Unintended Firing

Discussion in 'Gun Building, Modifications & Repairs' started by OutlawAirsoft, Jan 30, 2021.

  1. OutlawAirsoft

    OutlawAirsoft Active Member

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    Hey all,
    I am installing a very simple 3034 mosfet in an ar47 I recently purchased and am having an interesting issue. The 'fet works fine, but with everything put together, when I switch from semi auto to full auto it starts cycling. It only cycles about a quarter of the way, but it is anoying and sometimes that causes it to fire. I have looked inside the gearbox but don't see anything obviously wrong. I suspect the cutoff lever is bumping the trigger trolley enough to complete the circuit, but I can't tell for sure. It also occasionally fires full auto in semi auto mode.
    Any advice would be greatly appreciated.
     
  2. Guges Mk3

    Guges Mk3 Administrator Staff Member Administrator Supporting Member Lifetime Supporter

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    And why are we adding this FET to the system right now?
     

  3. wetpee

    wetpee Active Member

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    I think you have the right idea. It could be difficult to isolate the issue since the sector gear can affect the position of the cutoff. If you re-shimmed, try shimming the gears a bit higher or maybe try a stronger spring on the trolley. When I was first starting out, I was having issues shimming too low in standard mechanical switch systems, causing the sector to rub on the cutoff lever and not allow full range of motion. It would commonly block the trigger trolley completely or otherwise make a clicking noise if it was able to slip by OR just not allow the COL to engage the trolley at all and just shoot full auto in semi. At the time, I was having trouble getting good shimming on the gun (VFC avalon), so I opted for modding the trolley by sanding more of a slope on the surface that was meeting the cutoff lever so it could slide by. After this mod, it worked flawlessly, but this did not prove fruitful for me in another attempt, in which I opted to re-shim higher (Specna Arms Edge M4).
     
    Last edited: Feb 4, 2021
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  4. OutlawAirsoft

    OutlawAirsoft Active Member

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    It was not rated for LiPo batteries, and it is an extremely simple operation. Let me clarify that this was a "boneyard" product from Evike, so it was never working before now. I'll do some troubleshooting tonight.
     
  5. Guges Mk3

    Guges Mk3 Administrator Staff Member Administrator Supporting Member Lifetime Supporter

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    Umm...not rated for LIPO batteries is a Misnomer.

    Any AEG can run "any" battery as long as the voltage is within spec of the AEG's operating design. This can be batteries made of any chemistry. NICD, NIMH, NIZN, LIPO LIPO4FE, LION, PB...lemons....8 volts is 8 volts regardless of the battery chemistry.
     
  6. Raven1st

    Raven1st Well-Known Member

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    Does the selector plate have a metal plate on it? Some companies use that design to break the circuit when in placed in safe.
    It’s also is known to short if parts get bumped around a little too much.
     
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  7. Ben3721

    Ben3721 Well-Known Member

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    I would check for a sticky cutoff lever and trigger trolley and if the safety is hitting anything it shouldn't. Something is sticking or snagging. I would also check for any shorts if you have a voltmeter.
    Many just install a mosfet into every aeg they use. Less resistance can mean a couple more rps in some cases.
     
  8. OutlawAirsoft

    OutlawAirsoft Active Member

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    LiPo's tend to produce more amperage than NiMH's, and this increased amperage coupled with the jump from 9.6v to 11.1v can cause the trigger contacts to arc, which is not ideal.

    Yes it does. It does seem as if it is shorting through this, but I can't see how. Should I remove the plate? I could rig something up to bypass the plate, but would this be feasible?

    I did some diagnosing earlier today and found that it only misbehaves when the grip screws are tightened all the way. If I leave them backed off a little bit (enough to keep the grip firm, but not reefed on), it does not misfire. But when I was using it, apparently my holding it produces the same effect that tightening the grip screws does, and it fires again abnormally.
     
  9. Guges Mk3

    Guges Mk3 Administrator Staff Member Administrator Supporting Member Lifetime Supporter

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    This is not how electricity works. Electric items works on Draw...not Push, which you are implying in your statement.

    The arcing is due more to Voltage and not Amperage.

    Your phone charger is hooked up to a 110A line that is connected to a megawatt electric grid...yet it doesn't explode in a fireball when you plug it in.

    Here is my battery simulator video. This device could drop 100A into the line...but the AEG is only "Drawing" what it needs to run, regardless of voltage.

     
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  10. aotsukisho

    aotsukisho Well-Known Member

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    Not true - the internal resistance and chemistry needs to match the power delivery and usage demands of the application. "Voltage is voltage" is a flawed statement that only holds true in an open circuit

    As-is this implies AAA alkalines or Titan li-ion batteries are acceptable for AEG use, when in reality they are both laughably underpowered. On the other hand, we don't go out and buy 150C lipos for our bluetooth speakers because they don't need expensive batteries that can handle 500A bursts.

    Running your electric LMG on a car battery until you hit voltage cutoff is not desirable either. Even though it has great voltage and constant current specifications for an AEG, standard lead-acid batteries do not like being deep discharged and that means the chemistry is not suited for this application.

    True, although I don't think people take into consideration that a computerized MOSFET controller (like the Titan) results in a drop on RPS due to the complexity of the control circuit compared to a more direct implementation.

    Amperage is not produced by a battery, it is drawn by a load. Ohm's Law V=IR. Lipo batteries tend to have a lower internal resistance than NiMH, which means they can sustain higher current before overheating, and also why the motor will 'see' more effective voltage if all other variables are kept identical.

    Guges posted video proof that amperage does not increase in an AEG just because voltage changes.

