Converting negative signal into positive signal

Discussion in 'The Projects Forum' started by SilvrEclipse, Nov 21, 2007.

  1. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    Hey guys, I need some help. I am reconfiguring the ignition system on my car and need some help.
    We are dealing with pulse signals, basically power is provided to the coil pack. The ECU (engine control unit) provides a ground for these coil packs which allows them to fire the spark plugs.
    What I need to do is turn that ground into a positive 12v circuit limited to 500mA. This appears to be very easy to accomplish but my lack of knowledge in this field has left me with no solution. I appreciate all the help guys.
     
  2. Distort10n

    Active Member

    Dec 25, 2006
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    I limited knowledge of automobile electronics; however, I do believe the voltage used for auto electronics is 12V as you mentioned.

    Your issue is that you cannot invert ground (0V) potential to obtain a higher potential.

    What I do not know is what is the actually potential the spark plugs are referenced too?
     
  3. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    I have no idea what you are asking.
    Heres what I was thinking. Building a circuit that a 12v source would hook into, the "ground" from the ECU would also hook into this circuit. There would be a 3rd lead coming out that would be the 12v 500mA signal I am after. So whenever the ECU would "ground" this circuit try to fire the coil pack, I would get my 12v signal coming out the 3rd lead.

    This is just what I have come up with. Like I said I have very little knowledge about this stuff so thats what I need help with. The assembly and parts needed to build this thing. I have the rest of the system covered.
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
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    For R1, use 2.49 Ohm 1% or 2.4 Ohm 5%

    ignit.jpg
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Perhaps if you explain why you need to do this, it would help us understand your problem, and maybe provide a solution.
    Do you mean that each time the ECU provides the ground closure, you need a 12V pulse that is limited to 500mA to go the the coil? If so, I think we need some idea of what the output drive circuitry internal to the ECU looks like. Do you have a part number, or a make and model number?
     
  6. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    I didn't want to give to much info because I thought it might confuse some people. Basicaly my motor runs a "wasted spark" ignition system which means that the coil pack gets an all time 12volts and the ECU provides the ground. There are 2 seperate coils, 1 coil for every 2 cylinders. So when the ECU needs to fire say the "A" coil it grounds that circuit which fires the coil.

    Now the problem is that I need to be able to control the ignition timing which I cant do with this setup. So what I need to do is convert that ground into a 12v signal. I run this new signal into an aftermarket computer which will alter the signal to whatever I need it to be. This aftermarket computer will only take current less than 500mA or it fries. Then the altered signal comes out the computer and goes to a "transistor pack" which will be used from another car. I hook this pack to coils A and B and when I send the 12v signal to this pack is will ground one of them causing the coil to fire.

    This is almost correct, I need a 12v pulse but it wont go directly to the coil. I cant find any diagram of the actual ECU. I have a diagram of the ignition circuit. Here
    Here is the wiring diagram of a car with the transistor pack I will be using. The new ignition system should be close to this. Here

    So what about Thingmaker's circuit, would that not work?

    This process cycles 7.5 times/s at idle and 58 times/s at 7k rpms. Not sure if that would be a factor in the parts used.
     
  7. techroomt

    Senior Member

    May 19, 2004
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    a 12v relay could be wired to do the job, i believe one could be had to handle the cycle times you mention.
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I sort of understand what you want, but a datasheet for the aftermarket computer might help.
    (1) Are you replacing the powertrain control module in Ignition.gif with the power transistor module in Ignition2.gif?
    (2) Does the aftermarket computer provide a closure to +12V at its output?
    If the answer to both these questions is yes, then all you need is a 33 ohm series resistor (rated at 10 watts) between the two units, for each transistor. Since we don't know the actual circuitry at the inputs and outputs, it might be prudent to add a resistor of about 100 ohms, 1/4 watt, from each transistor input to ground, to insure that the transistors turn off rapidly. These resistors may already be present on the inputs and/or the outputs, but we don't know. That's why datasheets are so important.

