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Circuit to drive a fuel injectors with NE555

Discussion in 'Automotive Electronics' started by milkoni, Feb 8, 2017.

  1. milkoni

    milkoni Member

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    Hi all,
    I've decided to build a device for testing and cleaning a car fuel injectors (petrol).
    I've made a simple circuit to manage a four injectors.
    My question is - Is this circuit will work properly?
     

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  2. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Look okay, except you have Q2 upside down.
    The emitter should be the output and the collector goes to ground.

    But note that at such slow speed, the 555 can likely drive the MOSFET gates directly without the transistor driver.
    If you want to try that, remove R3 and connect R4 directly to the gates.
     
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  3. milkoni

    milkoni Member

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    Thank you for advise!
    You mean to change the circuit like this:
     

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  4. dave

    Dave New Member

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  5. spec

    spec Well-Known Member Most Helpful Member

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    Hi Milkoni,

    A pretty neat design.:cool:

    Just a general observation: when testing injectors, ignition systems etc, it is best to be able to vary the on (dwell) period and the frequency. I think this can be done with a single 555 chip.

    Even though a switching element (BJT, IGT, MOSFET) may be switching at a relatively low frequency, it is still important to minimize the switching element turn on and turn off time to minimize stress of the switching element.

    In addition to Cruts' observations, here are a few from me.:)

    (1) The BC548/BC558 transistors are not really up to the job. BC337/BC327 high conductance, higher current transistors would be more suitable. The effective gate capacitance of a single MOSFET is huge so four in parallel amount to quite a capacitance.
    (2) It would be better to have an independent gate shaping resistor for each NMOSFET. I would recommend 22 ohms.
    (3) R4 is a tough high. I would recommend 47 Ohms.
    (4) The standard 555 timer needs at least 220nF ceramic capacitors directly across its supply pins.
    (5) The NMOSFETs also need decoupling. I would recommend a 1000uF low frequency decoupling capacitor across the supply line and 220uF ceramic capacitors physically close to each NMOSFET

    By the way, I have not looked at the deign of the 555 part of the circuit, but my impression is that it could be simplified.

    Please do not think that I am criticizing your design as I said it is pretty neat, well drawn and well presented.:)

    spec
     
  6. spec

    spec Well-Known Member Most Helpful Member

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    That is what Cruts advised, but can I just say to connect a separate 47R resistor to each MOSFET gate. 10K Ohms is way too high, especially for a single resistor.

    spec
     
  7. milkoni

    milkoni Member

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    Hi Spec!

    Like always wise and perfect advise thanks!
    I think that I realized your recommendations right:
     

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  8. spec

    spec Well-Known Member Most Helpful Member

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    Wow- thanks for your kind words.

    Yes, your schematic is what I had in mind, but I did not make it clear about the 220nF capacitors.

    The 220nF capacitors should connect to the 12V power line physically connected to each injector rather than to the drain of the NMOSFET

    The transistors connections still have the problem that Cruts pointed out in post #2.

    spec
     
  9. spec

    spec Well-Known Member Most Helpful Member

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    Hi again Milkoni,

    Please do not think that I am criticizing your work, but below is how I suggest that your schematic should look (I have not analyzed the the 555 timer part). The red dots indicate changes.

    You may think it odd that I have rearranged the ground (0V) connection on the schematic, but this is intended to reflect the physical layout to a degree.

    While on the subject of physical layout, it would be wise to have a two star point arrangement, one star point for ground (0V) and one star point for 12V.

    spec

    2017_02_10_INJECTOR_TESTER.png
     
    Last edited: Feb 10, 2017
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  10. milkoni

    milkoni Member

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    It's Ok Spec - there is no problem.
    My idea was to publish my circuit here, to get the best advises from specialists like you Spec, Curt, Mike and others like this!
    I would like to make a proper circuit, which will work properly with your advises.
    So, thank you for your proposals!
    I've reworked the circuit as you recommend.
    Now I hope everything is fine!
    But I've decided to add a 4 push on/push off buttons to switch on/off each injector to my schematics.
    Something like this:
     

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  11. Tom Stark

    Tom Stark New Member

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    I cannot comment on the design as others here know FAR more than me, but I do have a question for you. If this is just for cleaning and testing injectors, why not just use a battery and momentary switch? When I have to test an injector I just hook it up to a fixture that I put WD40 into, add some pressure, and put the juice to the injector and look at the spray pattern. Not criticizing just wondering.
    Thank you
    Tom
     
  12. spec

    spec Well-Known Member Most Helpful Member

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    Hi TS,

    The spray pattern of an injector is vital, as you imply. Another important characteristic is cut off- does the injector seal completely when de-energized.

    But the dynamics of an injector also need to be tested so you need to exercise the injectors at high speed.

    Also, if you are cleaning an injector with a suitable solvent, the injector needs to be exercised for quite a while and it would be rather tiresome to have to keep switching the injector on and off with a switch.:)

    spec
     
  13. Tom Stark

    Tom Stark New Member

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    Thank you.
    Tom
     
  14. milkoni

    milkoni Member

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    May be this will work properly. I will add a on/off switch to each injector.
     

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  15. shortbus=

    shortbus= Well-Known Member

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    I still can't understand why people don't use one of the many injector driver chips that are available along with a 555 to do this. Over time there have been many questions asked about doing this same thing. But when using high value mosfets in a "test" fixture, is that really going to tell you what will happen in a real world environment? Where using a real injector driver should be closer to real world, and in the end faster and easier.
     
  16. spec

    spec Well-Known Member Most Helpful Member

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    Hmm, interesting observation.

    Can you buy injector drivers, or were you thinking of using an engine management system (EMS) from an automobile?

    spec
     
  17. shortbus=

    shortbus= Well-Known Member

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  18. spec

    spec Well-Known Member Most Helpful Member

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  19. milkoni

    milkoni Member

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    Thanks for information "SB"!
    I can't use this chip, because I can't find it here in Bulgaria. Otherwise, if I buy it by the net, will be so expensive to me.

    I heed help for a circuit, that I would like to use for switching injectors on/off.
    I attach the two circuits with simulations. First circuit works fine. In second, I'm adding a relay, but it doesn't working properly in simulation.
    I'm using a 5V relay model JQC-3F-1C-5VDC.
    Coil voltage: 5 VDC
    Coil resistance: 69 Ohms
    Pickup voltage: 3.75 VDC
    Release voltage: 0=5 VDC
    Coil power consumption: 0.36 W
    Operate tome: 10 ms
    Release time: 5 ms
     

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  20. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Perhaps this circuit (which I designed for another forum member and has been built) would help?
    As drawn the circuit needs negative-going ~ 12V pulses to trigger it. For each pulse, the injector initially draws high-current during a 'fire' period, then a lower-current during a 'hold' period. Both periods are adjustable with potentiometers.
    What are the inductance and resistance values of the injectors you have?
    Injector_driver.PNG
     

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  21. milkoni

    milkoni Member

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    Thank you Alec for your circuit.
    My petrol injectors are with 14.5 Ohm resistance (high impedance). I don't know their value of inductance.
    I will use my circuit to build a cleaning machine for injectors.
    I've added a little circuit to switch On and Off injector. Because I'll clean 4 injectors simultaneously, and sometime the cleaning process for some injector have to be longer than others.
    Open time is adjustable from about 3.2 ms to 50 ms.
    Close time - 13 ms to 28 ms.
    I'm attaching my circuit.
    I've tested it on experimental board and it works pretty good. Soon I'll try make a video.
    Thanks again for advises and collaboration to Spec!!!
     

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    Last edited: Mar 14, 2017

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