    Trigger contact arcing is caused by inductive back-EMF from the motors, which can be suppressed by kickback diodes connected to the motor terminals. Marui spec trigger switches can arc regardless of what you use to power it. Surface pitting caused by arcing leads to lifespan reduction of the switch assembly, but the higher constant current passing through may exceed the spec and melt the switch especially if lipo batteries are paired with a more powerful motor.

    The metal plate is due to Marui spec trigger switches having an electrical safety through the hole in the gearbox underneath it. If your trigger switch does not have contacts that the selector plate touches, then the metal plate on the selector is redundant and can be removed.

    Check carefully for pinched wires or damaged insulation around the bottom of the gearbox, potentially in the gearbox around where the motor enters. I am currently working on an MP5K that randomly goes into uncontrollable full auto likely because the trigger switch wires to the MOSFET got pinched.
     
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  11. Ben3721

    Ben3721 Well-Known Member

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    Figured it out. Your shorting out the contacts on the switch to the body of the gun. Hence why when its tight it shoots. You need to either resolder away from the outside or heat shrink/electrical tape to prevent the trigger tabs from protruding outwards. This also can really screw up a mosfet if its barely touching and basically bouncing with a lot of resistance on the gate.

    This is true, however many mini nimh (or heck a small li-ion battery) doesn't really put out a lot of amps, which can bottleneck a setup pretty low preventing contacts from burning out assuming the motor needs more amps to run at 100%, so stepping to a battery with a higher discharge rate and more voltage to meet a setups needs can increased stress on the electronics because now the motor is getting as much as it needs compared to maxing out a weak battery and having less trigger contact and motor comm wear. Yeah a motor will take only what it needs but some batteries definitely don't supply enough power for some setups.
    One could theoretically use a better trigger switch like lonex then use a schottky diode on the motor with a 30amp+ setup and not burn anything for a long time. But the resistance would be worse than using a mosfet, but for a mild setup it can just be more convenient.
     
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  12. Guges Mk3

    Guges Mk3 Administrator Staff Member Administrator Supporting Member Lifetime Supporter

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    Sorry Aotsukisho, I over simplified my battery statement

    Ben when you overdraw a NIMH you get voltage sag. I think that is what you are implying. Which, leads to less voltage that in turn leads to less arcing.
     
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  13. Raven1st

    Raven1st Well-Known Member

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    As others have said since it misfires when you adjust the grip screws it’s more than likely a short there.

    I would so check the wiring around the grip to make sure it’s not getting worn down by the pinion gear or has any breaks in the insulation from removing the motor.
     
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  14. aotsukisho

    aotsukisho Well-Known Member

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    Technically voltage has a factor due to Ohm's Law but an electromagnetic field is sustained in an inductor with current, and when that current is removed the collapsing EM field will cause a high voltage to appear across it. In the plumbing analogy, voltage is water pressure but amperage is how quickly the water is actually flowing...and back-EMF can I guess be compared to water hammer.

    NiMH can put out a lot of current (5-10C, sometimes more) but internal resistance of the smaller airsoft-sized batteries are too high when used in AEG systems with powerful motors and/or heavy load. Like Guges said the voltage sag is what is limiting power, a larger 1:10 RC NiMH pack with lower IR should have no problem dumping 50A or so since that's what the Tamiya RS540 Sport Tuned motor (which has been around forever) pulls in most applications. I've had an old 8c 9.6v NiMH battery sag to 2v under heavy load and the motor will obviously not have enough power at that point to spin.
     
  15. Ben3721

    Ben3721 Well-Known Member

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    Its a ak so the wires don't pass the pinion and those ak grips are plastic or wood, if it shorted out to the cage it would stop firing rather than fire. Tighting tho pulls down the gearbox which would explain the a short around the switch tabs to the gun body.
     
  16. OutlawAirsoft

    OutlawAirsoft Active Member

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    Technically it is an AR47, so it has an m4 style body but feeds ak style mags.

    To fix the issue, I'm going to try shimming the gearbox up so it doesn't short, and if that doesn't work, I'm going to try insulating everything that I can think of and hope that catches the short.

    I thought the same thing, but it is clean and undamaged there. Plus, it only fires when I mess with the fire selector.
     
  17. Ben3721

    Ben3721 Well-Known Member

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    ...well then... that changes a lot lol... excuse my ignorance. Are your grip screws too long and dig into the wires? You'll just have to do a complete wire short check to figure this out.
     
  18. OutlawAirsoft

    OutlawAirsoft Active Member

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    I don't think that is necessary for two reasons:
    1. It only misfires when I shift from semi auto to full auto, not randomly.
    2. The same effect is produced when I hold the gun normally, applying pressure to the grip.

    I suspect the issue is that when the grip is tight (or the gearbox is sucked to the bottom of the receiver), the fire selector shorts somewhere.
     
  19. Raven1st

    Raven1st Well-Known Member

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    I highly suggest pulling out the wiring harness and checking it for exposed wiring or small cracks in the insulation.

    I would also apply more shrink tubing to the motor connectors like others have said, it sounds like that area is lending a fair share to the problem.
     
  20. OutlawAirsoft

    OutlawAirsoft Active Member

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    I have discovered the issue; the metal plate on the fire selector was touching the shell and the bottom trigger contact at the same time, but only when it is in between semi auto and full auto. That means that there is a short between the other signal wire and the gearbox shell somewhere. I don't have time to find it today (I'm going to an airsoft birthday party in 2 hours), so I just removed the plate and soldered a tiny wire between the external contacts, making sure it wouldn't short to the shell. Here is a mock-up I did: Short Example Pic.png
    Some other time I will find the other short and fix it, but this appears to work for now.
     
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