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Thread: dc motors and diodes and mofets

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    Junior Member fyrhawse's Avatar
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    Default dc motors and diodes and mofets

    hi guys,
    please see circuit picture



    the trigger switches the motors on

    but another switch is needed to ONLY switch the M2 motor

    no power should go to M1 or the Mosfet

    also need to consider flyback diodes for both motors

    any clues please ?

    cheers



Look Here ->
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    More info required.
    What is the circuit for?

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    looks like a leg puller circuit to me ????

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    OK Fyr .... you're trying to tell us what you want and how we should do it. These two things are in conflict.

    It is often easiest just to say what you're trying to do with specific detail.
    What you've drawn is about a kindergarten level schematic diagram and it's difficult to actually understand what you have, what it does and how it works.

    So lets start with what size, voltage, current the motors require.
    What are each of the motors doing?
    What is their relationship. Do they share a drive shaft or differential etc?
    What is the "MOSFET"? Everybody here knows what an actual mosfet is, but you've drawn it as a mysterious box. Telling us exactly what this piece of equipment is, with make and part numbers will help.

    I would also ask that you try and clarify how you want it to work or what it needs to do.

    You've tried to do that, but the description is confusing.
    You have two switches and two motors..

    Code:
    SW1		OFF	ON	OFF	ON	
    SW2		OFF	OFF	ON	ON	
    Motor1		OFF	ON	ON	ON		
    Motor2 		OFF	OFF	ON	ON
    Is this how you want them to function?
    Last edited by trash; 07-08-18 at 07:16 PM.
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    Junior Member fyrhawse's Avatar
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    Default pics

    well since gods busy, lemme be less shy

    its for a toy gun jelly blaster, like mini paintballs about 8mm, no it wont take over the world or start WW4.

    1.
    the only reason for the mod is that the trigger switch burns out very quickly, as do motors.
    anyways, there is a motor which drives a gearbox to push ammo out via a simple plunger.
    as this happens, the magazine motor also turns to supply ammo.

    2.
    the ammo has to be prefed by the mag motor without the gearbox motor running for a few secs.
    this is on a switch, when released its ready to shoot.

    3.
    the motors get crazy hot, and so far lost 2 motors due to heat (no flyback diodes).
    burned 1 switch.

    4.
    motors run off 2s or 3s lipo batts-7.4v to 11.1v nominal DC
    MosFet off ebay

    please see photos:


    Last edited by fyrhawse; 07-08-18 at 08:27 PM. Reason: image server down

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    Not having a flyback diode doesn't cause your heating issue but may be causing switch burn a bit, but more like over current issues are causing it.
    Motors getting too hot is likely a) they're getting too much voltage (battery voltage too high) &/or b) they're not being allowed to turn freely enough (eg the gearing ratio on the devices poor).
    Both a) & b) will be causing excessive current flow, hence heating.


    So flyback diodes go ACROSS each motor's terminals. A 1A power diode like a 1N1001 will do flyback just fine. The band on the diode goes to the motor terminal getting +ve voltage from the battery(s).

    You need Nch MOSFETs (most common) & these have three pins a Gate, Drain & Source.
    The MOSFET should be wired between the motor -ve (drain) & the battery -ve (source). You'll need a resistor like 4.7k or 10k ohms from the gate to the source (or it won't turn off).
    The switch then goes from battery +ve to gate (with resistor connected).
    The flyback diode is still across the motor.
    Last edited by Redpacketboy; 07-08-18 at 10:27 PM.

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    Junior Member fyrhawse's Avatar
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    heres the question again as the first seems to have created confusion


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    Why not just cut one of the wires of the motor you DON'T want to run, and insert a switch there ?

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    My guess would be that the motors don't have enough grunt to get moving and a stalled motor is essentially a short circuit so the solution in that case is larger motors or gearing to allow them to move more freely. I don't see the need for 'Flyback' diodes in this application other than to protect the 'mosfet' which I'm assuming is a 'buck converter' type power supply?

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    Righto... so your motors are getting hot because they're being worked, or overworked.
    The back EMF is burning out your switch contacts and you're trying to replace it with a solid state switch.
    I'd be using a capacitor across the motors and switches and reducing their current.
    This is going to slow the action of the gun down, but it extend it's life.

    So how does the gun achieve the switching and timing when it is stock standard?
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    The OP has to work out why the motors are getting so hot.
    Unfortunately, no MOSFETs, capacitors, flyback diode, back emf solutions, etc are going to fix that ;-)
    It's most likely poor mechanical design & needs better gearing &/or higher load capable motors.
    If the existing gears are not well oiled, some extra might help, eg "Lanox" (leaves a film, find at Bunnings).

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