need to use an optocoupler to connect the logic with the power side to switch the MOSFET gate; you will need to have the comparator on the power side as well, and you will need another optocoupler on the output of the comparator to send the signal back to the logic side.

    There is another problem with your circuit; when the MOSFET turns off, there will be noplace for the transformer primary current to go, and it will blast the MOSFET with hundreds of volts, killing it. You need a diode connected across the transformer to take care of that.

    There is yet another problem that you will run into if you attempt to run the transformer at more than 50% duty cycle, and that is a phenomenon known as "flux walk". Google that term for more information.
     
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