I need some extra eyes on this circuit.

Discussion in 'The Projects Forum' started by jonfarrugia, Mar 8, 2016.

  1. jonfarrugia

    Thread Starter Member

    Feb 19, 2010
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    I've attached a circuit that I'd like to use in my car to drive high current loads from a PWM output of my arduino (D2/PWM).

    I have a 5v supply for the arduino and a 24v supply to drive the mosfet as a high side driver...and yes I want a high side mosfet. The 12v supply is the car battery.

    D2/PWM drives the circuit from an arduino output
    D24 is an input on the arduino to determine if the fuse has blown
    A1 is an analog input on the arduino used to measure the current flow through the shunt

    D2 runs a pwm which runs a PWM at 24v to the mosfet. The mosfet supplies 12Vdc to the load through a 0.001 ohm shunt resister. The 1NA169 circuit is used to amplify the shunt signal to the arduino analog input. D24 should always be high unless the fuse has blown.

    Now that I look at the circuit, is the mosfet high when D2 is low?

    How does this circuit look...any feedback would be great. I'm know engineer!

    I need to make sure there is very little current draw when the circuit is off to prevent draining my car battery. Also this circuit is intended to allow a max 100amp draw
     
  2. jonfarrugia

    Thread Starter Member

    Feb 19, 2010
    20
    1
    I forgot to mention that the TP1 jumper is used just to test the circuit by adding a jumper to that position to enable the circuit
    All power supllies reference the same ground.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Why dont you use a Pfet, and then you can still feed the positive supply, then you don't need the 24v.

    D24 will be low unless the fuse is blown, you need a pnp to reverse this.
     
    Last edited: Mar 9, 2016
  4. John P

    AAC Fanatic!

    Oct 14, 2008
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    I assume D24 is used to tell the processor that power is available.

    You didn't say what the frequency is, but it seems that the gate of the FET Q2 and10K resistor R9 will form an RC circuit, where the discharge will be fast, but charge time may be slow. If the FET spends a lot of time in the linear region, it may heat up more than you'd like it to.

    Should there be a fail-safe device to ensure that the FET isn't always on, if the processor freezes? It could be a simple charge pump that requires an active waveform on the PWM line. Actually I'm not totally comfortable with the idea that R9 makes the circuit "default on" but it's your call.
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The absolute maximum Vgs is 20V according to the IRF2204L datasheet, so 24V would probably let the magic smoke out if the FET source was held low for any reason (e.g. big load capacitor, transient, shorted load). Incidentally, that FET is listed as obsolete.
     
  6. Picbuster

    Member

    Dec 2, 2013
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    If I read your schematic I see that the xtor connected via 1K is producing a high current into the base.
    I do not understand the strap(jumper) this will connect the dp2/pwm input / output generator directly to 5V .
    This total circuitry is lets say strange and open to all type of problems.
    please tell us what you want to achieve and why.
     
  7. jonfarrugia

    Thread Starter Member

    Feb 19, 2010
    20
    1
    I didn't use a Pfet due to their price. I plan on making a bunch of these so I need to consider the cost.

    The Strap jumper to 5v on the base of Q1 is just used in the field to be able to turn the circuit on and off manually for setup purposes.

    The 24v supply is actually the output of a charge pump created from a PWM line of the arduino but I didn't show that circuit.

    I made a few changes based on the suggestion but I'm a bit confused how to get around the Vgs 20v max limit if the load is grounded. Wouldn't this blow the fuse before damaging the fet?
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Q4 is drawn wrong, its shown as a pnp with the C/E terminals reversed., why the extra transistor Q2 to feed the fet??
     
  9. jonfarrugia

    Thread Starter Member

    Feb 19, 2010
    20
    1
    Changed Q4 Drawing. For some reason, every PNP transistor in KiCAD is drawn that way. I made a new one.
    I added the extra trasnsitor to make sure the fet logic was the same as the arduino logic. Arduino High, Fet conducts. Arduino Low, Fet does not conduct.
    Is there a better way of doing this?

    After looking at it again, i figured instead of two npn transistors i could use 1 pnp transistors.
     
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    No. The FET reacts quickly, the fuse doesn't. So the FET is there to protect the fuse :).
     
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    A pnp won't work as its base is always lower than the emitter due to the psu difference voltage.,stick with the npn.
     
  12. jonfarrugia

    Thread Starter Member

    Feb 19, 2010
    20
    1
    OK I changed the circuit back to the npn transistors. Do the resistors look ok?

    How do I prevent damage to the fet if Vgs exceeds 20v?
     
  13. jonfarrugia

    Thread Starter Member

    Feb 19, 2010
    20
    1
    I think I have the final version. I ended up using a 12v zener between Vgs to protect for voltage above 12V.
     
  14. John P

    AAC Fanatic!

    Oct 14, 2008
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    If you're going to have 2 transistors controlling the FET gate, why not make one of them a PNP switching 24V, controlled by the other? That would satisfy my feeling that the FET should be actively, not passively, turned on.Then to meet the requirement that the gate voltage shouldn't be greater than +/- 20V, make the pull-down resistor on the gate be a voltage divider pulling the gate below 12V, but not all the way to Gnd. Then there wouldn't be any need for the zener.
     
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