    If the answer to either question is no, then we need more info (or at least I do).:confused:

    EDIT: I reread your posts. It appears that your aftermarket computer has a ground closure, and you need to invert that, providing 12V at approximately 500mA to the power transistor module. Is that correct?
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Since the minimum cycle time is 1/58=17 milliseconds, I suspect the delay and bounce of a relay would be unacceptable.
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
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    I might guess that the ground out of the module completes the current path through each individual ignition coil. The active device is probably a FET, with the drain to the coil and the source to ground.

    The problem with a relay is that the contact's time-of-flight is going to introduce a lag in the closure at anything but an idle. The line is changing between +12 and ground - use another FET. Look for Mil-Spec or at least certified at high temps.
     
  11. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    I think I have confused some of yall. I am using the stock ECU aka power control module. This unit provides a ground each time the coil needs to fire to complete the circuit. I am basically converting Ignition setup 1 into the 2nd one I posted.

    What I need is to build a circuit that will pulse a 12v 500mA signal EACH time that the ECU grounds this circuit. That signal will run into the aftermarket computer which will only advance or retard the signal very slightly. The signal will run from the aftermarket computer into the new transistor pack.
    I can draw a pic if it would make things easier. But how it works is not important I just need to build a circuit that will provide that 12v signal.

    EDIT - Aftermarket computer is Greddy Emanagement Blue
     
  12. Ron H

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    Apr 14, 2005
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    500mA seems like a huge waste of current for what appears to be just a control signal. I believe you said 500mA is the maximum. Do you know what the minimum is?
     
  13. beenthere

    Retired Moderator

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    I suspect that a signal going high will do the trick. An input into a computer should not ahve to supply more than a fraction of a milliamp of current. So, a small FET should do the trick - something like a 2N7000 with a moderate pullup to +12, like 100 ohms. Just place a 100 ohm resistor into the gate lead.
     
  14. thingmaker3

    Retired Moderator

    May 16, 2005
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    Without looking at your datasheet, my best guess is that your computer cannot source more than half an Amp. I suspect it does not actually require current limited inputs.

    If I'm wrong, simply use a 24 Ohm (or larger) resistor in series with the computer input.

    In either event, use any old transistor to invert the control signal. Lowest power waste would be by using a PNP or a P-channel FET.
     
  15. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    I appreciate all the help guys. I made a quick drawing to make sure I have everything right. Here

    So this PNP would be fine to use? PNP The PNP says 6a continuous so would I need a 2ohm resistor on the signal going to the PNP?
     
  16. Ron H

    AAC Fanatic!

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    I'm about to give up on this thread, because I still don't understand where in the larger circuit you intend to apply it. Can you put this PNP circuit into one of your larger schematics, so we can see where it goes?
     
  17. thingmaker3

    Retired Moderator

    May 16, 2005
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  18. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    Nah dude dont give up. This has got to be a super simple circuit to make. I made you a pic but its really not important how the circuit is used. I just need a 12v pulse whenever I send it a ground pulse. Here is the pic (left out that the transistor pack has a ground going to it which will be used to ground the coil packs. And there are 2 wire bc of the 2 coil packs. This circuit will have to be produced twice.)

    Thingmaker - I looked at your link, from what I can tell a PNP transistor will only let current through when a positive charge is applied? So this will not work for my application that I can see. Can you give me a diagram of how this would work?

    I found something called a ground-gate transistor. Isint this something like I need. When ever it is grounded it completes the connection? Haven't found to much info on these.
     
  19. beenthere

    Retired Moderator

    Apr 20, 2004
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    Any N channel FET with the gate driven off the coil ground signal - and perhaps a pull-up resistor to insure the output will be zero volts when the coil is not driven - will act as a signal inverter. I don't understand why a PNP transistor is so interesting.
     
  20. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    So on an n-channel FET I would use the 12v input as the source, send the ground to the gate and the 12v output would hook to the drain? From the info I have read regarding this FET it still seems like it will only allow current to pass through when voltage is applied to the gate. Am I just not understanding what yall keep saying? If I am wrong could someone make a simple drawing of this.

    And about the PNP transistor, it was mentioned that it could be used for this project so thats what all that talk is about.

    Also I dont know what you mean by a pull up resistor. Would it attach to the output of the FET and then to a ground to make sure the signal dies immediately?
     
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