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MOSFET use

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by malc9141, Sep 17, 2014.

  1. alec_t

    alec_t Well-Known Member Most Helpful Member

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    I'm sure it could be made to work just fine, but of course you'd have to do some coding and interface the CPU to the real world.. Not my forte, I'm afraid.
     
  2. malc9141

    malc9141 Member

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    OK Fair enough Danke
     
  3. malc9141

    malc9141 Member

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    Alec

    I hope this gets through.
    I am (regrettably) idle at present.
    But when you've a mo, could you show me your pulse-time image again, but with your R3 = 220Ω; R4 and R5 both 100Ω; and no R5, ie R5 = [​IMG]
    Thanks in advance

    Malcolm
     
  4. dave

    Dave New Member

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  5. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Here's the pulse timing with R3=220Ω, R4 and R7 both 100Ω, R5 removed and Trim2 set half-way:
    Trim2_halfway.PNG
    And here's the timing as above except with Trim2 set to zero:
    Trim2_zero.PNG
    The 'fire' pulse portion is missing here, because the inductor current doen't have enough time to build up.
    I don't advise removal of R5. It's there as a safety feature to keep the M1 FET switched off in the (unlikely) event that the 555 timer output became disconnected. Whether or not R5 is present makes virtually no difference to the pulse timing.
    Note that with R3 now =220Ω, if Trim2 is set close to zero then about 50mA could be flowing through the trimmer. Depending on the particular trimmer you have, that could be a tad on the high side.
    Here's a circuit mod which adds R11, R12 and C7 to ensure the trigger pulse at pin 2 of the 555 is very brief even if the supplied trigger signal from the engine gets stuck at 0V:
    InjectorFETdriver5PNG.PNG
     
    Last edited: Apr 18, 2016
  6. malc9141

    malc9141 Member

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    Many thanks Alec. I shall study this.
    A year ago I blew an expensive Injector and I wonder now if it was because R5 had been removed (though I think it was a clumsy 'dropped wire' by me.
     
  7. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Perhaps the injector blew because the FET driving it failed short-circuit. Note that my circuit doesn't include any 'hardening' components to protect it from voltage spikes, surges, static etc which typically occur in an automotive environment and might affect the input to the 12V regulator. Short-circuit failures of FETs are quite common. Circuit board layout is important. This article is worth a read.
     
  8. malc9141

    malc9141 Member

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    Hello Alec, once more.

    Please look at your circuit scheme #84.
    IT was OK.
    But after a period of non-use, I have a problem. I will describe one thing at a time. Here is #1 ---

    If the circuit gets the drop volt signal from the crank opto-sensor, the light goes on and a voltage spike appears if I test at the injector plug (+v and 0 v). So far sounds OK.
    But I notice if (carefully using only 4.5 v throughout) I leave the opto-sensor in place (ie seeing the reflector), then the light stays on. I thought the 555 would just allow a current only till it cancels via the feedback. Does this denote a problem? (Sad to say, there is a problem)

    Thank you

    Malcolm

    PS FETs seem OK, can't find any obvious fault elsewhere
     
  9. alec_t

    alec_t Well-Known Member Most Helpful Member

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    If the trigger input (from the opto) is held low, does the trigger voltage at pin 2 of the 555 also stay low?
    Do you mean you're powering the whole circuit from 4.5V? If so, it's won't work as designed.
     
  10. malc9141

    malc9141 Member

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    Trigger Low to Pin 2: I don't have an oscilloscope but I am pretty sure the answer is yes. I will try to make sure.

    4.5 v: no of course not, it used to be OK (no questions asked) on 15 v, but when I wondered what was wrong, I tested to see if the 555 would only give a short signal (yellow light flash), I used a low test voltage in case something went wrong & at a higher voltage, something might be damaged. But the light showed, implying 555 output, till I turned the reflector away.

    I was surprised that the light being on, ie the HOLD to Gate being live, was not determined by the (?) reset circuitry around the 555. Surely if the Pin 2 is kept low, the output to HOLD should not stay active?

    Thank you. Malc
     
  11. alec_t

    alec_t Well-Known Member Most Helpful Member

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    You can check with a DMM, since the opto output is being kept low.
    If pin 2 stays low, pin 3 stays high. That's the way a 555 works :D.
     
  12. malc9141

    malc9141 Member

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    1) Thank you that is reassuring because of course the pin stays low. I know it does.

    You have now told me that the 555 circuit is OK. That is important.

