PIC to control kitcar

Discussion in 'Embedded Systems and Microcontrollers' started by mtechautomotive, Feb 11, 2009.

  1. mtechautomotive

    Thread Starter Member

    Feb 11, 2009
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    Sorry to open an old thread.

    I am after some related info to the above scenario.

    I am currently trying to make a digital fuse box for my kitcar.

    I would like a PIC to control digital fuses of some sort, that reset at say 20A, and that point stop condicting until the user resests. The code side of things is fine, its just the hardware I am having issues with, i.e the current sense for a 20A at 12V circuit, as well as best method of limiting/switching it off.

    Any help much appreciated!

    Thanks,
    Matt
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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  3. mtechautomotive

    Thread Starter Member

    Feb 11, 2009
    10
    0
    Hi sorry,

    Looking to simply give an ADC read to the PIC for reference etc.

    Would need to have for example a 20Amp 'digital fuse' i.e needs to allow 20A to flow, but have some sort of notifying output (ie voltage for PICs ADC) such that the pic can then control another peice of hardware (such as a high power fet) to turn off the 20A circuit.

    such that if a short etc (or more than 20A) is drawn, the circuit is disabled. But allowed to be reset by ther PIC.

    The ability to change the limiting current by for example a volatge based signal would also be very beneficial. So the user could set a fuse to be say 10A rather than 20A etc

    The code I am fine with as well as intercitcuitry, just the main 'digital fuse' that am having issues with.

    Looking to simulate it all in proteus first as well.

    Does that make sense?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    A sensing resistor in the line will develop a voltage across it proportional to the current through it. That voltage can be amplified and applied to a voltage comparitor, which will send a digital signal to the PIC

    A .01 ohm resistor won't waste much power. Current-sensing resistors in this range are available.

    For "digital fuse" substitute "relay" or "FET". Recommendations could be less general if you can post up the circuit.
     
  5. mtechautomotive

    Thread Starter Member

    Feb 11, 2009
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    0
    Beenthere,

    Indeed, my only concern is of the high wattage R I would need to dissapate the heat of 30ish amps.

    I have no schematic as yet, imagine that we are simply replacing a 12V 30A fuse with a circuit that does the following:

    Monitors the current flow and outputs a signal when it goes over 30A (or indeed an analogue 0-5v signal) indicating current flow to a PIC ADC input

    ...bit of code in PIC which uis no issue for me etc...

    The PIC can then turn off this circuit, until the user solves the short etc.

    It would be incredibly good if an analogue signal could be given to the circuit to indicate the required current threshold, again driven by the PIC and a DAC chip. again coding/digital section I am fine with.

    Ideally there would be 30 user probammable value 'fuses' in the box, so simplicity/cost effectiveness is quite important.

    Any ideas?

    Regards,
    Matt
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    For 30 amps and a .01 ohm fuse - P = I^2 * R, or 900 * .01 = 9 watts. Might be a bit much, so use .001 ohms and reduce dissipation to .9 watts.

    Need a schematic for ideas on the current interruption method (sensing, too).
     
  7. mtechautomotive

    Thread Starter Member

    Feb 11, 2009
    10
    0
    Beenthere (sorry do you have a name :) )

    I havent really started any schematics at all.

    I assume for interuption a 40A'ish rated FET would do the trick?

    Managing the output current by an analogue controler would also be beneficial. How could I do that? PWM I have thought about, but would that cause noise?

    Matt
     
  8. mtechautomotive

    Thread Starter Member

    Feb 11, 2009
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    0
    In terms of monitoring, am thinking along similar lines of the original post that I wrote on, or would that not be sufficient for these higherr currents?

    My analogue electronics skills are weak to say to the least!

    Matt
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    Many FET's and IGBT's exist that will handle 40 amps - it is interesting to note that the current has increased by 10 with each post. We will have some problems past 60 amps.

    Can you expand on the concept of a "digital fuse"? And what's a "kitcar"?
     
  10. mtechautomotive

    Thread Starter Member

    Feb 11, 2009
    10
    0
    Sorry, The max fuse will be 30A, its just a case of having some leeway.

    The idea stems from people wiring up their own self build cars 'kit cars' check google for locost, westfield etc.

    People usually have to use normal automotive fuses and boxes, and it usually just ends up being a mess. My aim it to make a tidy neat install with digital circuit breakers that the user can reset etc all in one tidy place with an LCD etc.

    I have just had a thought regarding current limits/user setting fuse ratings. We can simply switch off the supply IGBT at a different sense voltage.

    Do you know fo any sense voltage circuits that I could use, calibrated to say 0-5V for 0-50A (for ease of calucaltion in software.)

    Cheers,
    Matt
     
  11. mtechautomotive

    Thread Starter Member

    Feb 11, 2009
    10
    0
    Note the proposed 'digital fuse'

    I am after the current sense system to give 0-5V feedback (or thereabouts) for 0-50A of current flow.

    Hope this illustrates it well?

    Thanks,
    Matt
     
  12. mtechautomotive

    Thread Starter Member

    Feb 11, 2009
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    And I guess ina related question to reduce costs, could I run 2x20A power transistors in parralell to allow 40A current flow (80p a tranny rather than £4 for a 40A device)

    Matt
     
  13. beenthere

    Retired Moderator

    Apr 20, 2004
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    Why bother with going to that trouble? - Digi-Key has an Infineon device #IDP800N06N G rated at 60 volts and 80 amps for $.80/ea.

    You arrange a resistance in line with the current and amplify the voltage across that resistance. You can both display it, and let a voltage comparator signal the controller if the current exceeds some limit. If need be, you can use switching to let one amp and comparator do it all.
     
  14. mtechautomotive

    Thread Starter Member

    Feb 11, 2009
    10
    0
    Thanks that is brilliant, really changed things price wise.

    If the device is dissapating around 45W, will it not get 'warm' so to speak? Plan on having around 10 'fuses'; of a maximum of 30A continuous, gving around 450W of dissapation. That would surely super heat the case etc?

    Also do you have a rough circuit for driving a FET from a PIC?

    Matt
     
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