    2) That means the fault must lie between C5 and R7 in your schematic. The board all looks OK. I don't have it here and will try to report tomorrow

    Malc
     
  13. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Probably not, if the LED is staying on.
    If pin 2 is staying low then check R11/R12/C7
     
  14. malc9141

    malc9141 Member

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    Sorry, Alec: I know the 555 is a switch.
    All I want confirmed at this stage is if the longest time the output can be on is determined by the RC timer system which includes variable R set pot 1.
    I presume if input at 2 is low, the output to the FET gates would be transient determined by set pot 1.

    Whereas it stays on as long as Optosensor is low.
     
  15. ChrisP58

    ChrisP58 Well-Known Member

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    R11, R12, and C7 create an edge detector. The R12 side of C7 should only go low for about 100 uSecond, even if the R11 side goes low and stays low. So if both sides of C7 are always at the same DC level, then something is wrong. Check for a short across C7.

    Or, if there is enough noise on the R11 side that couple multiple edges through C7, then it could be that the 555 is being constantly retriggered.

    Time to get the o'scope out and start poking around.
     
  16. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Your presumption is incorrect. If pin2 is kept low the timer output (pin 3) will stay high for as long as pin 2 is low, so will not be determined by the Trim 1 setting. That's why, as Chris has explained, you need to check C7 (and R11/12 as I mentioned in post # 92).
     
  17. malc9141

    malc9141 Member

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    Thank you. I understand.
    Of course its the cap!!!
    Its not there!

    That is curable. But next will be the problem of why the signal to the Fire FET gate isn't present, when the signal to the Hold gate is.
    Malc
     
    Last edited: Oct 28, 2016
  18. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Check C5, R7. Perhaps someone's nicked C5, too ! :D
     
  19. malc9141

    malc9141 Member

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    I have replaced C5 just in case but no different. I will test R7 value directly.
    It doesn't explain why Fire has stopped but I will lengthen the interval by adding another 100nf in parallel.
     
  20. malc9141

    malc9141 Member

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    Hi Alec

    I am frustrated and at a loss. I am not an "electronics guy" and did have good help years back. I have access to Sheffield Univ lab but the folks there are on a different wavelength. ("A 555? Sounds like an op-amp or something").
    Anyway: I cannot get the thing to work.
    I cannot understand whats wrong.

    I am using 12 v, not 15 v

    The layout is clean and I can't fault it. We got perfect wave forms to the two Gates and two Drains; did all the right things. I got a 12 V voltage drop at the Drain. Very exciting. But the injector would not "click" as it does when it Fires. The I suddenly can't get that 12 v drop. (Unbelievably, the Univ oscilloscope proved faulty on two lanes & they just say, Its getting old. But there was a functional third channel)
    I have a 5 A fuse so can't imagine anything is blown yet suddenly the Optosensors would not work.
    But if I short the Input to Lead 2, (across the Optosensor) the light flashes. But then no good wave form. Then there is a steady 4 v PD between 555 pin 4 and 0 v. What is going on?

    Then when I measure the PD between the posts to the injector, I get 6 v. And 6 v between Drain and 0 v. The diode checks OK. I can find no leak anywhere. Each bit seems perfect. I don't understand the FETs but to the extent that I do, they seem OK. I replaced the Fire one to see if it helped. I put a new 555 in and cleaned the saddle underneath tho' it looked & tested OK.

    I am ready to give up the whole project. It is a Mech Eng job for me, lots of engineering cleverness and detail but confounded by this inability to fire the injector.

    Now, I know you can't help online any more than you have done. But one last throw of the dice. Is it possible for me to meet you directly and see if you can solve the mystery? I would drive to Cardiff if that is where you are.

    Malcolm
     
  21. alec_t

    alec_t Well-Known Member Most Helpful Member

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    When you say you are now using 12V, are you inputting that to the 7812? (Is so, the rest of the circuit will be getting only about 10V if the regulator is working).
    If you are measuring only 4V at pin 4 something is clearly wrong. The pin 4 voltage should be the same as the pin 8 voltage and the regulator output voltage.
    Hopefully we can save you a road trip to the wilds of Wales by running a simple test or two. So, first test :
    1) disconnect the injector and in its place connect a 1k (or thereabouts) resistor as a dummy load,
    2) disconnect pin 3 of the 555 from R4 and C5,
    3) make a temporary connection from pin 2 of the 555 to ground (pin 1),
    3) with 15V at the regulator input, take measurements of the voltages (referenced to ground) at all eight pins of the 555 and report back. This should tell us if the regulator and 555 are still working.
     

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